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Recombination
Aum Sri Sai Ram
Amit Kumar Sahoo
I MSc BIOSCIENCES
15151

What Is Recombination?
 Recombination is a process by which pieces of DNA are broken and
recombined to produce new combinations of alleles.
 This recombination process creates genetic diversity at the level of genes
that reflects differences in the DNA sequences of different organisms.
 In eukaryotic cells, which are cells with a nucleus and organelles,
recombination typically occurs during meiosis.
 During the alignment, the arms of the chromosomes can overlap and
temporarily fuse, causing a crossover.
 Crossovers result in recombination and the exchange of genetic material
between the maternal and paternal chromosomes.
 As a result, offspring can have different combinations of genes than their
parents.

Genetic recombination
 Genetic recombination is the production of offspring with
combinations of traits that differ from those found in either parent.
 During meiosis in eukaryotes, genetic recombination involves the
pairing of homologous chromosomes. This may be followed by
information exchange between the chromosomes.
 Recombination may also occur during mitosis in eukaryotes where it
ordinarily involves the two sister chromosomes formed after
chromosomal replication.

Types of recombination
A+ B+ C+
A- B- C-
A+
B+ C+A-
B- C-
Homologous
or general
A B C
D E F
A B
CD E
F
Site-specific
att
att l
att att
l integrase
Replicative
recombination,
transposition
A B C A B C
transposase
l
E. coli

Homologous recombination
 Homologous recombination is a type of genetic recombination
in which nucleotide sequences are exchanged between two
similar or identical molecules of DNA. It is most widely used by
cells to accurately repair harmful breaks that occur on both
strands of DNA, known as double-strand breaks.
 These new combinations of DNA represent genetic variation in
offspring, which in turn enables populations to adapt during
the course of evolution.
 Homologous recombination is also used in horizontal gene
transfer to exchange genetic material between different strains
and species of bacteria and viruses.

Two Types of Homologous
Recombination.
 Double-strand break repair (DSBR)
pathway (sometimes called the double
Holliday junction model) .
 Synthesis-dependent strand annealing
(SDSA) pathway.

Holliday junction - corresponding strands of two aligned
homologous DNA duplexes are nicked and the nicked
strands cross over to pair with the nearly complementary
strands of the homologous duplex after which the nicks are
sealed.

Formation of Holliday Junction
Branch migration-4 strands exchange base
pairing partners

Resolution of Holliday
Junction occurs in 2
ways
1.The cleavage of the strands that did not
cross over exchanges the ends of the
original duplexes to form, after nick
sealing, traditional recombinant DNA
2.The cleavage of strands that
crossed over exchanges a pair of
homologous single-stranded
segments.

X-ray structure of the cross over event.
Electron micrographs
of intermediates in the
homologous
recombination of two
plasmids.

 The complex of Rec B, Rec C and Rec D
proteins recognizes the ends of a double
stranded break, and travels along the DNA
until reaching the closest Chi site.
 Rec D cleaves the backbone of one strand
And dissociates from the complex.
 Rec BC continue to unwind the DNA beyond
the chi site creating a stretch of single
Stranded DNA.

Site-specific recombination
 Site-specific recombination, is a type of genetic recombination in which
DNA strand exchange takes place between segments possessing only a
limited degree of sequence homology.
 Site-specific recombinases (SSRs) perform rearrangements of DNA
segments by recognizing and binding to short DNA sequences (sites).

Replicative recombination
 It is a type of recombination which generates a new copy of a
segment of DNA. Many transposable elements use a process of
replicative recombination to generate a new copy of the
transposable element at a new location.
 It is seen for some transposable elements, shown as red rectangles,
again using a specific enzyme, in this case encoded by the
transposable element.
 Step 1 - Replicative Integration
 Step 2 - Resolution

Mechanism Of recombination

Benefits of recombination
 Greater variety in offspring: Generates new combinations of
alleles
 Negative selection can remove deleterious alleles from a
population without removing the entire chromosome carrying
that allele
 Essential to the physical process of meiosis, and hence sexual
reproduction
 Yeast and Drosophila mutants that block pairing are also defective in
recombination, and vice versa!!!!

Application
 Homologous recombination employs zinc finger nucleases to
boost genomic integration and shows their usefulness in
efficient genome modification.
 Site-specific recombination systems are potent genome
modifiers.
 DNA transposition-based strategies provide means to
generate single-copy insertions.
 DNA Cloning Using In Vitro Site-Specific Recombination
 Used to map genes on chromosomes - recombination
frequency proportional to distance between genes.
 Making transgenic cells and organisms.

References
Molecular Biology of the Cell, 4th edition
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith
Roberts, andPeter Walter.
New York: Garland Science; 2002.ISBN-10: 0-8153-3218-1ISBN-
10: 0-8153-4072-9
Lodish, H. (2008). Molecular Cell Biology. W. H. Freeman
The Mechanism of Conservative Site-Specific Recombination
Annual Review of Genetics
Vol. 22: 77-105 (Volume publication date December 1988)
DOI: 10.1146/annurev.ge.22.120188.000453
N L Craig