Which Process Is Necessary To Prevent The Doubling Of Genome Size During Sexual Reproduction?

How does DNA recombination piece of work? It occurs frequently in many different cell types, and it has of import implications for genomic integrity, development, and human affliction.

This diagram shows specific alleles on knobbed or normal chromosomes in maize. A genetic cross between two strains of maize is shown above the associated crossover phenotypes for seed color and endosperm type in the progeny. Chromosomes are depicted as vertical black lines; knobbed chromosomes have a black circle at one end of the vertical line.

DNA recombination involves the substitution of genetic material either betwixt multiple chromosomes or between dissimilar regions of the aforementioned chromosome. This process is mostly mediated by homology; that is, homologous regions of chromosomes line upward in grooming for exchange, and some degree of sequence identity is required. Various cases of nonhomologous recombination do exist, notwithstanding.

One important instance of recombination in diploid eukaryotic organisms is the exchange of genetic data between newly duplicated chromosomes during the process of meiosis. In this instance, the outcome of recombination is to ensure that each gamete includes both maternally and paternally derived genetic data, such that the resulting offspring will inherit genes from all 4 of its grandparents, thereby acquiring a maximum corporeality of genetic diversity. Recombination is also used in Dna repair (particularly in the repair of double-stranded breaks), as well equally during DNA replication to assist in filling gaps and preventing stalling of the replication fork. In these cases, a sister chromatid serves equally the donor of missing fabric via recombination followed past DNA synthesis.

The function of recombination during the inheritance of chromosomes was first demonstrated through experiments with maize. Specifically, in 1931, Barbara McClintock and Harriet Creighton obtained evidence for recombination past physically tracking an unusual knob structure within certain maize chromosomes through multiple genetic crosses. Using a strain of maize in which one member of a chromosome pair exhibited the knob simply its homologue did not, the scientists were able to show that some alleles were physically linked to the knobbed chromosome, while other alleles were tied to the normal chromosome. McClintock and Creighton and then followed these alleles through meiosis, showing that alleles for specific phenotypic traits were physically exchanged between chromosomes. Evidence for this finding came from the fact that alleles first introduced into the cross on a knobbed chromosome later appeared in offspring without the knob; similarly, alleles initially introduced on a knobless chromosome subsequently appeared in progeny with the knob (Effigy 1).

Recombination too occurs in prokaryotic cells, and it has been especially well characterized in Due east. coli. Although bacteria do not undergo meiosis, they practice engage in a type of sexual reproduction chosen conjugation, during which genetic fabric is transferred from ane bacterium to some other and may be recombined in the recipient cell. As in eukaryotes, recombination also plays important roles in Dna repair and replication in prokaryotic organisms.

Models of Recombination

Panel A is an electron micrograph showing a Holliday junction. Panel B shows two diagrams of possible Holliday junction configurations.

A) © 1979 Common cold Bound Harbor Laboratory. Potter, H. & Dressler, D. Dna recombination: in vivo and in vitro studies. Cold Spring Harb. Symp. Quant. Biol. 43, 969–985 (1979). All rights reserved. B) © 2004 Nature Publishing Group. Liu, Y. et al. Happy Hollidays: 40th anniversary of the Holliday junction. Nature Reviews Molecular Cell Biology five, 940 (2004). All rights reserved.

Although mutual, genetic recombination is a highly complex process. It involves the alignment of ii homologous Deoxyribonucleic acid strands (the requirement for homology suggests that this occurs through complementary base-pairing, but this has not been definitively shown), precise breakage of each strand, exchange betwixt the strands, and sealing of the resulting recombined molecules. This process occurs with a loftier degree of accuracy at loftier frequency in both eukaryotic and prokaryotic cells.

The basic steps of recombination tin occur in two pathways, co-ordinate to whether the initial interruption is single or double stranded. In the single-stranded model, following the alignment of homologous chromosomes, a break is introduced into ane Deoxyribonucleic acid strand on each chromosome, leaving two free ends. Each finish then crosses over and invades the other chromosome, forming a structure called a Holliday junction (Figure 2). The next pace, chosen branch migration, takes identify as the junction travels down the Deoxyribonucleic acid. The junction is then resolved either horizontally, which produces no recombination, or vertically, which results in an substitution of DNA.

In the alternating pathway initiated by double-stranded breaks, the ends at the breakpoints are converted into single strands by the addition of 3' tails. These ends can then perform strand invasion, producing two Holliday junctions. From that indicate frontward, resolution proceeds as in the single-stranded model (Effigy iii). (Notation that a third model of recombination, synthesis-dependent strand annealing [SDSA], has besides been proposed to account for the lack of crossover typical of recombination in mitotic cells and observed in some meiotic cells to a lesser degree.)

Recombination Enzymes

© 2006 Nature Publishing Group Sung, P. et al. Machinery of homologous recombination: mediators and helicases take on regulatory functions. Nature Reviews Molecular Cell Biological science 7, 741 (2006). All rights reserved.

No matter which pathway is used, a number of enzymes are required to consummate the steps of recombination. The genes that code for these enzymes were first identified in E. coli by the isolation of mutant cells that were deficient in recombination. This enquiry revealed that the recA factor encodes a poly peptide necessary for strand invasion. Meanwhile, the recB, recC, and recD genes code for three polypeptides that join together to form a protein circuitous known as RecBCD; this circuitous has the capacity to unwind double-stranded Deoxyribonucleic acid and cleave strands. Two other genes, ruvA and ruvB, encode enzymes that catalyze branch migration, while Holliday structures are resolved by the protein resolvase, which is production of the ruvC cistron. Several enzymes involved in Deoxyribonucleic acid replication, such as ligase and Dna polymerase, also contribute to recombination (Clark, 1973).

In eukaryotes, recombination has been perhaps virtually thoroughly studied in the budding yeast Saccharomyces cerevisiae. Many of the enzymes identified in this yeast take also been establish in other organisms, including mammalian cells. Such studies reveal that the Rad genes (named for the fact that their activity was plant to be sensitive to radiation) play a cardinal role in eukaryotic recombination. In particular, the Rad51 factor, which is homologous to recA, encodes a protein (chosen Rad51) that has recombinase action. This gene is highly conserved, just the accompaniment proteins that assist Rad51 appear to vary among organisms. For example, the Rad52 protein is found in both yeast and humans, only it is missing in Drosophila melanogaster and C. elegans.

In eukaryotic cells, single-stranded DNA (ssDNA) becomes rapidly coated with the poly peptide RPA (replication protein A). RPA has a higher analogousness for ssDNA than Rad51, and it therefore can inhibit recombination past blocking Rad51'due south access to the single strand needed for invasion. In yeast, however, binding of Rad51 to ssDNA is enhanced by the proteins Rad52 and the circuitous Rad55-Rad57. Once admission has been gained, Rad51 polymerizes on the DNA strand to form what is called a presynaptic filament, which is a correct-handed helical filament containing six Rad51 molecules and 18 nucleotides per helical repeat. The search for DNA homology and germination of the junction betwixt homologous regions is then carried out within the catalytic center of the filament.

In addition to proteins that assist Rad51 activity, in that location are also 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 role in preventing recombination during Deoxyribonucleic acid replication when it is not needed.

In humans, the tumor suppressor genes BRCA1 and BRCA2 also play a role in regulating recombination. Individuals who are heterozygous for BRCA2 are field of study to increased gamble for breast and ovarian cancer; loss of both alleles causes Fanconi's anemia, a genetic disease characterized by predisposition to cancer, amongst other defects. BRCA2 appears to promote Rad51 binding to ssDNA (Li & Heyer, 2008; Modesti & Kanaar, 2001).

How Are Homologous Sequences Brought Together?

As previously described, the enzymes and mechanisms that carry out the process of homologous recombination are fairly well delineated. Not so 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 draw two alternate hypotheses, 1 of which they call the aught model. This model proposes that homologues find one some other through a passive process of diffusion, in which the DNA sequence at the broken end of a strand is sequentially compared to all of the other potential end sequences in the genome. In society for diffusion to account for the rapid repair of double-stranded breaks observed in yeast, notwithstanding, Barzel and Kupiec calculate that each homology search would have to proceed at a speed twoscore 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. Interim confronting this hypothesis is the finding that in induced recombination experiments, the broken ends of strands recombine with what are called ectopic homologues (areas of fortuitous sequence identity) as oftentimes as they recombine with their truthful homologous chromosomes. Furthermore, although homologous pairing has been observed in somatic cells of some organisms (due east.g., Drosophila, Neurospora), it is not widely seen in the cells of other organisms, including mammals. As Barzel and Kupiec (2008) point out, the absence of general homologous pairing does non necessarily mean random array. Instead, discrete sections of chromosomes may be required for homology. The utilise of subdomains for homology searches would reduce the time it takes to find a homologous partner. Despite such theories, the verbal mechanism responsible for locating and lining up homologous segments remains to exist determined.

References and Recommended Reading

Barzel, A., & Kupiec, M. Finding a lucifer: How exercise homologous sequences assemble for recombination? Nature Reviews Genetics 9, 27–37 (2008) doi:10.1038/nrg2224 (link to article)

Clark, A. J. 1973. Recombination deficient mutants of East . coli and other bacteria. Annual Review of Genetics 7, 67–86 (1973) doi:ten.1146/annurev.ge.07.120173.000435

Li, X., & Heyer, Due west. D. Homologous recombination in DNA repair and DNA damage tolerance. Cell Research eighteen, 99–113 (2008)

Liu, Y., & West, South. C. Happy Hollidays: Fortieth anniversary of the Holliday junction. Nature Reviews Molecular Jail cell Biological science 5, 937–944 (2004) doi:ten.1038/nrm1502 (link to commodity)

Modesti, M., & Kanaar, R. Homologous recombination: From model organism to human disease. Genome Biology 2, 1014.one–1014.five (2001)

Sung, P., & Klein, H. Mechanism of homologous recombination: Mediators and helicases take on regulatory functions. Nature Reviews Molecular Cell Biology 7, 739–750 (2006) doi:10.1038/nrm2008 (link to article)