Bacterial genetics

1. Bacterial genetics

2. Growth and Division • The rate at which bacteria grow and divide depends in large on the nutritional status of the environment • Division is by binary fission DNA replication: Replication of the DNA is a precise process resulting in each daughter cell acquiring an exact copy of the parental genome • Initiation: Begins at a site on the chromosome called origin of replication • unwinding of the DNAs to separate into two strands • Enzymes – called helicase and topoisomerase (e.g. DNA gyrase) are utilized • Elongation • Separated DNA strands each serve as a template for DNA polymerase • incorporation of deoxyribonucleotides to form complementary DNA strand which correctly base pair with the template DNA • The two copies of the total genome produced during replication • comprise one parental strand and one newly synthesized strand of DNA • Termination: specialized termination zone ; specialized binding proteins stall the replication process Formation of Daughter cells • 2 chromosomes segregate, cell wall septum forms between them • Division to give 2 daughter cells

3. Bacterial genetic material DNA: where? • Chromosome • Circular, double stranded DNA • Contains essential genes for replication, metabolism • Single copy per cell • Haploid (one copy of each gene) • Plasmids • Transposons • Bacteriophages

4. Plasmids • Extrachromosomal genetic material • Replicate autonomously • Circular, double stranded DNA • Multiple copies may be present • Can be gained or lost from a bacterial cell • Plasmids contain genes for • replication • mediating their own transfer between bacteria (tra- genes) • variety of additional functions e.g. • resistance to antimicrobials • resistance to toxic metals • bacteriocin production • toxin production

5. Classification of plasmids by function • Fertility-(F-) plasmids • contain only tra-genes. • function is to initiate conjugation • Resistance-(R-)plasmids, • which contain genes for antibiotic resistance • Col-plasmids • contain genes that code for  colicines (bacteriocins) -proteins that can kill other bacteria • Degradative plasmids • genes for the digestion of organic compounds & other unusual substances e.g toluene & salicylic acid • Virulence plasmids • Genes for virulence factors

6. Transposons • “Jumping genes” • Small segments of DNA • jump from one chromosomal site • Move from chromosome into a plasmid • May carry genes for antibiotic resistance • When they land in a gene they may alter its function therefore cause mutation

7. Bacteriophages • Viruses that infect bacterial cell • Consists of a protein coat or head (capsid) which surrounds nucleic acid which may be • Genome is either DNA or RNA – never both

8. Lifecycle of Bacteriophages • 2 pathways • Virulent pathway:- • Takes over the protein synthesis machinery of the cells for viral replication • Formation of new virus particles • Release of new viruses  cell lyses • Lysogenic pathway:- • Occurs in temperate bacteriophages • integrate with bacterial chromosome & replicate within the bacterial chromosome • prophage state • result in expression of new characteristics by the cell • Under adverse conditions, prophages are activated & go into the lytic pathway

9. Regulation of gene expression • Bacteria adopt to their environment by controlling gene expression • Show a remarkable ability to adopt to changes in their environment • Predominantly achieved by controlling gene expression – ensuring when and for what proteins are produced • Most common way of altering gene expression is to change the amount of mRNA transcription • Expression of many virulence determinants are highly regulated

10. Regulation of gene expression • Inducible genes: • expression is turned on by the presence of some substance • Examples • Lactose induces expression of the lac genes • An antibiotic induces the expression of a resistance gene • Repressible genes • expression is turned off by the presence of some substance (co-repressor) e.g. Tryptophan represses the trp genes

11. MUTATION • Any change in structure of the genetic material (sequence of the DNA) • Mutation can occur:- • spontaneously or • by a variety of chemicals (mutagens) • Mutation can be:- • Unstable: frequently revert to original state • Stable: causes some changes in the characteristic of the organism • Lethal: cell unable to tolerate the loss of function & die

12. Type of mutations at the level of DNA Sequence

13. Mutation at the level of the organism • Auxotrophic or Nutritional mutants: • Mutation occurs in a biosynthetic gene • cell will not grow unless missing nutrient is provided – auxotrophs • Temperature sensitive mutant: • Gene expression will only occur at particular temperature

14. Gene transfer and recombination • New genotypes arise when genetic material is transferred from one bacterium to another • Transferred DNA can • Recombine with the genome of the recipient cell • Remain on self replicating plasmid • DNA can be transferred from donor cell to a recipient cell by:- • transformation • transduction • conjugation • transposition

15. Transformation • Uptake of free DNA present in the environment • Short pieces of the DNA • Integrated into recipient chromosome

16. Transduction Transfer of genetic material via bacteriophage

17. Conjugation • Transfer of genetic material via plasmids • encode for the bacterial cell to produce a sex pilli • The donor or male cell makes contact with the recipient cell or female cell • DNA is transferred from one bacterium to another

18. Conjugation