Bacterial Genetics (Ch18)
Genetic diversity caused by
Rapid reproduction
Mutation
Recombination
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
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
Bacterial DNA
replication
Replication fork
Origin of
replication
Both genome
& plasmid replicate
in this manner
Termination
of replication
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 genes
then (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
Phage DNA
A+ B+
A+ B+
Donor
cell
A+
Crossing
over
A+
A- B–
Recipient
cell
A+ B–
Recombinant cell
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_4
F plasmid
Bacterial chromosome
F+cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjugation and transfer of F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
Hfr cell
Formation of Hfr (high frequency of recombination) cell
F– cell
Temporary
Recombinant F–
partial
bacterium
diploid
Conjugation and transfer of part of the bacterial chromosome from an
Hfr donor to an F– recipient, resulting in recombiination
Questions???