Chromosome pairing and Crossing over .docx

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1. Chromosome Pairing
• Chromosome pairing refers to the lengthwise alignment of
homologous chromosomes at the prophase stage of meiosis.
• Most sexually reproducing organisms have two sets of chromosomes,
one set inherited from each parent.
• For these organisms to produce cells with a single set of
chromosomes, the sets have to be separated such that the daughter
cells have one copy of each chromosome.
• The responsible cell division is meiosis and the mechanism is
pairing/synapsis and subsequent separation of homologous
chromosomes.
• ‘Pairing’ refers to the juxtaposition of a pair of homologs at meiotic
prophase, and ‘synapsis’ refers to the even closer alignment of the
homologs, usually via the parallel alignment of the meiotic
chromosome cores that form the synaptonemal complex.
• Close alignment of homologs can occasionally also be observed in
somatic cells, particularly in dipteran insects.
Types of Chromosome Pairing
Chromosome pairing may be divided into three main types, viz.,
primary pairing, secondary association and non-homologous
association, which may be further classified.

2. (i) Primary Pairing:
Primary pairing is observed at interphase, mitosis and meiosis. It
involves the synapsis of chromosomal regions which are
genetically homologous.
 Interphase Pairing
During interphase, homologous chromosomes in the salivary
gland cells of the Dipteran larvae remain paired lengthwise.
Pairing occurs throughout the entire length of the
chromosomes: this leads to the appearance of specific and
constant patterns of transverse bands. The condensed bands
(densely stainable) are called chromosomes.
 Mitotic or Somatic Pairing
This type of chromosome pairing is found in the cells
undergoing mitosis for the development of the germ line,
gonads and also of the soma. Mitotic pairing differs from
meiotic pairing. When more than two homologues are
present, such as, in polysomics and auto-polyploids, all the
homologous chromosomes are paired throughout their
length.

3.  Meiotic Pairing
This type of pairing occurs in the cells undergoing meiosis.
Synapsis occurs between the homologous chromosomes
throughout their length in the zygotene-pachytene stage of
prophase I. Pairing in initiated near the ends of
chromosomes and proceeds in a zipper -like fashion. After
pachytene, i.e., at diplotene, diakinesis and MI, the
homologous chromosomes are held together by chiasmata
(chiasmate meiosis).
In certain species of plants and animals, chiasmata are not
formed and the homologous chromosomes are associated
by a persistent pairing force during the spindle stage of
metaphase 1; such type of meiosis is called achiasmate
meiosis.

4. 2. Secondary association:
 It is observed in some polyploid species which form
bivalents. In these species, genetically similar (homologous)
bivalents are often found in pairs.
 This type of association occurs after the lapse of primary
pairing and therefore, it is called the secondary association.
 It has also been found that the bivalents which are
genetically similar may lie close to one another on the
equatorial plate more frequently than that expected for
random distribution.
 This type of association has no effect on the segregation of
chromosomes. Secondary association is used as a criterion
of remote polyploidy.
3. Non-homologous Association:
There are two main types of non-homologous association, one
related to the heterochromatin and the other related to the
euchromatin.
@ Association of heterochromatic segments:
(i) Chromocentres: Heterochromatin has a tendency to fuse with
other heterochromatic entities. Chromocentres consist of

5. heterochromatin and are positively heteropycnotic.
Centromeric Chromocentres are formed in many Dipteran
nuclei possessing polytene chromosomes. Thus this type of
pairing involves lateral or terminal juxtaposition or fusion of
heterochromatic segments.
(ii) Terminal association: This type of pairing is observed at MI of
meiosis in some cases. It refers to the terminal associations
between univalent chromosomes, which may be due to
heterochromatic fusions.
(b)Association of euchromatic chromosome segments:
This type of association of non-homologous chromosomes occurs
during meiosis in zygotene and pachytene stages. Such pairing may also
lead to chiasma formation. This may indicate some duplication in the
genome. In mono-haploids, non-homologous pairing has been observed
in some plants, such as, maize, barley and Antirrhinum majus etc.
Molecular Basis of Chromosome Pairing
• A new era in the studies of chromosome pairing was ushered in with
the introduction of electron microscope to biological research.
• Moses (1956) working with the spermatocytes of crayfish discovered
a tripartite ribbon at the site of synapsis; this structure is called
synaptonemal complex or synaptinemal complex.
• This complex occupies the space between the paired homologous
chromosomes and appears to be a advice to bring together and allign
the DNA molecules from paired homologous chromosomes which
facilitates crossing over between them.
• After pachytene, the synaptinemal complex dissolves and the
homologous chromosomes begin to de-synapse; this stage is called
diplotene during which homologues are associated by chiasmata.

6. Synaptinemal complex may not get fully dissolved in some cells and its
remnants may persist through the second meiotic division and form
aggregates called poly-complexes.
Synaptonemal complex (SC)
• Synapsis of homologous chromosomes during meiotic prophase I
involves the assembly of the synaptonemal complex (SC), a meiotic-
specific structure.
• The SC was initially described in longitudinal sections of animal
pachytene spermatocytes under electron microscopy.
• This structure has been found in almost all sexually reproducing
organisms, although typically it is absent in males of some species of
fruit flies.
• The SC is a tripartite proteinaceous zipper-like structure that is
sequentially arranged during zygotene along the entire length of
chromosomes when they are attached to the nuclear envelope and
becomes fully assembled in pachytene bivalents (synapsis).

7. Synaptomere-Zygosome Hypothesis of the Formation of
Synaptonemal Complex
• This hypothesis states that homologous chromosomes are attached to
the nuclear membrane with their ends and that two substances,
namely, synaptomere (made up of polysegments) and zygosomes
(composed of protein molecules) are involved in chromosome pairing,
as shown by King in 1970.
• Synaptomeres are distributed along the length of synapsed
chromosomes; each synaptomere consists of three segments
designated as A (lateral), B (central) and C (lateral segments arranged in
the following order, ABC, CBA, ABC, CBA.
• The B segment of the synaptomere is directed towards the central
element and acts as the site for zygosome attachment.
• The zygosome is rod-shaped subunit and its one end (head) is
attached to the B segment, while the other end (tail) is attached to the
tail of the other zygosome attached to the homologous chromosome
generating a ladder-like arrangement. The tails of B segment may be
charged sites.

8. Crossing Over
• Crossing over is a basic concept of genetics and cell biology,
often called recombination. It occurs during meiosis.
• Crossing – over takes place during pachytene sub – stage of
prophase I of meiosis, but visible at diplotene sub – stage.
• The term crossing over was coined by Morgan.
• Crossing over is the exchange of chromosome segments
between non – sister chromatids during the production of
gametes.
Mechanism of Crossing Over
Mechanism of crossing over can be described under following
headings:
1. Synapsis
• The paternal and maternal Chromosome of a
homologous pair comes close to each other and start
paring along their length during zygotene stage of
Prophase-I.
• This Pairing is called Synapsis.
• The paired homologous chromosomes are called
bivalent.
• Synaptonemal complex help in synapsis.

2. Duplication of chromosomes
• Synapsis is followed by duplication of chromosomes .

9. • Here each of the chromosome in a bivalent splits
longitudinally into two sister chromatids during Diplotene
stage.
• Thus the bivalent know consists of four chromatids which
is called tetrad.
3. Chiasmata formation
• Chiasmata are the points of attachment between two
homologous chromosomes, while crossing over takes
place.
• Number Of chromosomes depends upon the length of
chromosome.
• crossing may be take place at one end or several points.
• This may result in the formation of one or more chiasma.
4. Terminalisation
• After the crossing over, the non – sister chromatids starts
repelling each other due to lack of attraction force
between them.
• The repulsion of chromatids Starts from centromere
towards the Chiasmata & the chiasmata itself move
towards the end of tetrad.
• Due to terminalisation the homologous chromosomes,
separate from each other.

11.  Crossing over, leading to recombination of linked genes, is
due to interchange of sections of homologous
chromosomes.
 At meiosis, the homologous chromosomes come together
and pair, or synapse, during prophase.
 The pairing is remarkably precise and is evidently brought
about by mutual attraction of the parts of the
chromosomes that are similar or homologous because
they contain allellic genes.
 The enzymes involved in crossing over are:
1.Recombinase is the major enzyme regulating recombination
event.
2) Endonuclease is responsible for breakage of 2 non – sister
chromatids at corresponding sites.
3) Ligase enzyme – exchanged segments are joined or the gap is
filled by ligase enzyme.
Types of crossing over
1.Somatic crossing over- Pairing of homologous chromosomes
occurs in germinal cells but some times in somatic cells.
Somatic crossing over is reported in Drosophila by Curt Stern
(1935). Somatic crossing over occurs rarely.
2.Meiotic crossing over or germinal crossing over- This type of
crossing over takes place in germinal cells during
gametogenesis.

12. Kinds or Crossing over
• Single crossing over (only one chiasmata is form).
• Double crossing over (two chiasmata are form).
• Multiple crossing over. (more than two chiasmata are
formed).


Significance of Crossing Over
• Exchange of Chromosome material results in the variations.
• Plays a Crucial function in the evolution process.
• Recombinations or new gene combinations are produced due
to crossing over, which change genetic pool by changing of gene
frequency and this provide a way of evolution (micro evolution).
• Crossing over generates genetic difference within a
population
• The Creation of linkage maps is also based on cross over
frequencies.
• It is a evidence of a chromosomes linear Organization of
Connected genes.