Genetic recombination

1. Genetic recombination
Harish. R
Assistant Professor
Department of Biosciences,
Mar Thoma College, Thiruvalla
hareesh@marthomacollege.org

2. 1. General or homologous recombination : occurs
between DNA molecules of very similar sequence,
such as homologous chromosomes in diploid
organisms.
Is an integral part of the complex process
of meiosis in sexually reproducing organisms. Results
in crossing over.
In Bacteria: recombination during F-factor mediated
conjugal transfer of parts of chromosomes.
In Bacteriophages Recombination between two
phage during a mixed infection of bacteria
 Breaking and rejoining of two parental DNA molecules
to produce new DNA molecules
Genetic recombination

3. 2. Illegitimate or nonhomologous recombination
occurs in regions where no large-scale sequence
similarity is apparent, (i.e due to translocations
between different chromosomes or deletions
that remove several genes along a chromosome).
3. Site-specific recombination occurs between
particular short sequences (about 12 to 24 bp)
present on otherwise dissimilar parental
molecules. Eg. integration of bacteriophages.

4. 3. Replicative recombination: generates a new
copy of a segment of DNA. New DNA
molecules carry genetic information from both
parental molecules.
Ex: transposable elements, Mu Phages.

6. Mechanism of Recombination
• Endonuclease cuts DNA in the middle of a strand,
and nicks one strand of the donor DNA.
• This nicked strand is separated from the other
strand by proteins with helicase activity
• The resulting single-stranded segment binds to
SSbps and then RecA. This results in a complex
that promotes base pairing with the
complementary sequence in the recipient DNA
molecule.
• This base pairing in turn displaces the other
strand of the recipient DNA molecule (strand
invasion).
• Heteroduplex regions, with each strand
originated from a different chromosomes.
• Resolvase (RuvC protein): specifically cleave
Holliday intermediates have also been isolated
from bacteria and yeast.
• COMPLEMENTATION

7. Rec A Protein
• A 38 kilodalton protein essential for the repair and maintenance ofDNA.
• Activated during DNA Damage
• The RecA protein binds strongly to ssDNA to form a nucleoprotein
filament.
• Has more than one DNA binding site, and thus can hold a both the
strands together.
• This feature makes it possible to catalyze a DNA synapsis reaction
between a DNA double helix and a complementary region of single
stranded DNA.

8. Watson, J.D, Baker T.A, Bell, S.P, Gann, A., Levine, M and Losik R ( 2004).
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References

9. Dale, J.W and Park S.F (2004). Nucleic Acid Structure and Function,
Molecular Genetics of Bacteria (4th ed). John Wiley & Sons. Ch.1 (1-
33).
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Genetics of Bacteria (4th ed). John Wiley & Sons. Ch.2 (37-62).
Dale, J.W and Park S.F (2004). Gene Transfer, Molecular Genetics of
Bacteria (4th ed). John Wiley & Sons. Ch.6 (165-187)
References