BACTERIAL GENETICS DEFINITION CENTRAL DOGMA EXTRACHROMOSOMAL GENETIC ELEMENTS MUTATION TRANSMISSION OF GENETIC MATERIAL

1. BACTERIAL GENETICS
DR.KAVYA.P

2. • DEFINITION
• CENTRAL DOGMA
• EXTRACHROMOSOMAL GENETIC ELEMENTS
• MUTATION
• TRANSMISSION OF GENETIC MATERIAL
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3. GENETICS……
defines & analyses heredity i.e., constancy and variation
in the physiologic functions, that form the properties of an organism.
The term was coined by British biologist William Bateson in 1906.
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4. 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

5. Gregor Mendel ….. Formulated the basic rules of heredity in 1865.
Thomas Morgan (1866-1945)… the founder of modern genetics, showed
chromosomes contain genes & won Nobel prize, 1933.

6. Watson - Crick
Discovery of DNA

7. Beginning of Bacterial Genetics
Since 1940s principles of genetics were applied to bacteria &
viruses.
Better understanding of genetic process has led to the birth of a
new branch of science, MOLECULAR BIOLOGY..

8. Central Dogma
of Molecular Genetics
DNA------- mRNA------ protein
transcription translation

9. Genetic Materials
DNA & RNA
DNA -deoxyribonucleic acid
RNA -ribonucleic acid
Nucleotides
◦ Phosphate group
◦ Pentose sugar
◦ Nitrogenous base

10. Structure of DNA
Double stranded (double helix)
Chains of nucleotides
5’ to 3’ (strands are anti-parallel)
Complimentary base pairing
◦ A-T
◦ G-C

11. DNA Structure
Phosphate-P
Sugar-blue
Bases-ATGC

12. DNA Structure
◦ The double stranded structure stabilized by hydrogen bonding b/n nitrogenous bases
of opposite strands.
◦ Purine bases are Adenine, Guanine
◦ Pyrimidine bases are Thymine, Cytosine.
◦ A+T/G+C ratio remains constant for each bacterial species.
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13. Bacterial Chromosome
Bacterial chromosome consists of a double stranded molecule of
DNA arranged in a circular form
1000µm in length
1kb=1000 base pairs
Bacterial DNA about 4000kb
Human genome about 3 million kb long

14. DNA Replication
Semiconservative type of DNA replication
 In daughter cell, one strand is derived from the mother cell &
other strand is newly synthesized

15. DNA Replication-occurs at the replication
fork
5’ to 3 ‘
DNA helicase-unzips + parental DNA strand that is used as a template
◦ Leading strand (5’ to 3’-continuous)
*DNA polymerase-joins growing DNA strand after nucleotides are aligned
(complimentary)
◦ Lagging strand (5’ to 3’-not continuous)
*RNA polymerase (makes short RNA primer)
*DNA polymerase (extends RNA primer then digests RNA primer and replaces it with DNA)
*DNA ligase (seals Okazaki fragments-the newly formed DNA fragments)

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18.  Replication of bacterial circular DS-DNA begins at the ori
locus
 Chromosome replication termination regions are called
ter.
 They are located at opposite points on the circular DNA
chromosome

19. RNA
Single stranded
It contains sugar Ribose
It contains the nitrogenous base Uracil in place of Thymine in
DNA

21. 3 Types of RNA
1. mRNA (messenger RNA)
◦ Contains the codons
2. r RNA (ribosomal RNA)
◦ Ribosomes (70S)
3. t RNA (transfer RNA)
◦ Transfer amino acids to the ribosomes for protein synthesis
◦ Anti-codon

22. Template strand DNA strand which is transcribed to give rise to
m-RNA
Coding strand the opposite DNA strand which has the same
sequence of that of m-RNA
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23. Basic principles of molecular biology
Central dogma of molecular biology
DNA
Transcription
RNA
Translation
POLYPEPTIDE
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24. The template strand of DNA is transcribed into a SS m-RNA by DNA
depended RNA polymerase
Steps
Initiation
Elongation
Termination
Post transcriptional modification

25. Transcription
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26. INITIATION
Promoter region : certain specific areas on the DNA to which RNAP
attaches.
Transcription unit : The region b/n promoter & terminator sites.
TATA Box : it act as the signal for the start region
Enhancers : increase the rate of transcription.

27. ELONGATION
The RNAP moves along the DNA template & new nucleotides are
incorporated in the nascent m-RNA.
Transcription bubble is formed by
RNAP, DNA & nascent RNA

28. TERMINATION
Rho dependent…when Rho
factor attaches to the RNA , the
RNAP cannot move further &
enzyme dissociates from DNA &
newly formed m-RNA is released

29. TERMINATION
Rho independent….it is an
interrupted invert repeat
DNA sequence &
is transcribed
The m-RNA produced will
form intrachain base pairing
to make a hairpin structure
this obstruct further movement of
RNAP

30. POST TRANSCRIPTIONAL MODIFICATION
In eukaryotes, the primary transcript undergoes extensive editing to become
mature m-RNA.
Poly A tailing, 5`capping, removal of introns & connection of exons.
In bacteria m-RNA is not changed & translation starts even before completion
of transcription.

32. TRANSFER RNA
Transfer amino acids from cytoplasm to
the ribosomal protein synthesizing
machinery
Structure
Acceptor arm at 3` end…carries amino acid
Anticodon arm…recognizes the triplet
nucleotide codon in m-RNA
DHU arm…recognition site for enzyme
which adds the amino acid
Pseudo uridine arm…binds t-RNA to
ribosomes

33. RIBOSOMAL RNA
They are structural components of ribosomes
Ribosomal assembly is the protein synthesizing machinery
Bacteria has 70S ribosomes with 30S & 50S subunits
Bacterial ribosomes contain 3 types of r-RNA

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

36. GENETIC CODE
Genetic information in DNA is stored as a triplet code, which consists of 3
bases.
Each triplet code is transcribed to m-RNA & it has codons which specifies for a
single amino acid
Codon is degenerate…more than one codon may exist for same amino acid
Genetic Code is Universal…the codons are the same for the same amino acid in
all species

37. CODON
Codons code for a specific amino acid
3 base code - 4 bases ( A,U,G,C )
64 possible combinations ( 43)
20 amino acids
Nonsense / terminator codons…do not code for any amino acid
E.g. : UAA, UAG, UGA
Initiator codon…AUG act as the initiator codon

39. How bacterial Genome differs from Higher
forms of Life
Several stretches of DNA don't appear to function as codons, occurs between
the coding sequences of gene called as INTRONS
Coded are called as EXONS
In transcription introns are excised when form RNA before translated by
ribosomal proteins.

40. TRANSLATION

41. REGULATION OF GENE EXPRESSION
Synthesis of proteins under the influence of gene is called gene expression
All genes are not always being expressed
Genes turned on all the time - Constitutive
Enzymes are synthesized irrespective of
the conc. of the substrate
Other genes can be regulated by either induction / turned on or repression / turned off

42. LAC OPERON
Lac - Lactose (disaccharide)
◦ Glucose and Galactose
Operon - series of structural genes, all under the control of a Regulatory Gene
lac operon is normally turned off
lac operon is an Inducible operon

43. LAC OPERON

45. TRYPTOPHAN OPERON
Tryptophan - amino acid
Operon - series of structural genes, all under the control of a regulatory gene
Tryptophan operon is normally turned on
Tryptophan operon is a repressible operon

48. • DEFINITION
• CENTRAL DOGMA
• EXTRACHROMOSOMAL GENETIC ELEMENTS
• MUTATION
• GENE TRANSFER

49. EXTRACHROMOSOMAL 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, able to produce toxins

50. PLASMIDS
They are circular DNA molecule in the cytoplasm of bacteria
Capable of autonomous replication (independent replicons)
Can transfer genes from one cell to other
act as vectors in genetic engineering
Can also be present in yeasts

51. PLASMIDS
Plasmid seem to be ubiquitous in bacteria, May encode genetic information for properties
Resistance to Antibiotics
Bacteriocin production
Enterotoxin production
Enhanced pathogenicity
Reduced sensitivity to mutagens
Degrade complex organic molecules

52. Can be integrated with Chromosomal DNA
Episomes -Integrated form of plasmid with DNA

53. CLASSIFICATION
Based on restriction endonuclease fingerprints from purified plasmid DNA
The pattern of fragments produced is dependent on the distribution of the
specific DNA sequences recognized by the enzyme
Closely related plasmids will produce same or very similar fingerprints
Unrelated plasmids will produce different fingerprints

54. By incompatibility testing
Based on the fact that, closely related plasmids are unable to coexist stably in the same
bacterial cell
Plasmids belonging to the same incompatibility group are closely related & interfere with one
another’s replication
Unrelated plasmids can coexist stably & belong to different incompatibility group

55. By using specific virulent bacteriophages
They will adhere only to the type of pilus produced by a particular group of
plasmids
Lysis by such a phage allows the identification of the group to which the
plasmid contained in the cell belongs

56. GENOTYPE PHENOTYPE
The sum total of the genes that make up the
genetic apparatus of a cell.
The physical expression of the genotype in a given
environment
Genotypic variations are stable, heritable & not
influenced by the environment.
They are temporary, not heritable & influenced by
the environment.
E.g.: mutation, transformation, transduction,
conjugation
E.g.: loss of flagella of typhoid bacilli in phenol agar.

57. • DEFINITION
• CENTRAL DOGMA
• EXTRACHROMOSOMAL GENETIC ELEMENTS
• MUTATION
• TRANSMISSION OF GENETIC MATERIAL

58. MUTATION
A random, undirected, heritable variation caused by an alteration in
the nucleotide sequence at some point of the DNA of the cell.
Macro lesions ( Multisite mutations) involve extensive
chromosomal rearrangements such as deletion, duplication,
inversion.
Micro lesions (Point mutations) affects one or very few nucleotides

59. POINT MUTATION
BASE PAIR SUBSTITUTION:
TRANSITION -- when a purine is replaced by another purine or a pyrimidine by another pyrimidine.
TRANSVERSION -- It is the substitution of a purine for a pyrimidine and vice versa in a base pairing

60. FRAMESHIFT MUTATION
Frame shift (deletion)
(leu) (ser) (arg)
Normal AAT AGT GCC
(leu) (val) (pro)
Mutant AAT AGT GCC A
Frame shift (insertion)
(leu) (ser) (arg)
Normal AAT AGT GCC
(leu) (glut) (cyst)
Mutant AAT CAGT GCC

64. MUTATIONS
Suppressor mutation: reversal of mutant phenotype by another mutation at
a position distinct from that of original position
Lethal mutation : involve vital functions & such mutants are nonviable
E.g.: a conditional lethal mutant—able to live under certain permissive
conditions like temp sensitive mutant

65. MUTAGENIC AGENTS
U V rays
Alkylating agents
Acridine dyes
5-bromouracil
2-aminopurine

66. • DEFINITION
• CENTRAL DOGMA
• EXTRACHROMASOMAL GENETIC ELEMENTS
• MUTATION
• GENE TRANSFER

67. GENE TRANSFER
Mechanisms of gene transfer
1.VERTICAL INHERITANCE
inheritance of parental genes
2. LATERAL (HORIZONTAL) gene transfer
DNA can be transferred from one organism to another & cause permanent
stable change in recipient’s genetic composition.
E.g.: Transformation, Transduction & Conjugation

68. TRANSFORMATION
The transfer of genetic information through the agency of free DNA
Cells that are able to take up DNA from their environment & incorporate it into
the genome are said to be competent
Competence involves the ability to bind DNA to the cell surface & then
transport it through the cell envelope into the cytoplasm prior to incorporation
into the genome

69. TRANSFORMATION

70. Cont.….
Natural competence  shown by
S . Pneumoniae , B .subtilis & H. influenzae
S . pneumoniae & B .subtilis take up heterospecific & homospecific bacterial DNA
H.influenzae & Neisseria gonorrhoea  homospecific (closely related) DNA only.
In systems that rely on natural competence, transformation with homospecific
chromosomal DNA is much more efficient

71. Cont.….
Artificial competence commonly seen in E.coli , P.aeruginosa, S.aureus
Induced by treatment of bacteria with calcium chloride at 0 degree C
Cell suspensions are mixed with free DNA and are subject to electrical discharge
through the suspension, termed Electroporation
Application in recombinant DNA technology

72. TRANSFORMATION
Ist observation of bacterial transformation was by Griffith 1928
GRIFFITH’S 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

73. TRANSFORMATION
The nature of the transforming principle was identified as DNA by
Avery, Macleod & McCarty in 1944

74. GRIFFITH’S EXPERIMENT

75. TRANSDUCTION
The transfer of a portion of the DNA from one bacterium to another by a bacteriophage
2 forms
Generalized …..any part of the donor genome can be transferred to a suitable recipient
Specialized…..specific sequences are transferred

77. RESTRICTED TRANSDUCTION
Lambda phage of E.coli
Prophage lambda inserts only b/n the genes determining
Galactose utilization (gal) & biotin synthesis (bio)
It transduces only either of these

78. BACTERIOPHAGE
A virus that parasitize bacteria
A nucleic acid core & a protein coat
2 types of lifecycle
Virulent / lytic & Temperate / non lytic cycles

81. LYSOGENIC CONVERSION
Prophage…phage DNA integrated with bacterial chromosome
Lysogenic bacteria…bacteria harboring prophages
Lysogeny…process of prophage multiplying synchronously with the host DNA &
transferring to daughter cells
Phage conversion…the prophage DNA confers new genetic information to a bacterium

82. Cont.…
Episomes & plasmids may also be transduced
The plasmids determining penicillin resistance in Staphylococci are transferred by
transduction
Most widespread mechanism of gene transfer among prokaryotes
Transduction provides an excellent tool for the genetic engineering

83. CONJUGATION
The transfer of DNA directly from one bacterial cell to another by a mechanism that requires
cell-to-cell contact
Discovered by Lederberg and Tatum (1946)
in a strain of E . coli K 12
Plasmids are the genetic elements most frequently transferred

84. Self transmissible plasmids contain the tra gene which encodes
the genetic functions for cell contact & transfer
DNA transferred during conjugation through the plasmid
include
1.Fertility (F) factor
2.Colicinogenic (Col) factor
3.Resistance transfer factor (RTF)

85. F factor
The maleness or donor status of a cell is indicated by a specialized fimbria (sex pili)
Sex pili act as conjugation tube
Retraction of pilus brings the cells into close contact & a pore forms in the adjoining cell
membrane
In plasmid DNA a single stranded nick occurs at oriT & the 5`end of a single strand is transferred
to the recipient

89. The F factor is actually an episome & in some cells they exist in the integrated state
They are Hfr cells
Have ability to transfer chromosomal genes to recipient cells with high frequency
Following conjugation with an Hfr cell, an F- only rarely becomes F+
This conversion of an F+ cell into the Hfr state is reversible

91. Hfr and F- bacterial conjugation

92. SEXDUCTION
When the F factor reverts from the integrated state to the free state; it may sometimes carry
some chromosomal genes from near its site of attachment
Such F factor is called F prime factor
The process of transfer of host genes through the F prime factor resembles transduction & so
named as sexduction

93. Col factor
Colicin production is determined by a plasmid called Col factor, which
resembles the F factor
Colicins…..Antibiotic like substances which are specifically & selectively
lethal to enterobacteriae
Similar substances are produced by bacteria other than coliforms also & so
collectively termed as Bacteriocin

94. RESISTANCE TRANSFER FACTOR
This plasmid leads to the spread of multiple drug resistance among bacteria
The plasmid R factor consists of 2 components
The RTF…..for conjugational transfer
Resistance determinant (r) ….for each of
the several drugs

95. RTF & the ‘r’ determinants may exist as separate plasmids
Then the drug resistance is not transferable
The host cell remains drug resistant
Transferable drug resistance is seen in…..Enterobacteriaceae, Vibrio, Pseudomonas, Pasteurella

96. The transfer of drug resistance is inhibited by…
1. Anaerobic conditions
2. bile salts
3. alkaline pH
4. the abundance of anaerobic Gram positive bacteria by minimizing
the contact b/n donor cells & recipient cells

97. MOBILE GENETIC ELEMENTS
Structurally & genetically discrete segments of DNA
They move around in a cut & paste manner b/n chromosomal &
extra chromosomal DNA molecules within cells

98. They are also called Jumping genes
Depend on chromosomal or plasmid DNA for replication
No genetic homology is needed for transfer of DNA molecule
by Transposition
Discovered by Barbara McClintock

99. A segment of DNA with one or more genes in the centre & the two ends with
inverted repeat sequences of nucleotides

100. 2 types
Insertion sequence elements (IS)
small (1-2kb) & encode only those functions needed for their own transposition
Structure
Unique central sequence encoding the transposition function.
Flanked by short inverted repeat sequence

101. Transposons
larger (4-25kb) & encode atleast one function unconnected to transposition, that alters
the cell phenotype
E.g.: antibiotic resistance…..
TEM-2- beta lactamases
2 types
Composite transposon
Complex transposon

102. Composite transposon
Constructed from recognizable simpler units
Unique central sequence encodes the specific functions & lack transposition functions
The terminal sequence repeats comprise a pair of IS elements with transposition functions
e.g. Tn5,Tn7

103. Complex transposon
They do not have modular structures
Both transposition & non transposition functions are located in the bulk of the sequence
Flanked by short terminal inverted repeats
TnA or Tn1

104. Some transposons move by replicative manner
E.g. Tn1
Others non replicative (conservative) i.e. they move without making a copy of themselves
eg:Tn7

105. Integrons
A mobile DNA element
They can replicate & carry gene, particularly those responsible for antibiotic resistance by site
specific recombination
5` conserved region have a gene int encoding integrase, a site specific recombinase
attl site besides int is recognised & utilized by the integrase to capture gene cassettes

106. The antibiotic resistance genes that integron capture are located on gene cassettes
These gene cassettes exist as free circular DNA
A recombination event occurs, integrating the cassette into the integron
They are classified based on the integrase gene they contain
E.g.: most recently identified antibiotic resistance gene cassettes that encode the MBL