Bacterial Genetics (Ch18)

Bacteria– One of the simplest genetic model systems to study the

mechanisms of molecular genetics

Escherichia coli (E. coli)– Intestinal flora – “lab rat” of molecular biology

Genetic diversity caused byRapid reproductionMutationRecombination

Replication of bacterial genome

• One circular DNA genome– Single origin of replication (Ori)– Bidirectional DNA replication

• May have plasmids– smaller circular DNA molecules

• Autonomously replicated (contain ori)•

• Bacteria divide by binary fission• asexual reproduction• Progeny are genetically identical to parent (clones)

LE 18-14

Origin ofreplication

Replication fork

Termination of replication

Bacterial DNAreplication

Both genome

& plasmid replicate

in this manner

Bacterial DNA Mutation

Caused spontaneously (mistakes in DNA synthesis)– Physicochemical forces (UV, X rays,

chemical mutagens, etc.)

•Since reproduction is quick e.g.doubling time=20 min

New mutations spread quickly

Calculation of incidence of mutations

If doubling time= 20 min, then 23cells/hr

Over 12 hr, 236 cells (~1010)produced from a single cell

If spontaneous mutation rate = 1 x 10-7 / gene,

then in 12 hr (day) (1010) (10-7)= 103 mutations/gene/day

If bacteria have ~4000 genesthen (4x103genes)(103)= 4 x 109 mutations/day

Take home message: bacterial mutations rare per gene

But due to rapid cell division, become frequent

Major contribution to genetic diversity and ability to adapt

Another source of bacterial genetic diversity:

Genetic Recombination

• Three processes bring bacterial DNA from different individuals together:– Transduction– Transformation– Conjugation

Transduction

• Bacteriophages (bacterial viruses) transfer bacterial genes from one host cell to another

LE 18-16

A+

Phage DNA

A+

Donorcell

B+

A+

B+

Crossingover

A+

B–

Recipientcell

A+ B–

Recombinant cell

A-

Transformation

• Alteration of a bacterial cell’s genotype and phenotype– by the uptake of naked, foreign DNA from the

surrounding environment

•For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells

•Uptake of ampicillin resistant/ GFP-carrying plasmid (done in lab) (pGLO)

Conjugation

• Direct transfer of DNA between live bacterial cells that are temporarily joined

• Transfer one-way:

– “Male” donates DNA– “Female” receives DNA

• “Maleness,”– Contains F (fertility) genes on plasmid or in genome

– Encode sex pilus• Forms passage way for DNA from donor to recipient

LE 18-17

Sex pilus 5 µm

• Donor cells containing the F plasmid: F+

• Recipient cells: F-

• Cells with F factor integrated into genome: Hfr cell

(high frequency of recombination)

• Hfr cells– Transfer some genomic DNA to recipient cell

LE 18-18_4F plasmid Bacterial chromosome

F+cellMatingbridge

F+ cell

F+ cellBacterial chromosome

F– cell

Conjugation and transfer of F plasmid from and F+ donor to an F– recipient

F+ cell Hfr cell

F factor

Hfr cell

F– cell

Temporarypartialdiploid

Recombinant F–

bacterium

Conjugation and transfer of part of the bacterial chromosome from anHfr donor to an F– recipient, resulting in recombiination

Formation of Hfr (high frequency of recombination) cell

Questions???