Bacteria such as E. coli are useful models for studying genetics due to their simple genetic systems and rapid reproduction. Their DNA is organized into a single circular chromosome as well as optional extrachromosomal elements like plasmids. Bacterial genomes differ from eukaryotes in that they lack introns. Genetic diversity in bacteria arises from high mutation rates due to rapid growth and horizontal gene transfer mechanisms like transduction, transformation, and conjugation. Transduction involves the transfer of bacterial genes by bacteriophages. Transformation is the uptake of naked DNA from the environment, while conjugation is direct DNA transfer between joined bacterial cells mediated by plasmids like the F factor.

1. Bacterial Genetics
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. Bacterial Chromosome
• Contains a Double stranded molecules of
DNA arranged in circular form.
• Length 1,ooo microns.
• Bacterial DNA contains about 4million
base pairs
• Humans have about 3 billion base
pairs.

3. How bacterial Genome differs
from Higher forms of Life
• Several stretches of DNA don't appear to
function as codons, occurs between the
coding sequences of Gene. called as
INTRONS.
• Coded are called as EXONS
• In transcription introns are excised when form
RNA before translated by ribosomal proteins.

4. Extra chromosomal
Genetic Elements
• Bacteria posses Extra chromosomal genetic
elements
• Not Essential for survival of Bacteria
• But makes the Bacteria Resistant to
antibiotics, and makes them survive
• Able to produce toxins

5. 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)

6. Origin of
replication
Replication fork
Termination
of replication
Bacterial DNA
replication
Both genome
& plasmid replicate
in this manner

7. 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

8. 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

9. bacterial mutations rare per gene
But due to rapid cell division, become frequent
Major contribution to genetic diversity and
ability to adapt

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

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

12. Bacteriophages
• Are viruses that
parasitize bacteria
and consists of
Nucleic acid core and
a protein coat
• A phage particle may
have at its core
besides its own
nucleic acid and a
segment of the Host
DNA

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

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Lytic Cycle

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Lysogenic Cycle
• All phage species can undergo a lytic
cycle
• Phages capable of only the lytic cycle
are called virulent
• The alternative to the lytic cycle is
called the lysogenic cycle: no
progeny particles are produced, the
infected bacterium survives, and a
phage DNA is transmitted to each
bacterial progeny cell when the cell
divides
• Those phages that are also capable
of the lysogenic cycle are called
temperate

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General Transduction
• A bacterial virus, or
bacteriophage, transfers the
DNA from one bacterial cell to
another
• During a LYTIC infection, a
transducing phage, such as
P1 infecting E. coli,
accidentally packages a piece
of the bacterial chromosome
into a virus particle instead of
its own viral DNA.
• The phage carrying the
bacterial DNA then delivers it
to the recipient cell when it
tried to infect again.
• The injected bacterial DNA
may then be inserted into
recipient chromosome by
homologous recombination

17. 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)

19. Variants of Streptococcus pneumoniae
• S = Virulent
• coated with a
polysaccharide
which makes it
infective, smooth
(S) appearance
• R = Avirulent
• lacking capsules
rough (R) colonies
• harmless

20. Griffith’s Mysterious
Transformation Experiment

21. Frederick Griffiths studied the R & S strains by injecting
them into mice
• S injected into mice -> pneumonia -> death
• R injected into mice -> harmless
• Also, boiled S injected into mice ->
harmless (bacteria killed by boiling)

22. The Griffiths did a strange experiment and got a
strange result:
• Boiled S + live R injected into mice ->
pneumonia -> death
• This was not expected because boiled S and
live R were harmless by themselves
• Took blood samples and found live S in the
dead mice
• Concluded that some factor, a "transforming
principle", from the dead S had converted
some R bacteria into S bacteria (a genetic
change)

23. Some bacteria have
specialized
membrane proteins to
bring DNA into their
cells
Ca stimulates uptake
of DNA into bacteria

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

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

26. LE 18-17
Sex pilus 5 µm

27. • 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

28. F plasmid Bacterial 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

30. Hfr Conjugation
• When it exists as a free plasmid, the
F plasmid can only transfer itself.
This isn’t all that useful for genetics.
• However, sometimes the F plasmid
can become incorporated into the
bacterial chromosome, by a crossover
between the F plasmid and the
chromosome. The resulting bacterial
cell is called an “Hfr”, which stands for
“High frequency of recombination”.
• Hfr bacteria conjugate just like F+ do,
but they drag a copy of the entire
chromosome into the F- cell.

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Hfr
• F factor can integrate into chromosome
via genetic exchange between IS
elements present in F and homologous
copy located anywhere in bacterial
chromosome
• Cells with the F plasmid integrated into
the bacterial chromosome are known
as Hfr cells
• When an Hfr cell undergoes
conjugation, the process of transfer of
the F factor is initiated in the same
manner as in an F+ cell
• However, because the F factor is part
of the bacterial chromosome, transfer
from an Hfr cell also includes DNA from
the chromosome
• Hfr = high frequency of recombination

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Hfr and Conjugation
• Transfer begins within an
integrated F factor and
proceeds in one direction
• A part of F is the first DNA
transferred, chromosomal
genes are transferred next,
and the remaining part of F is
the last
• The conjugating cells usually
break apart long before the
entire bacterial chromosome
is transferred, and the final
segment of F is almost never
transferred
The recipient cell
remains F-

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