Which Three Processes Are Methods of Genetic Recombination

Although mutual, genetic recombination is a highly circuitous process. It involves the alignment of two
homologous DNA
strands (the requirement for homology suggests that this occurs through
complementary
base-pairing, simply this has non been definitively shown), precise breakage of each strand, exchange betwixt the strands, and sealing of the resulting recombined molecules. This process occurs with a high degree of accuracy at high
frequency
in both eukaryotic and prokaryotic cells.

The basic steps of recombination can occur in two pathways, according to whether the initial intermission is single or double stranded. In the single-stranded
model, following the alignment of homologous chromosomes, a break is introduced into one DNA strand on each chromosome, leaving ii complimentary ends. Each end then crosses over and invades the other chromosome, forming a structure called a
Holliday junction
(Figure 2). The next step, chosen
branch
migration, takes identify equally the junction travels downward the DNA. The junction is then resolved either horizontally, which produces no recombination, or vertically, which results in an exchange of Dna.

In the alternating pathway initiated past double-stranded breaks, the ends at the breakpoints are converted into unmarried strands by the addition of 3′ tails. These ends can and so perform strand invasion, producing two Holliday junctions. From that signal forward, resolution proceeds equally in the unmarried-stranded model (Figure 3). (Notation that a 3rd model of recombination, synthesis-dependent strand annealing [SDSA], has besides been proposed to business relationship for the lack of crossover typical of recombination in mitotic cells and observed in some meiotic cells to a lesser degree.)

No thing which pathway is used, a number of enzymes are required to consummate the steps of recombination. The genes that lawmaking for these enzymes were start identified in
Eastward. coli
past the isolation of
mutant
cells that were deficient in recombination. This research revealed that the
recA
gene
encodes a
protein
necessary for strand invasion. Meanwhile, the
recB,
recC, and
recD
genes code for three polypeptides that join together to course a protein complex known as RecBCD; this complex has the capacity to unwind double-stranded DNA and cleave strands. Two other genes,
ruvA
and
ruvB, encode enzymes that catalyze
branch migration, while Holliday structures are resolved past the protein
resolvase, which is production of the
ruvC
gene. Several enzymes involved in DNA replication, such as ligase and
DNA polymerase, likewise contribute to recombination (Clark, 1973).

In eukaryotes, recombination has been perhaps most thoroughly studied in the budding
yeast
Saccharomyces
cerevisiae. Many of the enzymes identified in this yeast have as well been found in other organisms, including mammalian cells. Such studies reveal that the
Rad
genes (named for the fact that their activity was institute to be sensitive to
radiation) play a key role in eukaryotic recombination. In item, the
Rad51
gene, which is homologous to
recA, encodes a protein (called Rad51) that has recombinase activity. This gene is highly conserved, simply the accessory proteins that assist Rad51 announced to vary among organisms. For example, the Rad52
poly peptide
is found in both yeast and humans, simply it is missing in

Drosophila melanogaster

and
C. elegans.

In eukaryotic cells, single-stranded Deoxyribonucleic acid (ssDNA) becomes chop-chop coated with the protein RPA (replication protein A). RPA has a higher affinity for ssDNA than Rad51, and information technology therefore can inhibit recombination by blocking Rad51’s access to the unmarried strand needed for invasion. In yeast, however, bounden of Rad51 to ssDNA is enhanced by the proteins Rad52 and the complex Rad55-Rad57. Once access has been gained, Rad51 polymerizes on the DNA strand to form what is chosen a presynaptic filament, which is a right-handed helical filament containing six Rad51 molecules and 18 nucleotides per helical repeat. The search for Deoxyribonucleic acid homology and formation of the junction between homologous regions is then carried out within the catalytic eye of the filament.

In addition to proteins that assist Rad51 activity, there are besides some proteins that inhibit it. In yeast, for instance, the
helicase
Srs2 dismantles the Rad51-ssDNA complex, while the proteins Sgs1 and BLM inhibit the complex. It is thought that these proteins play a part in preventing recombination during DNA replication when it is non needed.

In humans, the
tumor suppressor
genes
BRCA1
and
BRCA2
besides play a office in regulating recombination. Individuals who are
heterozygous
for
BRCA2
are subject to increased
run a risk
for chest and ovarian
cancer; loss of both alleles causes Fanconi’s anemia, a genetic
disease
characterized past predisposition to cancer, among other defects.
BRCA2
appears to promote
Rad51
binding to ssDNA (Li & Heyer, 2008; Modesti & Kanaar, 2001).

As previously described, the enzymes and mechanisms that acquit out the process of
homologous recombination
are adequately well delineated. Not then well understood is the important question of how homologous sequences come to be in proximity so that recombination can proceed. In their 2008 review, Barzel and Kupiec describe two alternate hypotheses, one of which they call the
null model. This model proposes that homologues find one another through a passive process of diffusion, in which the DNA sequence at the cleaved finish of a strand is sequentially compared to all of the other potential end sequences in the
genome. In club for improvidence to account for the rapid repair of double-stranded breaks observed in yeast, however, Barzel and Kupiec summate that each homology search would accept to proceed at a speed 40 times faster than the rate at which DNA polymerase adds a single
nucleotide
to a replicating Dna chain, which seems unlikely (Barzel & Kupiec, 2008).

An alternate hypothesis proposes that homologous chromosomes reside in pairs constitutively. Acting against this hypothesis is the finding that in induced recombination experiments, the cleaved ends of strands recombine with what are chosen ectopic homologues (areas of fortuitous sequence identity) as frequently as they recombine with their true homologous chromosomes. Furthermore, although homologous pairing has been observed in somatic cells of some organisms (e.g.,
Drosophila,
Neurospora), it is non widely seen in the cells of other organisms, including mammals. As Barzel and Kupiec (2008) betoken out, the absenteeism of general homologous pairing does not necessarily mean random assortment. Instead, discrete sections of chromosomes may be required for homology. The employ of subdomains for homology searches would reduce the time it takes to find a homologous partner. Despite such theories, the exact mechanism responsible for locating and lining upward homologous segments remains to be adamant.