Bacterial genetics png 2011

Bacterial Genetics

1. BACTERIAL GENETICS
II MBBS
Mr. Gunjal Prasad NiranjanMr. Gunjal Prasad Niranjan
M.Sc. Medical MicrobiologyM.Sc. Medical Microbiology
PG Diploma in CRA,PG Diploma in CRA,
Assistant Professor, Assistant Professor,
Dept of Microbiology, Dept of Microbiology,
PDVVPF’s Medical College, PDVVPF’s Medical College,
Ahmednagar Ahmednagar

2. Bacterial Genetics

GeneticsGenetics is the study of heredity and variation.
The unit of heredity is GeneGene.

GeneGene is a segment of DNA, carrying codons specifying for a
particular polypeptide.
IntronsIntrons - - non coding sequences on a gene.
ExonsExons - - coding sequences on a gene.
Bacterial genetics is used as a model to understand
DNA replication,
Genetic characters their changes & transfer to next generations.

3. Nucleic Acids

DNA (Deoxy ribonucleic acid) :DNA (Deoxy ribonucleic acid) :
Stores information for protein synthesis.
Consists large number of genes.

RNA (Ribonucleic acid) : RNA (Ribonucleic acid) :
Transcription & translation of information for
protein synthesis.
Central Dogma : DNA RNA Protein

4. Structure Of DNA
Proposed by Watson & Crick.
A Double helix model.
Composed of 2 chains of polypeptides.
Each chain has a backbone of deoxyribose sugar and
phosphate residues arranged alternately.

4 nitrogenous bases: 4 nitrogenous bases: Adenine (A) Purine
Guanine (G)
Thymine(T) Pyrimidine
Cytosine (C)

5. One of these 4 nitrogenous bases is attached to each
deoxyribose sugar.
The two strands are held together by “Hydrogen
bonds” between nitrogenous bases on the opposite
strands.
This bonding is very specific, that hydrogen bonds can
only be formed between Adenine and Thymine (A=T)
and between Guanine and Cytosine (G=C).
Adenine and Thymine thus form one complementary
base pair and Guanine & Cytosine another.

6. Double helical structure of DNA by
Watson & Crick
Double helical structure of DNA by
Watson & Crick

8. Structure Of RNA
Structurally similar to DNA, except for 2
major differences:
 Ribose sugar
 Uracil in place of Thymine.
3 types of RNA

m RNA (messenger RNA)m RNA (messenger RNA)

t RNA ( transfer RNA )t RNA ( transfer RNA )

r RNA ( ribosomal RNA ) r RNA ( ribosomal RNA )

9. GENEGENE – –
A segment of DNA, that carries codons which specifies for a
particular polypeptide is called as a “GENE”.“GENE”.
CODONCODON – –
Genetic information is stored in DNA as a code.
Codon consist of sequence of three nucleotide bases i.e. the code
is triplet.
Each codon specifies for production of a single amino acid.
But more than one codon may exist for the same amino acid.
For e.g.- For e.g.- AGA codes for Arginine & CGU, CGC, CGA, & AGG can
also code for the same amino acid, Arginine.
IMPORTANT DEFINATIONS

10. IMPORTANT DEFINATIONS
NON SENSE CODON– NON SENSE CODON–
Three codons (UAA, UGA, & UAG) do not code for any amino acid
& act as a “Stop codon” “Stop codon” for terminating the message for the
synthesis of a polypeptide.
ANTICODONS-ANTICODONS-
The term anticodon used in connection with tRNA.
Specific for nitrogen base triplet known as Recognition site placed
on the exposed part of tRNA.
This anticodon is complementary to codon on mRNA with which it
associates during protein synthesis.

11. TRANSCRIPTION

Transfer of genetic information from a DNA to mRNA.Transfer of genetic information from a DNA to mRNA.
Part of DNA unzipped by breaking H
2
bond. Only one of the strand is
used as “Template” for synthesis of mRNA.mRNA.
In synthesis of mRNA Thymine is replaced by Uralic.
mRNA synthesis takes place in presence of enzyme
““RNA Polymerase”RNA Polymerase” following pairing of A=U & C G.
Role of mRNA is acting as “Messenger”.
It carries transcribed information from DNA to Ribosome, the actual
site of protein synthesis. This process is called as “Transcription”.“Transcription”.
mRNA
RNA polymerase
DNA

12. TRANSLATION
Actual process of protein synthesis, takes place in cytoplasm on
ribosome.
The mRNA passes into cytoplasm.
Then mRNA & tRNA come together on surface of
ribosomes containing rRNA.
Triplet base sequence on mRNA is known as “Codon”. “Codon”.
The base sequence on mRNA are recognized
by the anticodon on tRNA.anticodon on tRNA.
The tRNA contains the triplet (anticodon) at one end & amino acid at
other end.
Ribosome
With rRNA
tRNA &
anticodon
mRNA
Amino acid
codon

13. Ribosome moves along the mRNA until the entire mRNA
molecule has been translated into corresponding sequence of
amino acids.
The transferred amino acids are joined by peptide bonds in
presence of enzymes, present in ribosomes.
This process is called as “Translation”.
rRNA +protein
Amino acid
tRNA
Anti codon
mRNA
codon
Growing polypeptide

14. This process is terminated by one of the three
“Terminating codons i.e. UAG, UGA,UAG, UGA, UAAUAA called as
“Stop Codon” “Stop Codon” OR “Non sense Codon” . OR “Non sense Codon” .
Which do not code for any amino acid.
The tRNA, mRNA, & ribosome are dislocated.
tRNA goes in cytoplasm for reuse.
mRNA being a labile structure get dissolved in
cytoplasm, & peptide chain is released to form protein.
TERMINATION

15. Thus in protein synthesis DNA acts as an “Architect”.DNA acts as an “Architect”.

mRNA as “Blue print”.mRNA as “Blue print”.

tRNA as “Builders”.tRNA as “Builders”.
Which build up proteins by using amino acids as a raw
material under the supervision of ribosomes.
TERMINATION

16. SYNTHESIS OF POLYPEPTIDE –SYNTHESIS OF POLYPEPTIDE –
RNA polymerase
DNA
mRNA mRNA
rRNA +proteinrRNA +protein
Amino acid Amino acid
tRNA tRNA
Anti codonAnti codon
mRNA mRNA
Growing polypeptide
RNA polymerase
DNA
mRNA mRNA
with with
ribosome ribosome
& rRNA & rRNA
tRNA tRNA
RNA polymerase
DNA

17. Genetic Information In Bacteria
ChromosomeChromosome Carries properties like virulence,
Pathogenicity & resistance.
PlasmidPlasmid Extrachromosomal genetic
material in the cytoplasm.
Replicate independently.
BacteriophageBacteriophage Virus infecting bacteria.
Carries properties like Drug
resistance, toxicity etc.

18. EXTRACHROMOSOMAL GENETIC ELEMENTS
-PLASMIDSPLASMIDS

Extra chromosomal, circular DNA molecules.Extra chromosomal, circular DNA molecules.

Replicate independently. Replicate independently.
Transmissible & non transmissible.
An important vectors in genetic engineering.

EPISOME EPISOME

Plasmids integrated with chromosomal DNA of bacteria.Plasmids integrated with chromosomal DNA of bacteria.

PLASMIDSPLASMIDS

Properties like drug resistance, toxigenicity, conjugation Properties like drug resistance, toxigenicity, conjugation
etc. etc.

Fertility , maleness, Fertility , maleness,

Production of Bacteriocin, etc.Production of Bacteriocin, etc.

19. Types of Plasmids

Fertility plasmid: Fertility plasmid: Carries genes to produce
a sex pilus; replicates, and a copy is passed
to another cell.

Resistance plasmidResistance plasmid: : Carries genes that
make the cell resistant to antibiotics, heavy
metals.

Catabolic plasmidCatabolic plasmid: : For example, toltol
plasmid plasmid with genes for breaking down and
using toluene, an organic solvent.

20. Types of Plasmids
Bacteriocin plasmid: codes for bacteriocins, proteins that kill
related bacteria.
Virulence plasmid: has genes needed for the bacterium to
infect the host.
Tumor-inducing plasmid: The Ti plasmid found in
Agrobacterium tumefaciens = Codes for plant growth hormones.

When the bacterium infects the plant cell, the plasmid is passed
to the plant cell and the genes are expressed, causing local
overgrowth of plant tissue.
Very useful plasmid for cloning genes into plants.

21. Genotypic & Phenotypic Variations
Phenotype –
Physical expression of the genotype in a given environment.
Genotype –
Genetic constitution of a cell that is transmitted to its progeny.
Variations
1.Phenotypic variations –
influenced by the environment
temporary & not heritable
1.Genotypic variations –
Not influenced by the environment
Stable & heritable

22. PHENOTYPIC VARIATIONS PHENOTYPIC VARIATIONS ––
•Bacteria exhibit different phenotypic appearance.
•Reversible Reversible
•E.g. E.g.
•Synthesis of flagella of Synthesis of flagella of Salmonella typhi - Salmonella typhi -
• Salmonella typhi generally possess flagella, but when grown in
phenol agar the flagella are not formed.

• This effect is reversed when subcultured from phenol agar to
nutrient broth.

23. Mechanisms Of Genetic Variations
Mutation – Mutation –

Is random, un-directed heritable variation.Is random, un-directed heritable variation.
Occur due to any change in nucleotide sequence of a
gene (DNA molecule).
Irrespective of a detectable change in cell phenotype.

Transfer or exchange of genetic materialTransfer or exchange of genetic material
1.1.TransformationTransformation
2.2.TransductionTransduction
3.3.Lysogenic conversionLysogenic conversion
4.4.ConjugationConjugation

24. Mutation
Random, un-directed heritable variation.
Caused by a change in the nucleotide base sequence of the
DNA.

Types of mutation:Types of mutation:
1.1.Point mutation Point mutation (Base pair substitution, Frame shift).
2.2.Multisite mutations Multisite mutations

Mutagens - Mutagens - Agents which can induce mutation.
e.g. UV rays, 5 bromouracil, alkylating agents, etc.

25. 1. Point Mutation1. Point Mutation
Caused due to addition, deletion or substitution of
one or more bases.
Types – Base pair substitution.
Transition : A purine base is replaced by a purine
base OR OR a pyrimidine base is replaced by another
pyrimidine base. Most common type. i.e. A replaced
by G OR T is replaced by C.

Transversion : Substitution of a purine base by a
pyrimidine base & vice versa i.e. A is replace by T OR
C is replaced by G .

26. FRAME SHIFT MUTATION -
During replication one OR a few adjacent base pair
(nucleotides) have been inserted to OR deleted from the
DNA.
 This shifts the normal transitional “ reading frame” of coded
message, from the point on forming an entirely new set of triplet
codon.
 So, during translation, the coded message is read correctly up to
the point of deletion or addition, but the subsequent codons will

specify the wrong amino acids.

28. Multi site mutations -

29. OTHER TYPES OF MUTATIONS
Nonsense mutation – Converts a codon that
specifies an amino acid into a termination codon.

30.  Mutation which results in turning vital functions
in the death of the organism – nonviable
mutation.
•A conditional lethal mutant may be able to live
under certain conditions – permissive conditions.
•Commonest type of conditional mutant is the
temperature sensitive (t
s
) mutant which is able to
live at the permissive temperature of 35°C but not
at the restrictive temp (39°C).
Lethal mutations –

31. Suppressor MutationSuppressor Mutation
A mutation (second) that restores the function of a
gene altered by previous mutation (first) is called as
“Suppressor Mutation”.

32. GENE TRANSFER GENE TRANSFER
Ways that bacteria can acquire new genetic
information -
Transformation -
Taking up of “naked DNA” from solution.
Transduction -
Transfer of DNA one to cell to another by a
Bacteriophage.
Conjugation -
“Mating”: transfer of DNA from one bacterium to another
by direct contact / bridge formation.

33. Transformation (Griffith, 1928)
Transfer of genetic information by free DNA. i.e. by
direct uptake of donor DNA by the recipient.

Live noncapsulated (R) Pneumococci +
Heat killed capsulated (S) Pneumococci

Injected into mice

Death of mice
Blood culture
Live capsulated Pneumococcus isolated from the
blood of mice.

35. Transduction
Transfer of a portion of the DNA from one bacterium to another
by a Bacteriophage.
Packaging error within the infected bacteria during the
assembly of progeny phages – presence of a segment of host
DNA along with the phage nucleic acid in the core of phage
Infection of another bacterium
Transfer of host bacterial DNA to the new bacterium
Acquisition of new characteristics coded by the donor DNA.

37. Transduction
Types of Transduction –
1.Generalized - When it involves any segment of the
DNA.
2.Restricted - When a specific bacteriophage
transduces only a particular genetic trait.
Episomes & plasmids can also be transduced.
Most widely used mechanism of gene transfer among
prokaryotes.

38. Lysogenic Conversion
Phage DNA itself act as the new genetic element.
Bacteriophages – 2 Types of life cycles.
Lytic or virulent cycle – Progeny viruses build
up inside host bacterium, which rupture to
release them.
Temperate OR Nonlytic OR Lysogenic cycle –
Host bacterium is unharmed.

40. This process is called Lysogeny & the bacteria harboring
these prophages are called Lysogenic bacteria.
Prophage behaves as an additional segment of bacterial
chromosome, coding for new characteristics.
This process by which prophage confers genetic
information to a bacterium is called Lysogenic
conversion .

By lysogenic conversion Diphtheria bacilli acquire
toxigenicity & therefore virulence.
Elimination of phage renders a toxigenic strain non-
toxigenic.

41. Conjugation
First described by Lederburg & Tatum in 1946 in a strain of E.coli
called K12.
A donor or male bacterium transfers DNA directly to a recipient or
female bacterium by a conjugation tube/ bridge (sex pili).
The recipient / female bacterium attains donor status & in turn can
conjugate with other recipient / female cells.
Maleness is determined by the presence of a plasmid which codes for
sex pili.
The plasmid is called the sex factor or fertility factor (F factor).
R (resistance) factor can also be transferred by conjugation.

42. Process of Conjugation