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• Two DNA molecules exchange genetic information, resulting in
the production of a new combination of alleles.
• This recombination process creates genetic diversity at the level
of genes that reflects differences in the DNA sequences of
• 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.
• Recombination can occur both during mitosis and meiosis
• During meiosis in eukaryotes, genetic recombination involves
the pairing of homologous chromosomes. This may be
followed by information exchange between the
• Recombination may also occur during mitosis in eukaryotes
where it ordinarily involves the two sister chromosomes
formed after chromosomal replication.
Cross-overs during meiosis I
Zygotene: Homologous chromosomes,
each with 2 sister chromatids, pair to
form bivalents (line=duplex DNA)
Pachytene: Cross-overs between
Diplotene: homologous chromosomes
separate partially but are held together at
Anaphase I: Cross-overs resolve to
allow homologous chromosomes to
separate into separate cells
• It is a physical phenomenon where exchange of
sequence occur with no net gain or loss of nucleotides
• It is based on sequence complementarity.
• Occurs between sequences that are nearly identical
(e.g., during meiosis)
• Homologous recombination is extensively studied in
• At least 25 proteins are involved in recombination in
• Illegitimate or nonhomologous recombination: occurs
in regions where no large scale sequence similarity is
• Site-specific recombination: occurs between particular
short sequences (about 12 to 24 bp) present on
otherwise dissimilar parental molecules. Good examples
are integration of some bacteriophage, such as λ, into a
bacterial chromosome and the rearrangement of
immunoglobulin genes in vertebrate animals. Integration
of bacterial, viral or plasmid DNA takes place.
• Replicative recombination: which generates a new
copy of a segment of DNA. Many transposable
elements use the process of replicative recombination
• RECIPROCAL RECOMBINATION: Equal exchange of
• The process resulting in new DNA molecules that
carry genetic information derived from both parental
• The number of alleles of each gene remains the
same in this products of recombination, only their
arrangement has changed.
• GENE CONVERSION / NON-RECIPROCAL
RECOMBINATION: one way transfer of genetic
information, resulting in an allele of a gene on
one chromosome being changed to the allele on
the homologous chromosome.
• The number of types of alleles has changed in the
products of this recombination.
• transfer of genetic material from a ‘donor’
sequence to a highly homologous ‘acceptor’
THE CLASSICAL ANALYSIS OF
• In each chromosome the genes are arranged in
linear series, and corresponding groups of genes
are exchanged in crossing over.
• Exchanges are complementary and involve the
physical exchange of material.
• Each exchange event involves only two
chromatids, one from each chromosome.
• Meiotic recombination does not occur between
sister chromatids; or if it does, it does not
interfere with recombination between homologs.
The Holliday Model
• In 1964, Robin Holliday proposed a model that
accounted for heteroduplex formation and gene
conversion during recombination.
• Single strand invasion:
• Endonuclease nicks at corresponding regions of the
same strands of homologous chromosomes
• Ends generated by the nicks invade the other,
• Ligase seals nicks to form a joint molecule.
• (“Holliday intermediate” or “Chi structure”)
• Branch migration expands heteroduplex region.
Resolution of joint molecules
• Can occur in one of two ways
• The Holliday junction can be nicked in the
same strands that were initially nicked =
“horizontal resolution.” This results in NO
recombination of flanking markers.
• The Holliday junction can be nicked in the
strands that were not initially nicked =
“vertical resolution.” This results in
RECOMBINATION of flanking markers
The steps in the Holliday Model illustrated are
(1) Two homologous chromosomes, each composed of duplex
DNA, are paired with similar sequences adjacent to each other.
(2) An endonuclease nicks at corresponding regions of
homologous strands of the paired duplexes.
(3) The nicked ends dissociate from their
complementary strands and each single strand
invades the other duplex. This occurs in a
reciprocal manner to produce a heteroduplex
region derived from one strand from each
(4) DNA ligase seals the nicks. The result is a
stable joint molecule, in which one strand of
each parental duplex crosses over into the
other duplex. This X-shaped joint is called a
Holliday intermediate or Chi structure.
(5) Branch migration then expands the region
of heteroduplex. The stable joint can move
along the paired duplexes, feeding in more of
each invading strand and extending the region
(6) The recombination intermediate is then
resolved by nicking a strand in each duplex and
Limitation of Holliday Model
•Although the original Holliday model accounted for many important aspects of
recombination (all that were known at the time), some additional information requires
changes to the model.
•For instance, the Holliday model treats both duplexes equally; both are the invader and the
target of the strand invasion. Also, no new DNA synthesis is required in the Holliday model.
•However, subsequent work showed that one of the duplex molecules is the used
preferentially as the donor of genetic information. Hence additional models, such as one
from Meselson and Radding, incorporated new DNA synthesis at the site of the nick to
make and degradation of a strand of the other duplex to generate asymmetry into the two
duplexes, with one the donor the other the recipient of DNA.
•These ideas and others have been incorporated into a new model of recombination
involving double strand breaks in the DNAs
The Double-Strand Break Repair Model
• Given by Jack Szostak and colleagues in 1983.
• New features in this model (contrasting with the
Holliday model) are initiation at double-strand
breaks, nuclease digestion of the aggressor duplex,
new synthesis and gap repair.
DSBs probably most severe form of DNA
damage, can cause loss of genes or even
cell death (apoptosis)
DSBs caused by:
- ionizing radiation
- certain chemicals
- some enzymes (topoisomerases,
- torsional stress
Enzymes involved are
• DNA ligase
• Rec A
• Ruv A
• A Complex Enzyme complex
with endonuclease and helicase
• Essential for 99% of recombination
events occurring at double-
stranded breaks in bacteria.
• Binds double stranded break
• Unwinds and degrades DNA
• 38 kDa protein
• Catalyzes strand exchange, also an ATPase
• Also binds DS DNA, but not as strongly as
• Catalyses in strand transfer
• Eukaryotes have multiple homologs of
• RecA can generate Holliday junction
• By its strand transfer &displacement
• DNA recombination studies used to map
genes on chromosomes. recombination
frequency proportional to distance between
• Making transgenic cells and organisms
• Modern Genetic Analysis- Antony J.F.Griffths,
William.M.Gelbart & Jefrey.H.Miler
• An Introduction to Genetic Analysis-
David.T.Suzuki & Antony.J.F.Griffths
• Genetics- Benjamin Pierce
• Essential Cell Biology- Hopkins & Johnson
• Advanced Genetic Analysis- R.Scott Hawley &