Lecture20BactGen decryption for MBBS students.ppt

bac genetics

1. 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 by
Rapid reproduction
Mutation
Recombination

2. 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)

3. LE 18-14
Origin of
replication
Replication fork
Termination
of replication
Bacterial DNA
replication
Both genome
& plasmid replicate
in this manner

4. 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

5. Calculation of incidence of mutations
If doubling time= 20 min, then 2
3
cells/hr
Over 12 hr, 2
36
cells (~10
10
)produced from a single cell
If spontaneous mutation rate = 1 x 10
-7
/ gene,
then in 12 hr (day) (10
10
) (10
-7
)= 10
3
mutations/gene/day
If bacteria have ~4000 genes
then (4x10
3
genes)(10
3
)= 4 x 10
9
mutations/day

6. 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

7. Another source of bacterial genetic diversity:
Genetic Recombination
•Three processes bring bacterial DNA
from different individuals together:
–Transduction
–Transformation
–Conjugation

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

9. LE 18-16
A
+
Phage DNA
A
+
Donor
cell
B
+
A
+
B
+
Crossing
over
A
+
B
–
Recipient
cell
A
+
B
–
Recombinant cell
A-

10. 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)

11. Conjugation
•Direct transfer of DNA between live bacterial
cells that are temporarily joined
•Transfer one-way:
–“Male” donates DNA
–“Female” receives DNA

12. •“Maleness,”
–Contains F (fertility) genes on plasmid or in genome
–Encode sex pilus
•Forms passage way for DNA from donor to recipient

13. LE 18-17
Sex pilus 5 µm

14. •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

15. LE 18-18_4
F plasmidBacterial chromosome
F+cell
Mating
bridge
F+ cell
F+ cell
Bacterial
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
Temporary
partial
diploid
Recombinant F
–
bacterium
Conjugation and transfer of part of the bacterial chromosome from an
Hfr donor to an F
–
recipient, resulting in recombiination
Formation of Hfr (high frequency of recombination) cell

16. Questions???