Chromosome Final Today.ppt Thirunahari Ugandhar

Thirunahari Ugandhar

1. CHROMOSOMESCHROMOSOMES
By
Dr. Thirunahari
Ugandhar
Associate Prof of Botany
Department of Botany
Kakatiya Govt College (A)
Hanamkonda

5. What Exactly is a chromosome?What Exactly is a chromosome?
Chromosomes are the Chromosomes are the rod-shapedrod-shaped, , filamentous filamentous
bodies bodies present in the present in the nucleusnucleus, which become , which become
visible visible during cell divisionduring cell division. .
They are the They are the carriers of the gene carriers of the gene or unit of or unit of
heredity.heredity.
Chromosome are Chromosome are not visible not visible in active nucleus due in active nucleus due
to their to their high water contenthigh water content, but are clearly , but are clearly
seen during cell division. seen during cell division.

8. History of Chromosomes

Chromosomes were first described by Chromosomes were first described by
Strausberger Strausberger in in 18751875. .

The term “Chromosome”, however was The term “Chromosome”, however was
first used by first used by Waldeyer Waldeyer in in 18881888. .

They were given the name chromosome They were given the name chromosome
(Chromo = colour; Soma = body) due to (Chromo = colour; Soma = body) due to
their marked their marked affinity for basic dyesaffinity for basic dyes. .

Their number can be counted easily only Their number can be counted easily only
during during mitotic metaphase.mitotic metaphase.

9. 
Chromosomes are composed of Chromosomes are composed of thin thin
chromatin chromatin threads called threads called Chromatin Chromatin
fibersfibers. .

These fibers undergo These fibers undergo foldingfolding, , coiling coiling and and
supercoiling supercoiling during prophase so that the during prophase so that the
chromosomes become progressively chromosomes become progressively
thicker and smaller. thicker and smaller.

Therefore, chromosomes become readily Therefore, chromosomes become readily
observable under light microscope. observable under light microscope.

At the end of cell division, on the other At the end of cell division, on the other
hand, the fibers uncoil and extend as hand, the fibers uncoil and extend as
fine chromatin threads, which are not fine chromatin threads, which are not
visible at light microscopevisible at light microscope

10. DEFINITION OF CHROMOSOME
It is a combination of two words, i.e., “It is a combination of two words, i.e., “ChromaChroma”-means ‘”-means ‘colourcolour’ ’
and “and “SomesSomes”-means ‘”-means ‘bodybody’.’.
So the coloured thread like bodies present in the nucleoplasm of So the coloured thread like bodies present in the nucleoplasm of
the living cells, which helps in the inheritance (transmission) of the living cells, which helps in the inheritance (transmission) of
characters in form of Genes from generation to generation are characters in form of Genes from generation to generation are
known as known as CHROMOSOMES.CHROMOSOMES.

11. NUMBER OF CHROMOSOMESNUMBER OF CHROMOSOMES
The number of chromosomes per organism is always a definite number, Which
is said as Diploid (2n) no., but gametes, sperms, ova etc. carry Haploid (n)
number. Some examples are given below.
Name of theDiploid No.Name of theDiploid No.
organism (2n) organism (2n)
Human beings ---- 46 Onions ----- 16
Cat ---- 38 Corn ----- 20

12. PHYSICAL STRUCTUREPHYSICAL STRUCTURE

Size varies from 1 to 30 micron in length and diameter from 0.2 to
2 micron.
CENTROMERE:- The non-stainable part of the chromosome making
a primary constriction.
CHROMATIDS:- Two chromatids join at the centromere to form a
chromosome.
CHROMONEMA:- In each chromatid, there are two longitudinal
chromonemata coiled with each other.
CHROMOMERES:- In each chromonemata, there are “bead” like
chromomeres present through out the coil.
GENES:- Each chromomeres contains genes, the unit of inheritance of
character.
SATELLITE:- In some chromosomes a round and elongated satellite is
present.
CONSTRICTION:- Presence of centromere shows the primary
constriction. But in some cases there is an additional Secondary
Constriction.
SURFACE VIEW

13. CHEMICAL STRUCTURECHEMICAL STRUCTURE
Chemically the chromosomes are made of proteins
and nucleic acids.
PROTEINS It is mainly Protamines, Histones and smaller amount of
acidic proteins.
NUCLEIC ACIDS It is de-oxy ribose Nucleic Acids (DNA). Genes are
nothing but the segments of DNA.
NB:- For brief notes about DNA structure, “Open the
Hyperlink at Right End.”
HYPERLINK
CLICK

14. Number of chromosomesNumber of chromosomes

Normally, all the individuals of a Normally, all the individuals of a species have the species have the
same number same number of chromosomes. of chromosomes.

Closely related species usually have similar Closely related species usually have similar
chromosome numbers. chromosome numbers.

Presence of a whole sets of chromosomes is called Presence of a whole sets of chromosomes is called
euploidyeuploidy. .

It includes haploids, diploids, triploids, tetraploids It includes haploids, diploids, triploids, tetraploids
etc.etc.

Gametes normally contain only one set of Gametes normally contain only one set of
chromosome – this number is called chromosome – this number is called HaploidHaploid

Somatic cells usually contain two sets of Somatic cells usually contain two sets of
chromosome - chromosome - 2n : Diploid2n : Diploid

15. STATES OF CHROMOSOMESSTATES OF CHROMOSOMES
3n – triploid3n – triploid
4n – tetraploid4n – tetraploid
The condition in which the chromosomes sets The condition in which the chromosomes sets
are present in a multiples of “n” is are present in a multiples of “n” is PolyploidyPolyploidy
When a change in the chromosome number does When a change in the chromosome number does
not involve entire sets of chromosomes, but not involve entire sets of chromosomes, but
only a few of the chromosomes - is only a few of the chromosomes - is
Aneuploidy.Aneuploidy.

Monosomics (2n-1)Monosomics (2n-1)

Trisomics (2n+1)Trisomics (2n+1)

Nullisomics (2n-2)Nullisomics (2n-2)

Tetrasomics (2n+2)Tetrasomics (2n+2)

16. Organism No. chromosomes
Human 46
Chimpanzee 48
Dog 78
Horse 64
Chicken 78
Goldfish 94
Fruit fly 8
Mosquito 6
Nematode 11(m), 12(f)
Horsetail 216
Sequoia 22

Round worm Round worm 22

17. Organism No. chromosomes

OnionOnion 1616

MoldMold1616

Carrot Carrot 2020

Tomato Tomato 2424

Tobacco Tobacco 4848

Rice Rice 2424

Maize Maize 2020

Haploppus gracilis Haploppus gracilis 44

Crepis capillaris Crepis capillaris 6

18. NUMBER OF CHROMOSOMESNUMBER OF CHROMOSOMES

On the extreme, On the extreme, round worm round worm shows only shows only two two
chromosomeschromosomes, while the other extreme is , while the other extreme is
represented by represented by Protozoa Protozoa having having 300 or more 300 or more
chromosomeschromosomes. .

However, most organisms have numbers However, most organisms have numbers
between between 12 to 5012 to 50. .

3-8 3-8 in fungiin fungi

From From 8 – 16 8 – 16 in Angiosperms (Most common in Angiosperms (Most common
number being number being 1212).).

19. Chromosome SizeChromosome Size

In contrast to other cell organelles, the size of In contrast to other cell organelles, the size of
chromosomes shows a remarkable variation depending chromosomes shows a remarkable variation depending
upon the stages of cell division. upon the stages of cell division.

Interphase:Interphase: chromosome are longest & thinnestchromosome are longest & thinnest

Prophase:Prophase: there is a progressive decrease in their length there is a progressive decrease in their length
accompanied with an increase in thicknessaccompanied with an increase in thickness

Anaphase: Anaphase: chromosomes are smallest.chromosomes are smallest.

Metaphase: Metaphase: Chromosomes are the most easily observed Chromosomes are the most easily observed
and studied during metaphase when they are very thick, and studied during metaphase when they are very thick,
quite short and well spread in the cell.quite short and well spread in the cell.


TTherefore, chromosomes measurements are generally herefore, chromosomes measurements are generally
taken during mitotic metaphase. taken during mitotic metaphase.

20. The size of the chromosomes in mitotic phase of animal The size of the chromosomes in mitotic phase of animal
and plants sp generally varies between and plants sp generally varies between 0.5 µ and 32 µ 0.5 µ and 32 µ in in
length, and between length, and between 0.2 µ and 3.0 µ 0.2 µ and 3.0 µ in diameter. in diameter.
The longest metaphase chromosomes found in The longest metaphase chromosomes found in TrilliumTrillium - -
32 µ32 µ. .
The giant chromosomes found in The giant chromosomes found in diptera diptera and they may be and they may be
as long as as long as 300 µ 300 µ and up to and up to 10 µ 10 µ in diameter.in diameter.
In general, In general, plants have longer chromosomes plants have longer chromosomes than animal than animal
and species having lower chromosome numbers have and species having lower chromosome numbers have
long chromosomes than those having higher long chromosomes than those having higher
chromosome numberschromosome numbers
Among plants, Among plants, dicots dicots in general, have a higher number of in general, have a higher number of
chromosome than monocots.chromosome than monocots.
Chromosomes are longer in monocot than dicots. Chromosomes are longer in monocot than dicots.

21. KARYOTYPEKARYOTYPE

In order to understand chromosomes and their In order to understand chromosomes and their
function, we need to be able to discriminate among function, we need to be able to discriminate among
different chromosomes. different chromosomes.

First, chromosomes differ greatly in sizeFirst, chromosomes differ greatly in size

Between organisms the size difference can be over Between organisms the size difference can be over
100-fold, while within a sp, some chromosomes are 100-fold, while within a sp, some chromosomes are
often 10 times as large as others.often 10 times as large as others.

In a species In a species KaryotypeKaryotype, a pictorial or photographic , a pictorial or photographic
representation of all the different chromosomes in a representation of all the different chromosomes in a
cell of an individual, chromosomes are usually cell of an individual, chromosomes are usually
ordered by size and numbered from largest to ordered by size and numbered from largest to
smallest. smallest.

22. Can distinguish chromosomes by “painting” – using DNA
hybridization + fluorescent probes – during mitosis

23. KARYOTYPE IDIOTYPEKARYOTYPE IDIOTYPE

KaryotypeKaryotype: is the general morphology of the somatic : is the general morphology of the somatic
chromosome. Generally, karyotypes represent by arranging chromosome. Generally, karyotypes represent by arranging
in the descending order of size keeping their centromeres in in the descending order of size keeping their centromeres in
a straight line.a straight line.

IdiotypeIdiotype: the karyotype of a species may be represented : the karyotype of a species may be represented
diagrammatically, showing all the morphological features of diagrammatically, showing all the morphological features of
the chromosome; such a diagram is known as the chromosome; such a diagram is known as Idiotype. Idiotype.

24. 
Chromosomes may differ in the position of the Chromosomes may differ in the position of the CentromereCentromere, the , the
place on the chromosome where spindle fibers are attached during place on the chromosome where spindle fibers are attached during
cell division. cell division.

In general, if the centromere is near the middle, the chromosome is In general, if the centromere is near the middle, the chromosome is
metacentricmetacentric

If the centromere is toward one end, the chromosome is If the centromere is toward one end, the chromosome is acrocentric acrocentric
or or submetacentricsubmetacentric

If the centromere is very near the end, the chromosome is If the centromere is very near the end, the chromosome is
telocentrictelocentric. .

26. TYPES OF CHROMOSOMESTYPES OF CHROMOSOMES
1.TELOCENTRIC :- The centromere is
present at the end of the chromosomes.
2. ACROCENTRIC :-The centromere
is almost terminal. It has one large and
another very small arm.
LONG ARM
SHORT ARM
CENTROMERE
LONG ARM
CENTROMERECENTROMERE

27. TYPES OF CHROMOSOMESTYPES OF CHROMOSOMES
(CONTINUED)(CONTINUED)
3. SUB-METACENTRIC :- Here the centromere
is not at the middle position of the chromosomes.
So the arms are unequal and it is ‘L-Shaped’ in
appearance.
4. METECENTRIC:- The centromere
is at the middle position. So the arms
are equal and it is ‘V-Shaped’ in
appearance.
LONG ARM
CENTROMERE
SHORT ARM
CENTROMERE
TWO EQUAL ARMS

28. 
The centromere divides the chromosome into two arms, so The centromere divides the chromosome into two arms, so
that, for example, an acrocentric chromosome has one that, for example, an acrocentric chromosome has one
short and one long arm, short and one long arm,

While, a metacentric chromosome has arms of equal length. While, a metacentric chromosome has arms of equal length.

All house mouse chromosomes are telocentric, while All house mouse chromosomes are telocentric, while
human chromosomes include both metacentric and human chromosomes include both metacentric and
acrocentric, but no telocentricacrocentric, but no telocentric. .

29. Autosomal pairAutosomal pair Sex chromosome Sex chromosome
DiploidDiploid No. of No. of No. of No. of X Y X Y
(2n) metacentrics acrocentric or telocentric(2n) metacentrics acrocentric or telocentric
CatCat 3838 1616 22 M M M M
DogDog 7878 00 3838 M A M A
PigPig 3838 1212 66 M M M M
GoatGoat 6060 00 2929 A A M M
Sheep Sheep 5454 33 2323 A A M M
Cow Cow 606000 2929 M M M M
HorseHorse6464 1313 1818 M A M A
M – Metacentric; A – AcrocentricM – Metacentric; A – Acrocentric

30. Satellite DNAsSatellite DNAs
When the DNA of a prokaryote, such as E.coli, is isolated, When the DNA of a prokaryote, such as E.coli, is isolated,
fragmented and centrifuged to equilibrium in a Cesium fragmented and centrifuged to equilibrium in a Cesium
chloride (CsCl) density gradient, the DNA usually forms a chloride (CsCl) density gradient, the DNA usually forms a
single band in the gradient.single band in the gradient.
On the other hand, CsCl density-gradient analysis of DNA from On the other hand, CsCl density-gradient analysis of DNA from
eukaryotes usually reveals the presence of one large band of DNA eukaryotes usually reveals the presence of one large band of DNA
(usually called “(usually called “MainbandMainband” DNA) and one to several small bands. ” DNA) and one to several small bands.
These small bands are referred to as “These small bands are referred to as “Satellite DNAsSatellite DNAs”.”.
For e.g., in Drosophila virilis, contain three distinct satellite DNAs.For e.g., in Drosophila virilis, contain three distinct satellite DNAs.

31. Euchromatin and HeterochromatinEuchromatin and Heterochromatin

Chromosomes may be identified by regions that stain in a Chromosomes may be identified by regions that stain in a
particular manner when treated with various chemicals. particular manner when treated with various chemicals.

Several different chemical techniques are used to identify Several different chemical techniques are used to identify
certain chromosomal regions by staining then so that they certain chromosomal regions by staining then so that they
form form chromosomal bands.chromosomal bands.

For example, darker bands are generally found near the For example, darker bands are generally found near the
centromeres or on the ends (telomeres) of the chromosome, centromeres or on the ends (telomeres) of the chromosome,
while other regions do not stain as strongly.while other regions do not stain as strongly.

The position of the dark-staining areThe position of the dark-staining are heterochromatic heterochromatic
region region oror heterochromatin heterochromatin..

Light staining are Light staining are euchromatic region euchromatic region or or euchromatineuchromatin. .

32. 
Heterochromatin is classified into two groups: (i) Constitutive Heterochromatin is classified into two groups: (i) Constitutive
and (ii) Facultative. and (ii) Facultative.

Constitutive heterochromatin remains permanently in the Constitutive heterochromatin remains permanently in the
heterochromatic stage, i.e., it does not revert to the euchromatic heterochromatic stage, i.e., it does not revert to the euchromatic
stage. stage.

In contrast, facultative heterochromatin consists of In contrast, facultative heterochromatin consists of
euchromatin that takes on the staining and compactness euchromatin that takes on the staining and compactness
characteristics of heterochromatin during some phase of characteristics of heterochromatin during some phase of
development. development.

..

33. 
Not only the genomes of eukaryotes are more complex than Not only the genomes of eukaryotes are more complex than
prokaryotes, but prokaryotes, but the DNA of eukaryotic cell the DNA of eukaryotic cell is also organized is also organized
differently from that of prokaryotic cells. differently from that of prokaryotic cells.

The genomes of prokaryotes are contained in The genomes of prokaryotes are contained in single single
chromosomeschromosomes, which are usually , which are usually circular circular DNA molecules.DNA molecules.

In contrast, the genomes of eukaryotes are composed of In contrast, the genomes of eukaryotes are composed of
multiple chromosomesmultiple chromosomes, each containing a , each containing a linear molecular linear molecular of of
DNADNA. .

34. 
Although the Although the numbers and sizes numbers and sizes of chromosomes of chromosomes vary vary
considerably between different species, their basic considerably between different species, their basic
structure is the same in all eukaryotesstructure is the same in all eukaryotes
OrganismOrganism Genome Genome ChromosomeChromosome
Size (Mb)Size (Mb)
aa
number number
aa
Arabidopsis thalianaArabidopsis thaliana 70 70 5 5
CornCorn 5000 5000 10 10
OnionOnion 15,000 15,000 8 8
LilyLily 50,000 50,000 12 12
Fruit flyFruit fly 165 165 4 4
ChickenChicken 50,000 50,000 39 39
Mouse 1,200 20Mouse 1,200 20
Cow 3000 30Cow 3000 30
Human 3000 23Human 3000 23
a – both genome size and chromosome numbers are for haploid cellsa – both genome size and chromosome numbers are for haploid cells

35. 
The DNA of eukaryotic cell is The DNA of eukaryotic cell is tightly bound tightly bound to small to small
basic proteins (histones) basic proteins (histones) that package the DNA in an that package the DNA in an
orderly way in the cell nucleus. orderly way in the cell nucleus.

This task is substantial (necessary), given the DNA content This task is substantial (necessary), given the DNA content
of most eukaryotesof most eukaryotes

For e.g., the total extendedFor e.g., the total extended length of DNA in a human cell length of DNA in a human cell
is nearly is nearly 2 m2 m, but this must be fit into a nucleus with a , but this must be fit into a nucleus with a
diameter of only diameter of only 5 to 10µm. 5 to 10µm.

Although DNA packaging is also a problem in bacteria, the Although DNA packaging is also a problem in bacteria, the
mechanism by which prokaryotic DNA are packaged in mechanism by which prokaryotic DNA are packaged in
the cell appears distinct from that eukaryotes and is not the cell appears distinct from that eukaryotes and is not
well understood. well understood.

36. Prokaryotic chromosomeProkaryotic chromosome

The prokaryotes usually have The prokaryotes usually have only one only one
chromosomechromosome, and it bears little , and it bears little
morphological resemblance to morphological resemblance to
eukaryotic chromosomes. eukaryotic chromosomes.

Among prokaryotes there is Among prokaryotes there is
considerable variation in considerable variation in genome genome
lengthlength bearing bearing genesgenes. .

The genome length is smallest in The genome length is smallest in RNA RNA
virusesviruses

In this case, the organism is provided In this case, the organism is provided
with only a with only a few genes few genes in its in its
chromosome. chromosome.

The number of gene may be as high as The number of gene may be as high as
150150 in some larger in some larger bacteriophage bacteriophage
genome.genome.

37. 
In In E.coliE.coli, about , about 3000 to 4000 genes 3000 to 4000 genes are organized are organized
into its one circular chromosome. into its one circular chromosome.

The chromosome exists as a The chromosome exists as a highly folded highly folded and and coiledcoiled
structure dispersed throughout the cell. structure dispersed throughout the cell.

The folded nature of chromosome is due to the The folded nature of chromosome is due to the
incorporation of RNA with DNA.incorporation of RNA with DNA.

There are about There are about 50 loops 50 loops in the chromosome of in the chromosome of
E.coli. E.coli.

These loops are highly These loops are highly twisted twisted or or supercoiled supercoiled
structure with about structure with about four million nucleotide pairsfour million nucleotide pairs. .

Its molecular weight is aboutIts molecular weight is about 2.8 X10 2.8 X10
99

During replication of DNA, the coiling must be During replication of DNA, the coiling must be
relaxed. relaxed.

DNA gyrase DNA gyrase is necessary for the unwinding the coils. is necessary for the unwinding the coils.

38. Bacterial ChromosomeBacterial Chromosome
Single, circular DNA molecule located in the
nucleoid region of cell

39. SupercoilingSupercoiling

40. SupercoilingSupercoiling
The helix twists
on itself; twists
to the right
Helix twists on
itself in the opposite
direction; twists to
the left
Most common type
of supercoiling

41. Mechanism of folding of a bacterial chromosomeMechanism of folding of a bacterial chromosome
There are many supercoiled loops (~100 in E. coli) attached to a
central core. Each loop can be independently relaxed or condensed.
Topoisomerase enzyme – (Type I and II) that introduce or remove
supercoiling.

42. ChromatinChromatin

The complexes between eukaryotic DNA and proteins are The complexes between eukaryotic DNA and proteins are
called called ChromatinChromatin, which typically contains about twice as , which typically contains about twice as
much protein as DNA. much protein as DNA.

The major proteins of chromatin are the The major proteins of chromatin are the histones histones – small – small
proteins containing a high proportion of basic aminoacids proteins containing a high proportion of basic aminoacids
((arginine and lysinearginine and lysine) that facilitate binding negatively ) that facilitate binding negatively
charged DNA molecule . charged DNA molecule .

There are There are 5 major types 5 major types of histones: of histones: H1, H2A, H2B, H3, H1, H2A, H2B, H3,
and and H4 H4 – which are very similar among different sp of – which are very similar among different sp of
eukaryotes. eukaryotes.

The histones are extremely abundant proteins in eukaryotic The histones are extremely abundant proteins in eukaryotic
cells.cells.

Their mass is approximately equal to that of the cell’s DNATheir mass is approximately equal to that of the cell’s DNA

43. The major histone proteins:The major histone proteins:
Histone Mol. WtHistone Mol. WtNo. of No. of PercentagePercentage
Amino acidAmino acidLys + ArgLys + Arg
H1H1 22,500 22,500244244 30.830.8
H2AH2A 13,960 13,960129129 20.220.2
H2BH2B 13,774 13,774125125 22.422.4
H3H3 15,273 15,273135135 22.922.9
H4H4 11,236 11,236102102 24.524.5
The DNA double helix is bound to proteins called histones.  The
histones have  positively charged (basic) amino acids to bind the
negatively charged (acidic) DNA.  Here is an SDS gel of histone
proteins, separated by size

44. 
In addition, chromatin contains an approximately equal In addition, chromatin contains an approximately equal
mass of a wide variety of mass of a wide variety of non-histone chromosomal non-histone chromosomal
proteinsproteins. .

There are more than a There are more than a thousand different types thousand different types of these of these
proteins, which are involved in a range of activities, proteins, which are involved in a range of activities,
including including DNA replication DNA replication and and gene expressiongene expression. .

The DNA of prokaryotes is similarly associated with The DNA of prokaryotes is similarly associated with
proteins, some of which presumably function as histones proteins, some of which presumably function as histones
do, packing the DNA within the bacterial cell. do, packing the DNA within the bacterial cell.

Histones, however are unique feature of eukaryotic cells Histones, however are unique feature of eukaryotic cells
and are responsible for distinct structural organization of and are responsible for distinct structural organization of
eukaryotic chromatineukaryotic chromatin

45. 
The basic structural unit of chromatin, the The basic structural unit of chromatin, the nucleosomenucleosome, was , was
described by described by Roger Kornberg Roger Kornberg in in 1974. 1974.

Two types of experiments led to Kornberg’s proposal of the Two types of experiments led to Kornberg’s proposal of the
nucleosome model.nucleosome model.

First, First, partial digestion of chromatin with micrococcal nuclease partial digestion of chromatin with micrococcal nuclease (an (an
enzyme that degrades DNA) was found to yield DNA fragments enzyme that degrades DNA) was found to yield DNA fragments
approximately approximately 200 base pairs long200 base pairs long..

In contrast, a similar digestion of naked DNA (not associated with In contrast, a similar digestion of naked DNA (not associated with
protein) yielded a protein) yielded a continuous smear randomly sized fragmentscontinuous smear randomly sized fragments..

These results suggest that the binding of proteins to DNA in These results suggest that the binding of proteins to DNA in
chromatin protectschromatin protects the regions of DNA from the regions of DNA from
nuclease digestion, so that enzyme can nuclease digestion, so that enzyme can
attack DNA only at sites separated byattack DNA only at sites separated by
approximately 200 base pairs. approximately 200 base pairs.

46. 
Electron microscopy revealed that chromatin Electron microscopy revealed that chromatin
fibers have a beaded appearance, with the beads fibers have a beaded appearance, with the beads
spaced at intervals of approximately 200 base spaced at intervals of approximately 200 base
pairs. pairs.

Thus, both nuclease digestion and the electron Thus, both nuclease digestion and the electron
microscopic studies suggest that chromatin is microscopic studies suggest that chromatin is
composed of repeating 200 base pair unit, which composed of repeating 200 base pair unit, which
were called were called nucleosome.nucleosome.

47. individual nucleosomes = “beads on a string”

48. 
Detailed analysis of these nucleosome core Detailed analysis of these nucleosome core
particles has shown that they contain 146 base particles has shown that they contain 146 base
pairs of DNA wrapped 1.75 times around a pairs of DNA wrapped 1.75 times around a
histone core consisting of two molecules each of histone core consisting of two molecules each of
H2A, H2B, H3, and H4 (the core histones). H2A, H2B, H3, and H4 (the core histones).

One molecule of the fifth histone H1, is bound One molecule of the fifth histone H1, is bound
to the DNA as it enters and exists each to the DNA as it enters and exists each
nucleosome core particle. nucleosome core particle.

This forms a chromatin subunit known as This forms a chromatin subunit known as
chromatosomechromatosome, which consist of 166 base pairs , which consist of 166 base pairs
of DNA wrapped around histone core and held of DNA wrapped around histone core and held
in place by H1 (a linker histone)in place by H1 (a linker histone)

49. FUNCTION OF CHROMOSOMESFUNCTION OF CHROMOSOMES
[I]- The chromosomes are capable of self-
duplication. During duplication process the
DNA strands unwind. As unwinding starts,
each template of DNA forms its
complementary strand in double-helix
nature. The conversion of the old DNA
molecule into two new molecules, helps in
duplicating the chromosomes.
FUNCTION OF CHROMOSOMESFUNCTION OF CHROMOSOMES

50. SELF DUPLICATION OF DNA
MOLECULE
(IT HELPS IN THE DUPLICATION OFCHROMOSOMES )
Single DNA
molecule in
double helical
structure
Mother templates
unwind and new
complementary
strands originate
Unwinding continues
along with new
template formation
Two separate DNA
molecules formed having
an old and a new strand

51. Function of chromosomes (continued)…Function of chromosomes (continued)…
[II]- They help in expression of different characters in
an organism by synthesizing proteins in cells. A
definite protein is accumulated to produce a definite
character.
NB:- To see the process of protein synthesis by DNA of chromosome, CLICK the
“Hyperlink Button” below.
HYPERL
INK
CLICK

52. Function of chromosomes (continued)…Function of chromosomes (continued)…
[III]- As carrier of genes they transmit characters from
generation to generation , i.e. parents to offspring.
[IV]- The chromosomes control the physiological and
biochemical processes in the body of the organism.

59. Centromeres and TelomeresCentromeres and Telomeres

CentromeresCentromeres and and telomeres telomeres are two essential are two essential
features of all eukaryotic chromosomes.features of all eukaryotic chromosomes.

Each provide a unique function i.e., Each provide a unique function i.e., absolutely absolutely
necessary for the stability of the chromosomenecessary for the stability of the chromosome..

Centromeres are required for the segregation of Centromeres are required for the segregation of
the centromere during meiosis and mitosis.the centromere during meiosis and mitosis.

Teleomeres provide terminal stability to the Teleomeres provide terminal stability to the
chromosome and ensure its survival chromosome and ensure its survival

60. Centromere Centromere

The region where two sister chromatids of a chromosome appear The region where two sister chromatids of a chromosome appear
to be joined or “to be joined or “held togetherheld together” during mitatic metaphase is ” during mitatic metaphase is
called called Centromere Centromere

When chromosomes are stained they typically show a When chromosomes are stained they typically show a dark-dark-
stainedstained region that is the centromere. region that is the centromere.

Also termed as Also termed as Primary constrictionPrimary constriction

During During mitosismitosis, the centromere that is shared by the sister , the centromere that is shared by the sister
chromatids must divide so that the chromatids can migrate to chromatids must divide so that the chromatids can migrate to
opposite poles of the cell. opposite poles of the cell.

On the other hand, during the first meiotic division the On the other hand, during the first meiotic division the
centromere of sister chromatids must remain intactcentromere of sister chromatids must remain intact

whereas during meiosis II they must act as they do during mitosis. whereas during meiosis II they must act as they do during mitosis.

Therefore the centromere is an important component of Therefore the centromere is an important component of
chromosome structure and segregation.chromosome structure and segregation.

61. 
As a result, centromeres are the first parts of As a result, centromeres are the first parts of
chromosomes to be seen moving towards the chromosomes to be seen moving towards the
opposite poles during anaphase.opposite poles during anaphase.

The remaining regions of chromosomes lag The remaining regions of chromosomes lag
behind and appear as if they were being pulled behind and appear as if they were being pulled
by the centromere. by the centromere.

62. KinetochoreKinetochore

Within the centromere region, most species have Within the centromere region, most species have
several locations where spindle fibers attach, and several locations where spindle fibers attach, and
these sites consist of DNA as well as protein.these sites consist of DNA as well as protein.


The actual location where the attachment occurs The actual location where the attachment occurs
is called the is called the kinetochore kinetochore and is composed of and is composed of
both DNA and protein. both DNA and protein.

The DNA sequence within these regions is The DNA sequence within these regions is
called called CENCEN DNA DNA..

63. 
Typically CEN DNA is about Typically CEN DNA is about 120 base pairs120 base pairs
long and consists of several sub-domains, long and consists of several sub-domains, CDE-CDE-
II, , CDE-IICDE-II andand CDE-IIICDE-III. .

Mutations in the first two sub-domains have no Mutations in the first two sub-domains have no
effect upon segregation,effect upon segregation,

but a point mutation in the CDE-III sub-but a point mutation in the CDE-III sub-
domain completely eliminates the ability of the domain completely eliminates the ability of the
centromere to function during chromosome centromere to function during chromosome
segregation. segregation.

Therefore CDE-III must be actively involved in Therefore CDE-III must be actively involved in
the binding of the spindle fibers to the the binding of the spindle fibers to the
centromere. centromere.

64. 
The protein component of the kinetochore is The protein component of the kinetochore is
only now being characterized. only now being characterized.

A complex of three proteins called A complex of three proteins called Cbf-IIICbf-III
binds to normal CDE-III regions but can not binds to normal CDE-III regions but can not
bind to a CDE-III region with a point mutation bind to a CDE-III region with a point mutation
that prevents mitotic segregation. that prevents mitotic segregation.

65. TelomereTelomere

The two ends of a chromosomeThe two ends of a chromosome are known as are known as
telomeres.telomeres.

It required for the It required for the replication and stabilityreplication and stability of the of the
chromosome. chromosome.

When telomeres are damaged or removed due to When telomeres are damaged or removed due to
chromosome breakage, the damaged chromosome chromosome breakage, the damaged chromosome
ends can readily fuse or unite with broken ends of ends can readily fuse or unite with broken ends of
other chromosome. other chromosome.

Thus it is generally accepted that structural Thus it is generally accepted that structural
integrity and individuality of chromosomes is integrity and individuality of chromosomes is
maintained due to telomeres.maintained due to telomeres.

66. 
McClintock noticed that if two chromosomes were McClintock noticed that if two chromosomes were
broken in a cell, the end of one could attach to the broken in a cell, the end of one could attach to the
other and vice versa. other and vice versa.

What she never observed was the attachment of the What she never observed was the attachment of the
broken end to the end of an unbroken broken end to the end of an unbroken
chromosome. chromosome.

Thus the ends of broken chromosomes are sticky, Thus the ends of broken chromosomes are sticky,
whereas the normal end is not sticky, suggesting whereas the normal end is not sticky, suggesting
the ends of chromosomes have unique features. the ends of chromosomes have unique features.

67. Telomere Repeat Sequences Telomere Repeat Sequences
until recently, little was known about molecular structure of until recently, little was known about molecular structure of
telomeres. However, during the last few years, telomeres have telomeres. However, during the last few years, telomeres have
been isolated and characterized from several sp.been isolated and characterized from several sp.
SpeciesSpecies Repeat SequenceRepeat Sequence
Arabidopsis Arabidopsis TTTAGGGTTTAGGG
Human Human TTAGGGTTAGGG
OxytrichaOxytricha TTTTGGGGTTTTGGGG
Slime Mold Slime Mold TAGGGTAGGG
Tetrahymena Tetrahymena TTGGGGTTGGGG
TrypanosomeTrypanosome TAGGGTAGGG

68. 
The telomeres of this organism The telomeres of this organism
end in the sequence 5'-end in the sequence 5'-
TTGGGG-3'. TTGGGG-3'.

The The telomerasetelomerase adds a series adds a series
of 5'-TTGGGG-3' repeats to of 5'-TTGGGG-3' repeats to
the ends of the lagging strand. the ends of the lagging strand.

A hairpin occurs when unusual A hairpin occurs when unusual
base pairs between guanine base pairs between guanine
residues in the repeat form. residues in the repeat form.

Finally, the hairpin is removed Finally, the hairpin is removed
at the 5'-TTGGGG-3' repeat. at the 5'-TTGGGG-3' repeat.

Thus the end of the Thus the end of the
chromosome is faithfully chromosome is faithfully
replicated. replicated.
TetrahymenaTetrahymena - protozoa - protozoa
organism.organism.
RNA Primer  -  Short stretches of
ribonucleotides (RNA substrates) found on the
lagging strand during DNA replication. Helps
initiate lagging strand replication

69. Staining and Banding chromosome Staining and Banding chromosome
Staining procedures have been developed in the past two Staining procedures have been developed in the past two
decades and these techniques help to study the karyotype in decades and these techniques help to study the karyotype in
plants and animals. plants and animals.
1.1.Feulgen StainingFeulgen Staining: :
Cells are subjected to a mild hydrolysis in 1N HCl at Cells are subjected to a mild hydrolysis in 1N HCl at
6060
00
C for 10 minutes. C for 10 minutes.
This treatment produces a free aldehyde group in This treatment produces a free aldehyde group in
deoxyribose molecules.deoxyribose molecules.
When When Schiff’s reagent Schiff’s reagent (basic fuschin bleached with (basic fuschin bleached with
sulfurous acid) to give a deep pink colour. sulfurous acid) to give a deep pink colour.
Ribose of RNA will not form an aldehyde under Ribose of RNA will not form an aldehyde under
these conditions, and these conditions, and the reaction is thus specific for DNAthe reaction is thus specific for DNA

70. 22. Q banding:. Q banding:
The Q bands are the The Q bands are the fluorescent bands fluorescent bands observed observed
after after quinacrine mustard staining quinacrine mustard staining and observation with and observation with
UV light. UV light.
The distal ends of each chromatid are not stained by this The distal ends of each chromatid are not stained by this
technique. technique.
The Y chromosome become brightly fluorescent both in The Y chromosome become brightly fluorescent both in
the interphase and in metaphase.the interphase and in metaphase.
3. R banding: 3. R banding:
The R bands (from The R bands (from reversereverse) are those located in ) are those located in
the zones that do not fluoresce with the quinacrine the zones that do not fluoresce with the quinacrine
mustard, that is they are between the Q bands and can be mustard, that is they are between the Q bands and can be
visualized as green. visualized as green.

71. 4. G banding:4. G banding:
The G bands (from The G bands (from Giemsa) Giemsa) have the same have the same
location as Q bands and do not require fluorescent location as Q bands and do not require fluorescent
microscopy. microscopy.
Many techniques are available, each involving Many techniques are available, each involving
some pretreatment of the chromosomes. some pretreatment of the chromosomes.
In In ASG (Acid-Saline-GiemsaASG (Acid-Saline-Giemsa) cells are ) cells are
incubated in citric acid and NaCl for one hour at 60incubated in citric acid and NaCl for one hour at 60
00
C C
and are then treated with the Giemsa stain. and are then treated with the Giemsa stain.
5. C banding:5. C banding:
The C bands correspond to The C bands correspond to constitutive constitutive
heterochromatinheterochromatin. .
The heterochromatin regions in a chromosome The heterochromatin regions in a chromosome
distinctly differ in their stainability from euchromatic distinctly differ in their stainability from euchromatic
region. region.

72. VARIATION IN STRUTURE OF VARIATION IN STRUTURE OF
CHROMOSOMECHROMOSOME

73. Chromosomal AberrationsChromosomal Aberrations

The somatic (2n) and gametic (n) chromosome The somatic (2n) and gametic (n) chromosome
numbers of a species ordinarily remain constant. numbers of a species ordinarily remain constant.

This is due to the extremely precise mitotic and meiotic This is due to the extremely precise mitotic and meiotic
cell division. cell division.

Somatic cells of a diploid species contain two copies of Somatic cells of a diploid species contain two copies of
each chromosome, which are called homologous each chromosome, which are called homologous
chromosome. chromosome.

Their gametes, therefore contain only one copy of each Their gametes, therefore contain only one copy of each
chromosome, that is they contain one chromosome chromosome, that is they contain one chromosome
complement or genome. complement or genome.

Each chromosome of a genome contains a definite Each chromosome of a genome contains a definite
numbers and kinds of genes, which are arranged in a numbers and kinds of genes, which are arranged in a
definite sequence. definite sequence.

74. Chromosomal AberrationsChromosomal Aberrations

Sometime due to mutation or spontaneous Sometime due to mutation or spontaneous
(without any known causal factors), variation in (without any known causal factors), variation in
chromosomal number or structure do arise in chromosomal number or structure do arise in
nature. - Chromosomal aberrations.nature. - Chromosomal aberrations.

Chromosomal aberration may be grouped into Chromosomal aberration may be grouped into
two broad classes: two broad classes:
1. Structural and 2. Numerical1. Structural and 2. Numerical

75. Structural Chromosomal Aberrations Structural Chromosomal Aberrations

Chromosome structure variations result from Chromosome structure variations result from
chromosome breakagechromosome breakage. .

Broken chromosomes tend to Broken chromosomes tend to re-join;re-join; if there is more if there is more
than one break, rejoining occurs at than one break, rejoining occurs at randomrandom and not and not
necessarily with the correct ends. necessarily with the correct ends.

The result is structural changes in the chromosomes. The result is structural changes in the chromosomes.


Chromosome breakage is caused by Chromosome breakage is caused by X-rays, various X-rays, various
chemicalschemicals, and can also occur spontaneously., and can also occur spontaneously.

76. 
There are There are four four common type of structural common type of structural
aberrations:aberrations:
1. Deletion or Deficiency 1. Deletion or Deficiency
2. Duplication or Repeat2. Duplication or Repeat
3. Inversion, and 3. Inversion, and
4. Translocation.4. Translocation.

77. 
Consider a normal chromosome with genes in Consider a normal chromosome with genes in
alphabetical order: alphabetical order: a b c d e f g h ia b c d e f g h i
1. Deletion1. Deletion:: part of the chromosome has been part of the chromosome has been
removed: removed: a b c g h i a b c g h i
2. Dupliction2. Dupliction:: part of the chromosome is duplicated: part of the chromosome is duplicated:

a b c d e f a b c d e f d e fd e f g h i g h i
3. Inversion3. Inversion:: part of the chromosome has been re- part of the chromosome has been re-
inserted in reverse order: inserted in reverse order: a b c f a b c f e e d g h id g h i
ringring:: the ends of the chromosome are joined the ends of the chromosome are joined
together to make a ring together to make a ring

78. 4. Translocation4. Translocation:: parts of two non-homologous parts of two non-homologous
chromosomes are joined: chromosomes are joined:
If one normal chromosome is If one normal chromosome is a b c d e f g h ia b c d e f g h i
and the other chromosome is and the other chromosome is u v w x y z,u v w x y z,
then a translocation between them would be then a translocation between them would be
a b c d e f x y za b c d e f x y z and and u v w g h i.u v w g h i.

80. Deletion or deficiencyDeletion or deficiency
Loss of a chromosome segment is known as deletion Loss of a chromosome segment is known as deletion
or deficiency or deficiency
It can be It can be terminal deletionterminal deletion or or interstitial or intercalaryinterstitial or intercalary deletion. deletion.
A single break near the end of the chromosome would be A single break near the end of the chromosome would be
expected to result in expected to result in terminal deficiencyterminal deficiency. .
If two breaks occur, a section may be deleted and an If two breaks occur, a section may be deleted and an
intercalary deficiencyintercalary deficiency created. created.
Terminal deficiencies might seem less complicated.Terminal deficiencies might seem less complicated.
But majority of deficiencies detected are intercalary type within But majority of deficiencies detected are intercalary type within
the chromosome. the chromosome.
Deletion was the first structural aberration detected by Bridges Deletion was the first structural aberration detected by Bridges
in 1917 from his genetic studies on X chromosome of in 1917 from his genetic studies on X chromosome of
Drosophila. Drosophila.

81. 
Deletion generally produce striking Deletion generally produce striking genetic and genetic and
physiological effects. physiological effects.

When homozygous, most deletions are lethal, because When homozygous, most deletions are lethal, because
most genes are necessary for life and a homozygous most genes are necessary for life and a homozygous
deletion would have zero copies of some genes. deletion would have zero copies of some genes.

When heterozygous, the genes on the normal When heterozygous, the genes on the normal
homologue are homologue are hemizygoushemizygous: there is only 1 copy of : there is only 1 copy of
those genes.those genes.

Crossing over is absent in deleted region of a Crossing over is absent in deleted region of a
chromosome since this region is present in only one chromosome since this region is present in only one
copy in deletion heterozygotes. copy in deletion heterozygotes.

In Drosophila, several deficiencies induced the mutants In Drosophila, several deficiencies induced the mutants
like Blond, Pale, Beaded, Carved, Notch, Minute etc.like Blond, Pale, Beaded, Carved, Notch, Minute etc.

82. Deletion in Prokaryotes:Deletion in Prokaryotes:
Deletions are found in prokaryotes as well, e.g., Deletions are found in prokaryotes as well, e.g.,
E.coli, T4 phage and Lambda phage.E.coli, T4 phage and Lambda phage.
In E.coli, deletions of up to 1 % of the bacterial chromosome In E.coli, deletions of up to 1 % of the bacterial chromosome
are known. are known.
In lambda phage, however 20% of the genome may be missing In lambda phage, however 20% of the genome may be missing
in some of the deletions. in some of the deletions.
Deletion in Human:Deletion in Human:
Chromosome deletions are usually lethal even as Chromosome deletions are usually lethal even as
heterozygotes, resulting in zygotic loss, stillbirths, or infant heterozygotes, resulting in zygotic loss, stillbirths, or infant
death. death.
Sometimes, infants with small chromosome deficiencies Sometimes, infants with small chromosome deficiencies
however, survive long enough to permit the abnormal however, survive long enough to permit the abnormal
phenotype they express. phenotype they express.

83. Cri-du-chat (Cat cry syndrome):Cri-du-chat (Cat cry syndrome):
The name of the syndrome came from a catlike mewing cry The name of the syndrome came from a catlike mewing cry
from small weak infants with the disorder.from small weak infants with the disorder.
Other characteristics are microcephaly (small head), broad face Other characteristics are microcephaly (small head), broad face
and saddle nose, physical and mental retardation. and saddle nose, physical and mental retardation.
Cri-du-chat patients die in infancy or early childhood. Cri-du-chat patients die in infancy or early childhood.
The chromosome deficiency is in the short arm of The chromosome deficiency is in the short arm of chromosome chromosome
55 . .
Myelocytic leukemiaMyelocytic leukemia
Another human disorder that is associated with a chromosome Another human disorder that is associated with a chromosome
abnormality is chronic myelocytic leukemia. abnormality is chronic myelocytic leukemia.
A deletion of A deletion of chromosome 22chromosome 22 was described by P.C.Nowell and was described by P.C.Nowell and
Hungerford and was called Hungerford and was called “Philadelphia” (Ph’)“Philadelphia” (Ph’)
chromosome after the city in which the discovery was made. chromosome after the city in which the discovery was made.

84. DuplicationDuplication
The presence of an additional chromosome The presence of an additional chromosome
segment, as compared to that normally present in segment, as compared to that normally present in
a nucleus is known as a nucleus is known as DuplicationDuplication. .

In a diploid organism, presence of a chromosome In a diploid organism, presence of a chromosome
segment in more than two copies per nucleus is segment in more than two copies per nucleus is
called duplication. called duplication.

Four types of duplication:Four types of duplication:
1. Tandem duplication1. Tandem duplication
2. Reverse tandem duplication2. Reverse tandem duplication
3. Displaced duplication3. Displaced duplication
4. Translocation duplication4. Translocation duplication

85. 
The extra chromosome segment may be located The extra chromosome segment may be located
immediately after the normal segment in precisely immediately after the normal segment in precisely
the same orientation forms the the same orientation forms the tandem tandem

When the gene sequence in the extra segment of a When the gene sequence in the extra segment of a
tandem in the reverse order i.e, inverted , it is tandem in the reverse order i.e, inverted , it is
known as known as reversereverse tandem duplicationtandem duplication

In some cases, the extra segment may be located in In some cases, the extra segment may be located in
the same chromosome but away from the normal the same chromosome but away from the normal
segment – termed as segment – termed as displaced duplicationdisplaced duplication

The additional chromosome segment is located in The additional chromosome segment is located in
a non-homologous chromosome is a non-homologous chromosome is translocation translocation
duplicationduplication. .

86. OriginOrigin

Origin of duplication involves Origin of duplication involves chromosome breakage chromosome breakage and and
reunion of chromosomereunion of chromosome segment with its homologous segment with its homologous
chromosome. chromosome.

As a result, one of the two homologous involved in the As a result, one of the two homologous involved in the
production of a duplication ends up with a production of a duplication ends up with a deficiencydeficiency, ,
while the while the other has a duplication for the concerned other has a duplication for the concerned
segmentsegment. .

Another phenomenon, known as Another phenomenon, known as unequal crossing overunequal crossing over, ,
also leads to exactly the same consequences for small also leads to exactly the same consequences for small
chromosome segments. chromosome segments.

For e.g., duplication of the band For e.g., duplication of the band 16A of X 16A of X chromosome of chromosome of
Drosophila produces Bar eyeDrosophila produces Bar eye. .

This duplication is believed to originate due to unequal This duplication is believed to originate due to unequal
crossing over between the two normal X chromosomes of crossing over between the two normal X chromosomes of
female. female.

89. InversionInversion

When a segment of chromosome is oriented in the reverse When a segment of chromosome is oriented in the reverse
direction, such segment said to be direction, such segment said to be invertedinverted and the phenomenon and the phenomenon
is termed as is termed as inversioninversion. .

The existence of inversion was first detected by The existence of inversion was first detected by StrutevantStrutevant and and
PlunkettPlunkett in in 19261926. .

Inversion occur when parts of chromosomes become detached , Inversion occur when parts of chromosomes become detached ,
turn through 180turn through 180
00
and are reinserted in such a way that the genes and are reinserted in such a way that the genes
are in reversed order.are in reversed order.

For example, a certain segment may be broken in two places, and For example, a certain segment may be broken in two places, and
the breaks may be in close proximity because of chance loop in the breaks may be in close proximity because of chance loop in
the chromosome. the chromosome.

When they rejoin, the wrong ends may become connected. When they rejoin, the wrong ends may become connected.

The part on one side of the loop connects with broken end The part on one side of the loop connects with broken end
different from the one with which it was formerly connected. different from the one with which it was formerly connected.

This leaves the other two broken ends to become attached. This leaves the other two broken ends to become attached.

The part within the loop thus becomes turned around or inverted. The part within the loop thus becomes turned around or inverted.

90. 
Inversion may be classified into two types:Inversion may be classified into two types:
Pericentric - include the centromere
Paracentric - do not include the centromere

91. 
An inversion consists of two breaks in one An inversion consists of two breaks in one
chromosome. chromosome.

The area between the breaks is inverted (turned The area between the breaks is inverted (turned
around), and then reinserted and the breaks then around), and then reinserted and the breaks then
unite to the rest of the chromosome.unite to the rest of the chromosome.

If the inverted area includes the centromere it is If the inverted area includes the centromere it is
called a pericentric inversion. called a pericentric inversion.

If it does not, it is called a paracentric inversion.If it does not, it is called a paracentric inversion.

92. Inversions in natural populationsInversions in natural populations

In natural populations, pericentric inversions are In natural populations, pericentric inversions are
much less frequent than paracentric inversions. much less frequent than paracentric inversions.

In many sp, however, pericentric inversions are In many sp, however, pericentric inversions are
relatively common, e.g., in some relatively common, e.g., in some grasshoppersgrasshoppers..

Paracentric inversions appear to be very frequent Paracentric inversions appear to be very frequent
in natural populations of in natural populations of DrosophilaDrosophila. .

93. TranslocationTranslocation

Integration of a chromosome segment into a Integration of a chromosome segment into a
nonhomologous chromosome is known as nonhomologous chromosome is known as
translocationtranslocation. .

Three types: Three types:
1. simple translocation1. simple translocation
2. shift2. shift
3. reciprocal translocation. 3. reciprocal translocation.

94. 
Simple translocationSimple translocation: In this case, : In this case, terminal segment terminal segment of a of a
chromosome is chromosome is integrated integrated at one end of a non-at one end of a non-
homologous region. Simple translocations are rather homologous region. Simple translocations are rather rarerare. .

ShiftShift: In shift, an : In shift, an intercalary segment intercalary segment of a chromosome is of a chromosome is
integrated integrated within a non-homologous chromosome. Such within a non-homologous chromosome. Such
translocations are known in the populations of translocations are known in the populations of DrosophilaDrosophila, ,
Neurospora Neurospora etc. etc.

Reciprocal translocationReciprocal translocation: It is produced when two non-: It is produced when two non-
homologous chromosomes exchange segments – i.e., homologous chromosomes exchange segments – i.e.,
segments segments reciprocally reciprocally transferred. transferred.

Translocation of this type is most common Translocation of this type is most common

95. Non-DisjunctionNon-Disjunction

Generally during gametogenesis the homologous Generally during gametogenesis the homologous
chromosomes of each pair separate out (disjunction) chromosomes of each pair separate out (disjunction)
and are equally distributed in the daughter cells. and are equally distributed in the daughter cells.

But sometime there is an But sometime there is an unequal distribution unequal distribution of of
chromosomes in the daughter cells. chromosomes in the daughter cells.

The failure of separation of homologous The failure of separation of homologous
chromosome is called chromosome is called non-disjunctionnon-disjunction..

This can occur either during This can occur either during mitosis mitosis oror meiosis meiosis or or
embryogenesisembryogenesis. .

97. 
Mitotic non-disjunctionMitotic non-disjunction: The failure of separation of : The failure of separation of
homologous chromosomes during mitosis is called mitotic homologous chromosomes during mitosis is called mitotic
non-disjunction. non-disjunction.

It occurs after fertilization.It occurs after fertilization.

May happen during first or second cleavage. May happen during first or second cleavage.

Here, one blastomere will receive 45 chromosomes, while Here, one blastomere will receive 45 chromosomes, while
other will receive 47. other will receive 47.

Meiotic non-disjunctionMeiotic non-disjunction: The failure of separation of : The failure of separation of
homologous chromosomes during meiosis is called mitotic homologous chromosomes during meiosis is called mitotic
non-disjunctionnon-disjunction

Occurs during gametogensisOccurs during gametogensis

Here, one type contain 22 chromosome, while other will Here, one type contain 22 chromosome, while other will
be 24.be 24.

98. Variation in chromosome number
Organism with one complete set of chromosomes is
said to be euploid (applies to haploid and diploid
organisms).
Aneuploidy - variation in the number of individual
chromosomes (but not the total number of sets of
chromosomes).

The discovery of aneuploidy dates back to 1916 The discovery of aneuploidy dates back to 1916
whenwhen Bridges Bridges discovered XO male and XXY female discovered XO male and XXY female
DrosophilaDrosophila, which had 7 and 9 chromosomes , which had 7 and 9 chromosomes
respectively, instead of normal 8.respectively, instead of normal 8.

99. Nullisomy - loss of one
homologous chromosome
pair. (e.g., Oat )
Monosomy – loss of a
single chromosome
(Maize).
Trisomy - one extra
chromosome. (Datura)
Tetrasomy - one extra
chromosome pair.
More about Aneuploidy

100. Uses of AneuploidyUses of Aneuploidy

They have been used to determine the phenotypic They have been used to determine the phenotypic
effect of loss or gain of different chromosomeeffect of loss or gain of different chromosome

Used to produce Used to produce chromosome substitution chromosome substitution
lines. Such lines yield information on the effects of lines. Such lines yield information on the effects of
different chromosomes of a variety in the same different chromosomes of a variety in the same
genetic background.genetic background.

They are also used to produce They are also used to produce alien addition alien addition and and
alien substitution linesalien substitution lines. These are useful in gene . These are useful in gene
transfer from one species to another. transfer from one species to another.

Aneuploidy permits the location of a gene as well Aneuploidy permits the location of a gene as well
as of a linkage group onto a specific chromosome. as of a linkage group onto a specific chromosome.

101. Trisomy in HumansTrisomy in Humans
Down SyndromeDown Syndrome

The best known and most common chromosome related The best known and most common chromosome related
syndrome. syndrome.

Formerly known as “Formerly known as “MongolismMongolism””

1866, when a physician named 1866, when a physician named John Langdon Down John Langdon Down
published an essay in England in which he described a set published an essay in England in which he described a set
of children with common features who were distinct from of children with common features who were distinct from
other children with mental retardation he referred to as other children with mental retardation he referred to as
““MongoloidsMongoloids.”.”

One child in every 800-1000 births has Down syndromeOne child in every 800-1000 births has Down syndrome

250,000 in US has Down syndrome.250,000 in US has Down syndrome.

The cost and maintaining Down syndrome case in US is The cost and maintaining Down syndrome case in US is
estimated at $ 1 billion per year.estimated at $ 1 billion per year.

102. 
Patients having Down syndrome will Short in stature Patients having Down syndrome will Short in stature
(four feet tall) and had an epicanthal fold, broad (four feet tall) and had an epicanthal fold, broad
short skulls, wild nostrils, large tongue, stubby handsshort skulls, wild nostrils, large tongue, stubby hands

Some babies may have short necks, small hands, and Some babies may have short necks, small hands, and
short fingers.short fingers.

They are characterized as low in mentality.They are characterized as low in mentality.

Down syndrome results if the extra chromosome is Down syndrome results if the extra chromosome is
number 21.number 21.

103. Amniocentesis for Detecting AneuploidyAmniocentesis for Detecting Aneuploidy

Chromosomal abnormalities are sufficiently well Chromosomal abnormalities are sufficiently well
understood to permit genetic counseling. understood to permit genetic counseling.

A fetus may be checked in early stages of A fetus may be checked in early stages of
development by karyotyping the cultured cells development by karyotyping the cultured cells
obtained by a process called obtained by a process called amniocentesisamniocentesis. .

A sample of fluid will taken from mother and A sample of fluid will taken from mother and
fetal cells are cultured and after a period of two fetal cells are cultured and after a period of two
to three weeks, chromosomes in dividing cells to three weeks, chromosomes in dividing cells
can be stained and observed. can be stained and observed.

If If three No.21 chromosomesthree No.21 chromosomes are present, are present, Down Down
syndrome confirmed. syndrome confirmed.

104. 
The risk for mothers less than 25 years of age to The risk for mothers less than 25 years of age to
have the trisomy is about 1 in 1500 births.have the trisomy is about 1 in 1500 births.

At 40 years of age, 1 in 100 birthsAt 40 years of age, 1 in 100 births

At 45 years 1 in 40 births.At 45 years 1 in 40 births.

105. Other SyndromesOther Syndromes
Chromosome NomenclatureChromosome Nomenclature: 47, +13: 47, +13
Chromosome formulaChromosome formula: 2n+1: 2n+1
Clinical SyndromeClinical Syndrome: Trisomy-13: Trisomy-13
Estimated Frequency BirthEstimated Frequency Birth: 1/20,000: 1/20,000
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Mental deficiency and deafness, minor Mental deficiency and deafness, minor
muscle seizures, cleft lip, cardiac anomaliesmuscle seizures, cleft lip, cardiac anomalies

106. Other SyndromesOther Syndromes
Chromosome Chromosome NomenclatureNomenclature: 47, +18: 47, +18
Chromosome formulaChromosome formula: 2n+1: 2n+1
Clinical SyndromeClinical Syndrome: Trisomy-18: Trisomy-18
Estimated Frequency BirthEstimated Frequency Birth: 1/8,000: 1/8,000
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Multiple congenital malformation of many Multiple congenital malformation of many
organs, malformed ears, small mouth and nose organs, malformed ears, small mouth and nose
with general elfin appearance. with general elfin appearance.
90% die in the first 6 months. 90% die in the first 6 months.

107. Other SyndromesOther Syndromes
Chromosome Chromosome NomenclatureNomenclature: 45, X: 45, X
Chromosome formulaChromosome formula: 2n - 1: 2n - 1
Clinical SyndromeClinical Syndrome: Turner: Turner
Estimated Frequency BirthEstimated Frequency Birth: 1/2,500 female: 1/2,500 female
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Female with retarded sexual development, Female with retarded sexual development,
usually sterile, short stature, webbing of skin in usually sterile, short stature, webbing of skin in
neck region, cardiovascular abnormalities, neck region, cardiovascular abnormalities,
hearing impairment. hearing impairment.

108. Other SyndromesOther Syndromes
Chromosome NomenclatureChromosome Nomenclature: : 47, XXY, 48, XXXY, 47, XXY, 48, XXXY,
48,XXYY, 49, 48,XXYY, 49,
XXXXY, XXXXY, 50, XXXXXY50, XXXXXY
Chromosome formulaChromosome formula: 2n+1; 2n+2; 2n+2; 2n+3; 2n+4: 2n+1; 2n+2; 2n+2; 2n+3; 2n+4
Clinical SyndromeClinical Syndrome: Klinefelter: Klinefelter
Estimated Frequency BirthEstimated Frequency Birth: 1/500 male borth: 1/500 male borth
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Pitched voice, Male, subfertile with small Pitched voice, Male, subfertile with small
testes, developed breasts, feminine, long limbs. testes, developed breasts, feminine, long limbs.

109. 
Found in certain tissues e.g., Found in certain tissues e.g.,
salivary glands of larvae, gut salivary glands of larvae, gut
epithelium, Malphigian epithelium, Malphigian
tubules and some fat bodies, tubules and some fat bodies,
of some Diptera (of some Diptera (Drosophila, Drosophila,
Sciara, RhyncosciaraSciara, Rhyncosciara))

These chromosomes are very These chromosomes are very
long and thick (upto long and thick (upto 200 200
times their sizetimes their size during during
mitotic metaphase in the mitotic metaphase in the
case of Drosophila)case of Drosophila)

Hence they are known as Hence they are known as
Giant chromosomesGiant chromosomes. .
Giant chromosomesGiant chromosomes

110. 
They are first discovered by They are first discovered by BalbianiBalbiani in in 18811881 in in
dipteran salivary glands and thus also known as dipteran salivary glands and thus also known as
salivary gland chromosomessalivary gland chromosomes. .

But their significance was realized only after the But their significance was realized only after the
extensive studies by extensive studies by Painter Painter during 1930’s. during 1930’s.

Giant chromosomes have also been discovered Giant chromosomes have also been discovered
in suspensors of young embryos of many plants, in suspensors of young embryos of many plants,
but these do not show the bands so typical of but these do not show the bands so typical of
salivary gland chromosomes. salivary gland chromosomes.

111. 
He described the morphology in detail and He described the morphology in detail and
discovered the relation between salivary gland discovered the relation between salivary gland
chromosomes and germ cell chromosomes.chromosomes and germ cell chromosomes.

Slides of Drosophila giant chromosomes are Slides of Drosophila giant chromosomes are
prepared by squashing in prepared by squashing in acetocarmine acetocarmine the the
salivary glands dissected out from the larvae.salivary glands dissected out from the larvae.

The total length of The total length of D.melanogaterD.melanogater giant giant
chromosomes is about chromosomes is about 2,000µ2,000µ. .

112. 
Giant chromosomes are made up of Giant chromosomes are made up of
several dark staining regions called several dark staining regions called
““bandsbands”. ”.

It can be separated by relatively light It can be separated by relatively light
or non-staining “or non-staining “interbandinterband” regions. ” regions.

The bands in Drosophila giant The bands in Drosophila giant
chromosome are visible even without chromosome are visible even without
staining, but after staining they staining, but after staining they
become very sharp and clear. become very sharp and clear.

In Drosophila about In Drosophila about 5000 bands5000 bands can can
be recognized.be recognized.

113. 
Some of these bands are as thick Some of these bands are as thick
as as 0.5µ0.5µ, while some may be only , while some may be only
0.05µ 0.05µ thick.thick.

About About 25,00025,000 base-pairs are now base-pairs are now
estimated for each bandestimated for each band. .

All the available evidence clearly All the available evidence clearly
shows that each shows that each giant giant
chromosome is composed of chromosome is composed of
numerous strandsnumerous strands, each strand , each strand
representingrepresenting one chromatidone chromatid. .

Therefore, these chromosomes are Therefore, these chromosomes are
also known as “also known as “Polytene Polytene
chromosomechromosome”, and the condition ”, and the condition
is referred to as “is referred to as “PolytenePolytene””

114. 
The numerous strands of these chromosomes are The numerous strands of these chromosomes are
produced produced due to repeated replication of the paired due to repeated replication of the paired
chromosomeschromosomes without any nuclear or cell division. without any nuclear or cell division.

So that the number of strands (chromatids) in a So that the number of strands (chromatids) in a
chromosome doubles after every round of DNA chromosome doubles after every round of DNA
replicationreplication

It is estimated that giant chromosomes of It is estimated that giant chromosomes of
DrosophilaDrosophila have about have about 1,024 strands1,024 strands

In the case of In the case of ChironomousChironomous may have about may have about 4,096 4,096
strands. strands.

The bands of giant chromosomes are formed as a The bands of giant chromosomes are formed as a
result of stacking over one another of the result of stacking over one another of the
chromomeres of all strands present in them. chromomeres of all strands present in them.

115. 
Since chromatin fibers are highly coiled in Since chromatin fibers are highly coiled in
chromosomes, they stain deeply. chromosomes, they stain deeply.

On the other hand, the chromatin fibers in the On the other hand, the chromatin fibers in the
interband regions are fully extended, as a result interband regions are fully extended, as a result
these regions take up very light stain. these regions take up very light stain.

In Drosophila the location of many genes is In Drosophila the location of many genes is
correlated with specific bands in the connected correlated with specific bands in the connected
chromosomes. chromosomes.

In interband region do not have atleast In interband region do not have atleast
functional genesfunctional genes

116. 
During certain stages of development, specific During certain stages of development, specific
bands and inter band regions are associated with bands and inter band regions are associated with
them greatly increase in diameter and produced them greatly increase in diameter and produced
a structure called a structure called PuffsPuffs or or Balbiani ringsBalbiani rings. .

Puffs are believed to be produced due to Puffs are believed to be produced due to
uncoiling of chromatin fibers present in the uncoiling of chromatin fibers present in the
concerned chromomeres. concerned chromomeres.

The puffs are sites of active The puffs are sites of active RNA synthesisRNA synthesis..

118. Figure 3. Polytene chromosome map of Anopheles gambiae

119. Lampbrush ChromosomeLampbrush Chromosome

It It was given this name because it is similar in was given this name because it is similar in
appearance to the brushes used to clean lamp appearance to the brushes used to clean lamp
chimneys in centuries past. chimneys in centuries past.

First observed by First observed by FlemmingFlemming in 1882. in 1882.

The name lampbrush was given by The name lampbrush was given by RuckertRuckert in 1892.in 1892.

These are found in These are found in oocytic oocytic nuclei of vertebrates nuclei of vertebrates
(sharks, amphibians, reptiles and birds)as well as in (sharks, amphibians, reptiles and birds)as well as in
invertebrates (Sagitta, sepia, Ehinaster and several invertebrates (Sagitta, sepia, Ehinaster and several
species of insects). species of insects).

Also found in plants – but most experiments in Also found in plants – but most experiments in
oocytes. oocytes.

120. 
Lampbrush chromosomes are up to Lampbrush chromosomes are up to 800 µm 800 µm long; thus long; thus
they provide very favorable material for cytological they provide very favorable material for cytological
studies. studies.

The homologous chromosomes are paired and each The homologous chromosomes are paired and each
has duplicated to produce two chromatids at the has duplicated to produce two chromatids at the
lampbrush stage.lampbrush stage.

Each lampbrush chromosome contains a central axial Each lampbrush chromosome contains a central axial
region, where the two chromatids are highly condensedregion, where the two chromatids are highly condensed

Each chromosome has several chromomeres Each chromosome has several chromomeres
distributed over its length. distributed over its length.

From each chromomere, a pair of loops emerges in the From each chromomere, a pair of loops emerges in the
opposite directions vertical to the main chromosomal opposite directions vertical to the main chromosomal
axis. axis.

121. 
One loop represent one One loop represent one
chromatid, i.e., one chromatid, i.e., one
DNA molecule.DNA molecule.

The size of the loop The size of the loop
may be ranging the may be ranging the
average of 9.5 average of 9.5 µm µm to to
about 200 about 200 µm µm

The pairs of loops are The pairs of loops are
produced due to produced due to
uncoiling of the two uncoiling of the two
chromatin fibers chromatin fibers
present in a highly present in a highly
coiled state in the coiled state in the
chromomeres. chromomeres.

122. 
One end of each loop is thinner (thin end) than One end of each loop is thinner (thin end) than
the other end (thick end).the other end (thick end).

There is extensive RNA synthesis at the thin end There is extensive RNA synthesis at the thin end
of the loops, while there is little or no RNA of the loops, while there is little or no RNA
synthesis at the thick end. synthesis at the thick end.

123. Phase-contrast and fluorescent micrographs of
lampbrush chromosomes

124. Dosage CompensationDosage Compensation
Sex Chromosomes: females XX, males XY
Females have two copies of every X-linked gene;
males have only one.
How is this difference in gene dosage
compensated for? OR

How to create equal amount of X chromosome How to create equal amount of X chromosome
gene products in males and females?gene products in males and females?

125. 
Levels of enzymes or proteins encoded by Levels of enzymes or proteins encoded by
genes on the X chromosome are the same in genes on the X chromosome are the same in
both males and femalesboth males and females

Even though males have 1 X chromosome Even though males have 1 X chromosome
and females have 2.and females have 2.

126. 
G6PD, glucose 6 phosphate dehydrogenase, G6PD, glucose 6 phosphate dehydrogenase,
gene is carried on the X chromosomegene is carried on the X chromosome

This gene codes for an enzyme that breaks This gene codes for an enzyme that breaks
down sugardown sugar

Females produce the same amount of G6PD Females produce the same amount of G6PD
enzyme as malesenzyme as males

XXY and XXX individuals produce the same XXY and XXX individuals produce the same
about of G6PD as anyone elseabout of G6PD as anyone else

127. In cells with more than two X chromosomes,
only one X remains genetically active and all the
others become inactivated.

In some cells the paternal allele is expressed In some cells the paternal allele is expressed

In other cells the maternal allele is expressedIn other cells the maternal allele is expressed

In XXX and XXXX females and XXY males In XXX and XXXX females and XXY males
only 1 X is activated in any given cell the rest are only 1 X is activated in any given cell the rest are
inactivatedinactivated

129. Barr BodiesBarr Bodies

1940’s two Canadian scientists noticed a 1940’s two Canadian scientists noticed a
dark staining mass in the nuclei of cat brain dark staining mass in the nuclei of cat brain
cellscells

Found these dark staining spots in female Found these dark staining spots in female
but not malesbut not males

This held for cats and humansThis held for cats and humans

They thought the spot was a tightly They thought the spot was a tightly
condensed X chromosome condensed X chromosome

130. Barr bodies represent the inactive X chromosome and
are normally found only in female somatic cells.
Barr Bodies

131. A woman with the
chromosome
constitution 47,
XXX should have 2
Barr bodies in each
cell.
XXY individuals
are male, but have a
Barr body.
 XO individuals
are female but have
no Barr bodies.

132. 
Which chromosome is inactive is a matter of Which chromosome is inactive is a matter of
chance, but once an X has become inactivated , chance, but once an X has become inactivated ,
all cells arising from that cell will keep the same all cells arising from that cell will keep the same
inactive X chromosome. inactive X chromosome.

In the mouse, the inactivation apparently occurs In the mouse, the inactivation apparently occurs
in early in development in early in development

In human embryos, sex chromatin bodies have In human embryos, sex chromatin bodies have
been observed by the 16been observed by the 16
thth
day of gestation. day of gestation.

133. Mechanism of X-chromosome InactivationMechanism of X-chromosome Inactivation

A region of the p arm of the X chromosome
near the centromere called the X-inactivation
center (XIC) is the control unit.
This region contains the gene for X-inactive
specific transcript (XIST). This RNA
presumably coats the X chromosome that
expresses it and then DNA methylation locks
the chromosome in the inactive state.

134. 
This occurs about 16 days after fertilization in a This occurs about 16 days after fertilization in a
female embryo.female embryo.

The process is independent from cell to cell. The process is independent from cell to cell.

A maternal or paternal X is randomly chosen to be A maternal or paternal X is randomly chosen to be
inactivated.inactivated.

135. 
Rollin Hotchkiss first discovered methylated DNA Rollin Hotchkiss first discovered methylated DNA
in 1948.in 1948.

He found that DNA from certain sources He found that DNA from certain sources
contained, in addition to the standard four bases, a contained, in addition to the standard four bases, a
fifth: 5-methyl cytosine. fifth: 5-methyl cytosine.

It took almost three decades to find a role for it.It took almost three decades to find a role for it.

In the mid-1970s, Harold Weintraub and his In the mid-1970s, Harold Weintraub and his
colleagues noticed that active genes are low in colleagues noticed that active genes are low in
methyl groups or under methylated. methyl groups or under methylated.

Therefore, a relationship between under Therefore, a relationship between under
methylation and gene activity seemed likely, as if methylation and gene activity seemed likely, as if
methylation helped repress genes. methylation helped repress genes.

137. 
This would be a valuable means of keeping genes This would be a valuable means of keeping genes
inactive if methylation passed on from parent to inactive if methylation passed on from parent to
daughter cells during cell division. daughter cells during cell division.

Each parental strand retains its methyl groups, Each parental strand retains its methyl groups,
which serve as signals to the methylating which serve as signals to the methylating
apparatus to place methyl groups on the newly apparatus to place methyl groups on the newly
made progeny strand. made progeny strand.

Thus methylation has two of the requirements for Thus methylation has two of the requirements for
mechanism of determination:mechanism of determination:

1. It represses gene activity1. It represses gene activity

2. It is permanent.2. It is permanent.

138. 
Strictly speaking, the DNA is altered, since Strictly speaking, the DNA is altered, since
methyl groups are attached, but because methyl methyl groups are attached, but because methyl
cytosine behaves the same as ordinary cytosine, cytosine behaves the same as ordinary cytosine,
the genetic coding remain same. the genetic coding remain same.

A striking example of such a role of methylation A striking example of such a role of methylation
is seen in the inactivation of the X chromosome is seen in the inactivation of the X chromosome
in female mammal. in female mammal.

The inactive X chromosome become The inactive X chromosome become
heterochromatic and appears as a dark fleck heterochromatic and appears as a dark fleck
under the microscope – this chromosome said under the microscope – this chromosome said
to be lyonized, in honor of Mary Lyon who first to be lyonized, in honor of Mary Lyon who first
postulated the effect in mice. postulated the effect in mice.

139. 
An obvious explanation is that the DNA in the An obvious explanation is that the DNA in the
lyonized X chromosome is methylated, where as lyonized X chromosome is methylated, where as
the DNA in the active, X chromosome is not. the DNA in the active, X chromosome is not.

To check this hypothesis To check this hypothesis Peter Jones and Lawrence Peter Jones and Lawrence
Shapiro grew cells in the presence of drug 5-Shapiro grew cells in the presence of drug 5-
azacytosine, which prevents DNA methylation. azacytosine, which prevents DNA methylation.

This reactivated the lyonized the X chromosome.This reactivated the lyonized the X chromosome.

Furthermore, Shapiro showed these reactivated Furthermore, Shapiro showed these reactivated
chromosomes could be transferred to other cells chromosomes could be transferred to other cells
and still remain active. and still remain active.