2. • Bacterial genetics is the study of the mechanisms of heritable information in bacteria,
their chromosomes, plasmids, transposons and phages. Techniques that have enabled this
discipline are culture in defined media, replica plating, mutagenesis, transformation,
conjugation and transduction.
• Gene is the segment of DNA that specifies for a polypeptide.
• Genome is the sum total of genes in a cell.
• Codon is the triplet of bases that specifies for a single amino acid.
• Degenerative: More than 1 codon codes for for an amino acid.
• Stop codons are the codons that terminate message for synthesis of amino acid.
• Bacterial Genome or Prokaryotic Genome
• Bacterial chromosome: Double stranded DNA that is arranged in a circular form.
• When straightened measures 1000 µm.
• Bacterial DNA is about 4000 kb.
• Located in the cytoplasm in nucleoid region.
2
3. • Plasmids
• They are small, circular, extra chromosomal, double-stranded DNA molecules.
• They are self replicable.
• They can integrate with host chromosome , therefore used as a vector.
• They determine important characteristics like Antibiotic resistance, production of enzymes
& toxins etc.
• Not essential for cell survival.
3
4. • Types of plasmids
1. Conjugative plasmids
• Common in Gram negative bacilli.
• Relatively large plasmids .
2. Non conjugative plasmids
• Common in Gram positive.
• Usually small .
• Transposons
• Transposons are genetic elements with DNA sequences of several kbp (4-25kb).
• They can migrate from one genetic locus to another.
• Carry additional genes, Abiotic R ,toxin.
• They are not self replicating.
• Simplest transposons contains insertion sequences.
• Complex transposons carry other genes.
4
5. • Bacterial Variation
• There are 2 types of Bacterial variations:
1. Phenotypic variation
2. Genotypic variation
5
6. 1. Phenotypic Variation
• ‘Display’ expression.
• Reversible.
• Influenced by environment.
• Flagella synthesis in S. typhi.
• Enzyme synthesis by E.coli.
• β-galactosidase enzyme essential for lactose fermentation.
• Operon concept – Jacob & Monad .
2. Genotypic Variation
• Genotypic variation is done by
• Mutation.
• Gene transfer.
6
7. • Mutation
• Random, undirected, heritable variation in nucleotide sequence.
• Addition, deletion or substitution.
• Point mutations, multiple mutations.
• Any gene involved.
• Types of Mutation
• Spontaneous Mutation
• Error in DNA replication.
• Induced Mutation
• Mutagens, UV rays, ionising radiations, alkylating agents.
• Can be harmful, lethal, helpful, silent.
• Conditional lethal mutant
• ‘ts’ mutants - Vaccine production.
• Drug resistance.
7
8. • Gene transfer
1) Transformation
2) Transduction
3) Conjugation
1) Transformation
• Direct uptake of free DNA by a recipient cell.
• First eg. of genetic exchange in bacteria.
• It is discovered by Griffith in 1928.
• Bacillus, Neisseria, Haemophilus do transformation.
• For transformation to take place, the recipient bacteria must be in a state of competence.
• Competence develops towards the end of log phase.
• High salt, CaCl2, temp. – induce competence.
• 10-50 genes can be taken up.
8
9. 2) Transduction
• It is phage-mediated.
• In transduction, genetic material of donor bacterial cell is enclosed in a bacterial
virus (phage) and transferred to the recipient cell.
• Two forms of transduction:
• A) Generalized :Any piece of the bacterial genome can be transferred.
• B) Specialized: only specific pieces of the chromosome can be transferred.
9
10. • Conjugation
• In conjugation, a suitable donor cell comes near a recipient cell, establishes direct cell-to-
cell contact and transfers genetic material.
• Plasmids are most frequently transferred by conjugation.
• Mediated by a fertility or F factor which is carried on a plasmid.
• Sex pilus is responsible for the attachment of donor (F+) cell and recipient cell (F-) in
bacterial conjugation (mating) process.
• Example: Transfer of antibiotic resistance can occur by conjugation. 10
11. • Applied Bacterial Genetics
• Genetic Engineering : In vitro synthesis of new genetic material.
• GENE Probe : Labelled single stranded Pieces of DNA.
• Blotting techniques : For identifying DNA / RNA/ proteins.
• Polymerase Chain Reaction – PC : Rapid, easy, sensitive method of in-vitro enzymatic
amplification of DNA.
_______ 11
