Genetic recombination allows for the exchange of genes between DNA molecules, contributing to genetic diversity in populations. In bacteria, recombination occurs horizontally between cells through three main mechanisms: transformation, conjugation, and transduction. Transformation involves the uptake of naked DNA from the environment. Conjugation requires direct cell-to-cell contact and plasmid transfer. Transduction uses bacteriophages to move bacterial DNA between cells. These mechanisms spread beneficial combinations of genes and increase adaptation.

1. RECOMBINATION
POOJA CHAWAN.S
2nd M.Sc

2. RECOMBINATION
• Genetic recombination refers to the exchange of genes between
two DNA molecules to form new combinations of genes on a
chromosome.
• Like mutation, genetic recombination contributes to a populations
genetic diversity, which is the source of variation in evolution.
• In highly evolved organisms such as present-day microbes,
recombination is more likely than mutation to be beneficial because
recombination will less likely destroy a gene's function and may bring
together combinations of genes that enable the organism to carry
out a valuable new function.

3. BACTERIAL RECOMBINATION
• Vertical gene transfer – From parents to offspring.
• Horizontal gene transfer – From one microbe to another.
• Part of total DNA from Donor cell integrated into
Recipient cell.
• Remaining amount of DNA from donor cell degraded.
• Recipient cell with DNA from donor is called
Recombinant.
• 1% of population might undergo Recombination.

4. BACTERIAL RECOMBINATION
• Transformation.
• Conjugation.
• Transduction.

5. TRANSFORMATION
• Transfer of naked DNA from donor to recipient cell.
• Transformation experiment by Griffith showed that
DNA is the genetic material and can be transferred
between host and recipient DNA.
• E.coli cannot undergo transformation naturally, hence
it is made competent in the lab.
• The process is called “Artificial Transformation‟.

7. TRANSFORMATION
• Bacterial transformation done without mice.
• Broth containing non-encapsulated living bacteria, to
which dead encapsulated bacteria were added.
• After incubation, encapsulated living virulent bacteria
were found.
• This proves that non-encapsulated bacteria received
genes from dead encapsulated ones and got genes for
forming a capsule.

8. TRANSFORMATION
• The material responsible for transmission of this
character was not known.
• In 1944, Oswald T Avery, Colin M Macleod, Maclyn Mc
carty proved that DNA is the genetic material.

10. TRANSFORMATION
• After death, cell lysis leads to release of DNA
from bacteria.
• Other bacteria take up DNA and integrate into their
chromosomes by recombination.
• recA protein binds to donor and cells DNA and
causes exchange of strands.
• Recipient cell with this combination of genes will
now become a hybrid or recombinant.
• All its daughter cells will be recombinant.

11. TRANSFORMATION
• Bacillus, Haemophilus, Streptococcus,
Staphylococcus, Neisseria etc. undergo transformation
in nature.
• Transformation works best when both donor and
recipient are closely related.
• A small portion of DNA is transferred, which is still
large to cross the cell wall and membrane in the
recipient cell.

12. TRANSFORMATION
• Physiological ability to take up DNA is called
“Competence‟ and such cells are Competent cells.
• E.coli cannot undergo transformation naturally,
hence made competent in the lab.
• This procedure is comparatively easy and simple.
• Involves Calcium chloride or Electroporation.

13. TRANSFORMATION BY CALCIUM
CHLORIDE
• Mechanism is unclear.
• Cells are incubated in a solution containing divalent
cations like calcium in cold condition and a rapid heat
shock is given.
• Surface of E.coli is negatively charged
(Phospholipids, Lipopolysaccharides) as well as DNAis
negatively charged.

14. TRANSFORMATION BY CALCIUM
CHLORIDE
• The divalent cation shields the negative charges
and hence DNA adheres to cell surface.
• Divalent cations might also weaken cell surface
making it more permeable to DNA.
• Heat shock creates thermal imbalance within the
cell.
• DNA enters the cell either by pores on the surface
or damaged cell wall.

15. TRANSFORMATION BY
ELECTROPORATION
• Electric shock is given to the cells which creates
holes in the pores of the membrane.
• DNA enters through the pores.
• After the shock, pores are closed rapidly by repair
mechanisms of cell membrane

16. CONJUGATION
• Needs extra chromosomal elements called Plasmids.
• Plasmids replicate independently of chromosome.
• They carry non-essential genes for growth during normal
conditions.
• They give advantage for cells during stress.
• Ex:Antibiotic resistance genes.
• Plasmids can be transferred from one cell to another (
Conjugative or transferable plasmids).

17. CONJUGATION VS
TRANSFORMATION
• Conjugations needs direct cell to cell contact.
• Conjugating cells must be of opposite mating type.
• Donor cells carry plasmids, recipient cells don‟t.
• Gram negative bacteria produce sex pili which contacts
both cells directly.
• Gram positive bacteria produce sticky surface molecules
that bring two cells in contact.

18. CONJUGATION
• Single strand is transferred from donor to recipient.
• In the recipient the SS plasmid is replicated.
• In E.coli, Fertility (F) Factor was the first plasmid
observed to be transferred.
• Donor cells with F factor are F+ cells, recipients
without F factor are F- cells.
• Donor cells transfer F factor to recipient cell, hence
recipient cells become F+ cells.

19. CONJUGATION
• In donor cells, F factor may integrate into the host
chromosome becoming hfr (High Frequency of
Recombination).
• Thus F+ cells become hfr cells.
• Conjugation between hfr and F- cells results in
replication of the chromosome with F factor.
• A single parental strand is transferred from hfr cell
to the F- cells.

20. CONJUGATION
• Complete transfer of the chromosome does not
take place.
• Only a small piece of F factor leads the
chromosomal genes into F+ cells.
• The small strand containing chromosomal genes
recombines with the DNA of F- cells.
• Thus F- cells receive only a part of chromosomal
genes and hence do not get converted to F+ cells.

22. TRANSUDUCTION
Transduction is the process of moving bacterial DNA from
one cell to another using a bacteriophage.
• Bacteriophage or just “phage” are bacterial viruses.
• They consist of a small piece of DNA inside a protein
coat.
• The protein coat binds to the bacterial surface, then
injects the phage DNA.
• The phage DNA then takes over the cells machinery and
replicates many virus particles.

24. TRANSUDUCTION
 Phage attaches to the cell and injects its DNA.
 Phage DNA replicates, and is transcribed into
RNA, then translated into new phage proteins.
 New phage particles are assembled.
 Cell is lysed, releasing about 200 new phage
particles.
Total time = about 15 minutes.

26. GENERALIZIED TRANSDUCTION
• Some phages, such as phage P1, break up the bacterial
chromosome into small pieces, and then package it into
some phage particles instead of their own DNA.
• These chromosomal pieces are quite small.
• A phage containing E. coli DNA can infect a fresh host,
because the binding to the cell surface and injection of
DNA is caused by the phage proteins.

27. GENERALIZED
TRANSDUCTION
• After infection by such a phage, the cell contains
an exogenote (linear DNA injected by the phage)
and an endogenote (circular DNA that is the hosts
chromosome).
• A double crossover event puts the exogenotes
genes onto the chromosome, allowing them to be
propagated.

28. SPECIALIZED TRANSDUCTION
• Some phages can transfer only particular genes to
other bacteria.
• Phage lambda (λ) has this property. To understand
specialized transduction, we need to examine the
phage lambda life cycle.
• lambda has 2 distinct phases of its life cycle. The
“lytic” phase : the phage infects the cell, makes more
copies of itself, then lyses the cell to release the new
phage.

29. SPECIALIZED TRANSDUCTION
• The “lysogenic” phase of the lambda life cycle starts the same
way: the lambda phage binds to the bacterial cell and injects its
DNA.
• Once inside the cell, the lambda DNAcircularizes, then
incorporates into the bacterial chromosome by a crossover.
• Once incorporated into the chromosome, the lambda DNA
becomes quiescent: its genes are not expressed and it remains a
passive element on the chromosome, being replicated along
with the rest of the chromosome.
• The lambda DNA in this condition is called the “prophage”.

30. SPECIALIZED TRANSDUCTION
• After many generations of the cell, conditions might
get harsh. For lambda, bad conditions are signaled
when DNA damage occurs.
• When the lambda prophage receives the DNA
damage signal, it loops out and has a crossover,
removing itself from the chromosome. Then the
lambda genes become active and it goes into the lytic
phase, reproducing itself, then lysing the cell.