Bacterial genetics

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

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

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

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

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

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

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

Regulation of gene expression Bacteria adopt to their environment by controlling gene expression Bacteria adopt to their environment by controlling gene expression Show a remarkable ability to adopt to changes in their environment 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 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 Most common way of altering gene expression is to change the amount of mRNA transcription Expression of many virulence determinants are highly regulated Expression of many virulence determinants are highly regulated

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

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

12 Type of mutations at the level of DNA Sequence

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

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

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

Transduction Transfer of genetic material via bacteriophage Transfer of genetic material via bacteriophage

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

Conjugation