Bacterial genetics

information realated to bacterial genetics

1. Bacterial Genetics
Prabin Shah
BScMLT, MSc(Biochemistry)

2. Understanding Genetics
 We resemble and differ because of Genetic
configurations
 Parents - Son - Daughter, how they
resemble each other.
 They breed true from Generation to
Generation
 But vary in small proportions in progeny.

Bacteria too obey the laws of
Genetics

3. Watson - Crick
Discovery of DNA

4. DNA
A Complex Structure
Makes Life

5. Prokaryotes Vs Eukaryotes
Genetics
Prokaryotes Eukaryotes
Prokaryotes are haploid eukaryotes are often diploid
contain a single circular
chromosome.
eukaryotes have linear chromosomes,
usually more than 1
Prokaryotes often contain
“plasmids”.
Doesnot contain plasmids
In prokaryotes, translation is
coupled to transcription:
translation of the new RNA
molecule starts before
transcription is finished
In eukaryotes, transcription of genes
in RNA occurs in the nucleus, and
translation of that RNA into protein
occurs in the cytoplasm. The two
processes are separated from each
other.

6. DNA
( Deoxyribonucleic Acid )
 DNA is composed of Many Units of
Adenine – Thymine A - T
Guanine – Cytosine G - C
A+ T
G+C proportion differ for each species
DNA replicates first unwinding at one end to
form a fork
Each strand of fork acting as template for the
synthesis of complementary strand

7. DNA
A DNA molecule is
composed of two
chains of Nucleotides
wound together in the
form of a Double Helix
Each chain has back
bone of Deoxyribose
and Phosphates
residues arranged
alternatively

8. Structure of DNA
Attached to each
Deoxyribose and
phosphate residues
arranged alternatively
Attached to each
Deoxyribose are of four
nitrogen bases
Purines - Adenine,
Guanine
Pyrimidines
Thymidine and Cytosine

9. How RNA differs from DNA
RNA contains - Sugar Ribose instead of
Deoxyribose
Uracil is present instead of Thymine
Types of RNA
Messenger RNA mRNA
Ribosomal RNA rRNA
Transfer RNA tRNA

10. What is a Code in
Genetics?
Code is a unit consists of sequence of three
Bases
Code is triplet A-T- C
A code can make single Amino acid
More than one code present for making
similar sequence of Amino acid
AGA make Arginine
AGC, CGU, CGG, also code for similar Amino
acid
Some Codons UAA don't code for any Amino
acid called as Nonsense codon

11. What is a Gene?
Gene is a sequence of
DNA carrying codons
specifying for particular
polypeptide.
DNA contains many
Genes(combinations of
hundreds and
thousands of
Nucleotides )

12. Bacterial Chromosome
Contains a Double stranded molecules
of DNA arranged in circular form.
Length 1,000 microns.
Bacterial DNA contains about
4,000kilobases
1 kb = 1000 base pairs (A-T) (G-C)

13. Extra chromosomal Genetic
Elements
 Bacteria posses Extra chromosomal
genetic elements
 Not Essential for survival of Bacteria
But makes the Bacteria Resistant to
antibiotics, and makes them survive
& also able to produce toxins

14. Plasmids
Plasmids are circular
DNA molecules present
in the cytoplasm of the
Bacteria
Their size varies from 1
kbp to over 400
kilobase pairs (kbp).
Capable of Autonomous
replication
Can transfer genes from
one cell to other
Act as vectors in Genetic
engineering.

15. Plasmids
Plasmid seem to be ubiquitous in bacteria, May encode
genetic information for properties
1 Resitance to Antibiotics
2 Bacteriocins production
3 Enterotoxin production
4 Enhanced pathogenicity
5 Reduced Sensitivity to mutagens
6 Degrade complex organic molecules

16. Plasmids are classified

1. by their ability to be transferred to
other bacteria
Conjugative
 The sexual transfer of plasmids to another
bacterium through a pilus.
Non-conjugative
 Non-conjugative plasmids don’t initiate conjugaison.
They can only be transferred with the help of
conjugative plasmids.

17. Mobilisable
An intermediate class of plasmids are mobilisable, and
carry only a subset of the genes
 These plasmids can 'parasitise' another plasmid,
transferring at high frequency
 presence of a conjugative plasmid
Incompatibility groups:
Several types of plasmids could coexist in a single cell.
On the other hand, related plasmids are often
'incompatible', resulting in the loss of one of them from the
cell line.

18. 2. Function
1. Fertility-(F) plasmids,
They are capable of conjugation (they contains the genes for
the pili).
2.Resistance-(R) plasmids,
Contain gene (s) that can build resistance against one or
several antibiotics or poisons.
3.Col-plasmids,
Contain genes coding for colicines, proteins that can kill other
bacteria. .

19. 4.Degradative plasmids,
able to digest unusual substances, e.g., toluene or
salicylic acid
5. Virulence plasmids
turn a bacterium into a pathogen.
6.Addiction system.
These plasmids produce both a long-lived poison and a
short-lived antidote.
Daughter cells that retain a copy of the plasmid survive
 while a daughter cell that fails to inherit the plasmid dies
or suffers a reduced growth-rate because of the lingering
poison from the parent cell.

20. Plasmids
Can be integrated
with Chromosomal
DNA
Episomes
-Integrated form of
plasmid with DNA

21. Potentials of Plasmids
Plasmids can be self
transmissible and
Non transmissible
Transfers the Sex
and Drug resistance
with the help of
restriction end
nucleases

22. Genotypic & Phenotypic variation
Genome – Sum total of Gene that make
up the genetic apparatus of cell
established as Genotype.
Hereditary constitution of cell this
transmitted to its progeny
Phenotype – is the physical expression
of genotype in a environment & can
change according to environment.

23. Phenotypic variation
Exhibit – different phenotypes
Appearance differs in different situations.
Eg: Typhoid bacilli flagellated normally
But grown in Phenol agar don't grow flagella so
flagella are lost physical variation
Lactose fermentation in E.coli dependent on Beta
Galactosidase
When lactose present - test is positive
When lactose is absent - test turns negative

24. Genotypic variations
Stable, heritable and not influenced by
environment
May occur by
Mutations
Genotypic by transfer of genes
Transformation
Transduction(Lysogenic conversion)
Conjugation

25. Mutations
Mutation is a Random, Undirected,
Heritable variation
Caused by alteration in the Nucleotide
sequence at some point of DNA which can
occur due to
Addition
Deletion
Substitution
of one or more bases

27. Mutation Rate
Different types of mutations can occur at
different frequencies. For a typical
bacterium, mutation rates of 10
–7
to 10
–11

per base pair are generally seen
 Although RNA and DNA polymerases
make errors at about the same rate, RNA
genomes typically accumulate
mutations at much higher frequencies
than DNA genomes.

28. Mutations can occur in any
sequence, inveitable, useful for
Survival

29. Multiple Mutations
Causes extensive chromosomal rearrangement
Missense mutation
Triplet code is altered so as to specify an
aminoacid different from that normally located at
particular position in the protein.
Nonsense mutation
Deletion of nucleotide within a gene may
cause premature polypeptide chain termination
by nonsense codon
Transversion is Substitution of purine for
pyramidine or vice versa in the base pairing

30. Possible effects of bp
substitution

31. Replica Plating Method
A common way to find bacterial mutants is
replica plating, which means making two
identical copies of the colonies on a petri plate
under different conditions.
For instance, if you were looking for trp-
auxotrophs, one plate would contain added
tryptophan and the other plate would not have
any tryptophan in it.
Bacteria are first spread on the permissive
plate, the plate that allows both mutants and wild
type to grow, the plate containing tryptophan in
this case..

32. They are allowed to grow for a while, then a
copy of the plate is made by pressing a
piece of velvet onto the surface of the
plate
then moving it to a fresh plate with the
restrictive condition (no tryptophan).
The velvet transfers some cells from each
colony to an identical position on the
restrictive plate.

33. Colonies that grow on the permissive plate
but not the restrictive plate are (probably)
trp- auxotrophs, because they can only grow
if tryptophan is supplied

34. Replica Plating

35. Transmission of Genetic material
( Gene Transfer )
Different Mechanisms
Transformation
Transduction
Conjugation

36. Transformation
Transformation is defined as transfer of Genetic
information through the activity of free DNA
 first experiment that showed DNA was the
genetic information
Griffith experiment
Mice injected with
Live non capsulated ( R ) Pneumococci
& with heat killed capsulated (S) Pneumococci

Lead to death of Mice with isolation of Live
capsulated Pneumococci
It means that some factor from dead pneumococci
transferred to live non pathogenic Pneumococci

37. Griffith Phenomenon

38. Certain prokaryotes exhibit competence, a state
in which cells are able to take up free DNA
released by other bacteria.
Incorporation of donor DNA into a recipient cell
requires the activity of single-stranded binding
protein, RecA protein, and several other
enzymes.
Only competent cells are transformable

40. Conjugation
A process by which a Donor cell or male cell
makes contact with another cell, the recipient
or Female cell.
DNA is directly transferable
Plasmid Carry genetic information necessary for
conjugation to occur.
Only cell that contain such plasmids can act as
donor. the cell lacking a corresponding plasmid
act as recipient.
Requires direct contact between donor and
recipient

41. Conjugation - Transferring genes
with plasmids
Plasmids mediating
conjugation carry
genes coding for
properties, of 1-2
microns long protein
appendage termed
Pilus on the Donor
cell

43. Conjugation

44. Pilus helps Conjugation
Different types of Pilus
are specified by different
types of plasmids and
can help in aid of
plasmid classification.
Only one strand of
circular DNA of the
plasmid nicked upon at
a specific site and
passed into a recipient.
Spread to all other cells.

45. F factor
Transfer factor that
contains the genetic
information necessary
for synthesis of Sex
Pilus and for self
transfer without any
other identifiable
genetic materials such
as drug resistance

46. F
+
called as Donor bacteria can transform F
-
into F
+

cell
Can be Episomes able to exist in some cells in the
integrated state in the donor cell chromosome
Can transform chromosomal genes to recruitment
with high frequency are known as Hfr cells
Conversion of F
+
cells into Hfr state is reversible.
F factor incorporates some chromosomal genes
and is called as F’
Sexduction : process of transfer of host genes
through F’ factor

47. Transduction
Transduction is
defined as transfer of
portion of DNA from
one bacteria to
another by
Bacteriophages, is
known as
Transduction

48. 48
Historical
Lederberg & Zinder
Transduction was first discovered in 1952 by
Joshua Lederberg and Norton Zinder
Joshua Lederberg Norton
Zinder


Alcamo's and Prescott , google images

49. How transduction was discovered
49
Studied in Salmonella typhimurium
Plated two auxotrophic strains (LA-2 and LA-22)
individually on minimal medium, no cells grew.
Plated a mixture of the two auxotrophic strains on
minimal medium, cells grew into colonies.
Thus, genetic exchange was taking place between
the two cell types.
www.bio.ilstu.edu

50. U-tube Experiment
www.2dix.com
50

51. TYPES OF TRANSDUCTION
GENERALIZED TRANSDUCTION
SPECIALIZED TRANSDUCTION

52. GENERALIZED TRANSDUCTION
process by which any bacterial gene may be
transferred to another bacterium via a
bacteriophage.
typically carries only bacterial DNA and no
viral DNA

53. Generalised Transduction
53
1. A lytic bacteriophage adsorbs
to a susceptible bacterium.
2. The bacteriophage genome
enters the bacterium. The
genome directs the bacterium's
metabolic machinery to
manufacture bacteriophage
components and enzymes
www.2dix.com

54. 54
3. Occasionally, a
bacteriophage head or capsid
assembles around a fragment of
donor bacterium's nucleoid
instead of a phage genome by
mistake.
steps in Generalised Transduction (cont’d)
4. The bacteriophages are released.
www.2dix.com

55. steps in Generalised Transduction (cont’d)
55
5. The bacteriophage
carrying the donor
bacterium's DNA adsorbs
to a recipient bacterium
6. The bacteriophage inserts the
donor bacterium's DNA it is
carrying into the recipient
bacterium .
www.2dix.com

56. 56
steps in Generalised Transduction (contd)
7. The donor bacterium's DNA is exchanged for some
of the recipient's DNA.
www.2dix.com

57. SPECIALIZED TRANSDUCTION
 Discovered by Morse et al
 Process by which a restricted set of bacterial
genes are transferred to another bacterium
 Partially coded prophage DNA is called a
"heterogenote".

58. Specialised Transduction
58
1. A temperate bacteriophage
adsorbs to a susceptible
bacterium and injects its
genome .
2. The bacteriophage inserts its
genome into the bacterium's
nucleoid to become a prophage.
www.2dix.com

59. steps in Specialised Transduction (cont’d)
59
3. Occasionally during
spontaneous induction, a small
piece of the donor bacterium's
DNA is picked up as part of the
phage's genome in place of
some of the phage DNA which
remains in the bacterium's
nucleoid.
4. As the bacteriophage
replicates, the segment of
bacterial DNA replicates as part
of the phage's genome. Every
phage now carries that segment
of bacterial DNA.
www.2dix.com

60. steps in Specialised Transduction (cont’d)
60
5. The bacteriophage adsorbs to
a recipient bacterium and injects
its genome.
6. The bacteriophage genome
carrying the donor bacterial
DNA inserts into the recipient
bacterium's nucleoid.
www.2dix.com

61. Genetic Mechanisms of Drug
Resistance
Bacteria acquire drug resistance through several
Mechanisms
Mutations
Genetic transfer
Transformation,
Transduction
Conjugation
Several Biochemical Mechanisms
Decreasing permeability of drugs,
Attaining alternative pathways
Produce enzymes and inactivate drugs

62. Genetic Mechanisms in
Bacteria helps to spread the
Infectious diseases

63. Transposons and Insertion
Sequences

Transposons and insertion sequences are
genetic elements that can move from one
location on a chromosome to another by a
process called transposition.
 a type of site-specific recombination.

66. Transposition is linked to the presence of
special genetic elements called transposable
elements. Transposition can be either
replicative or conservative

69. Summary

A DNA molecule is composed of two chains of
Nucleotides wound together in the form of a Double
Helix
Gene is a sequence of DNA carrying codons specifying
for particular polypeptide
Plasmids are circular DNA molecules present in the
cytoplasm of the Bacteria
Phenotype variation – is the physical expression of
genotype in a environment & can change according to
environment
Genotype variation – Stable, heritable and not
influenced by environment

70. Mutation – alteration in the Nucleotide sequence at some
point of DNA which can occur due to Addition, Deletion,
Substitution of one or more bases

Replica plating
Transformation – is defined as transfer of Genetic
information through the activity of free DNA
Conjugation – process by which a Donor cell or
male cell makes contact with another cell, the
recipient or Female cell.

71. Transduction – is defined as transfer of portion of DNA
from one bacteria to another by Bacteriophages
Generalised transduction – process by which any
bacterial gene may be transferred to another bacterium
via a bacteriophage

Specialised transduction – Process by which a
restricted set of bacterial genes are transferred to another
bacterium

73. Reference
Ananthnarayan and Paniker`s Text book
of Microbiology 8
th
Edition
 C.P.Baveja Textbook of Microbiology