Genetic recombination (aka genetic overhaul) is the production of offspring with a combination of properties different from those found in both parents. In eukaryotes, genetic recombination during meiosis can lead to a series of new genetic information that can be passed from parent to offspring. Most recombination occurs naturally.
During meiosis in eukaryotes, genetic recombination involves a pair of homologous chromosomes. This may be followed by transfer of information between chromosomes. Information transfer may occur without physical exchange (parts of genetic material copied from one chromosome to another, without the donor chromosome being changed) (see SDSA path in Figure); or by splitting and rejoining DNA strands, which form new DNA molecules (see DHJ path in Fig.
Recombination can also occur during mitosis in eukaryotes where it usually involves two twin chromosomes that form after chromosome replication. In this case, a new allele combination is not produced because the brother's chromosome is usually identical. In meiosis and mitosis, recombination occurs between similar (homologous) DNA molecules. In meiosis, unpaired pair of homologous chromosomes are paired together so that recombination occurs between non-sister homologues. In meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in DNA repair.
Genetic recombination and recombinant DNA repair also occur in bacteria and archaea, which use asexual reproduction.
Recombination can be artificially induced in the laboratory (in vitro) setting, producing recombinant DNA for purposes including vaccine development.
V (D) A recombination in organisms with the adaptive immune system is a type of site-specific genetic recombination that helps immune cells rapidly diversify to recognize and adapt to new pathogens.
Video Genetic recombination
Sinapsis
During meiosis, synapses (homologous chromosome pairs) usually precede genetic recombination.
Maps Genetic recombination
Mechanism
Genetic recombination is catalyzed by many different enzymes. Recombination is a key enzyme that catalyzes the transfer step of the strand during recombination. RecA, the main recombinase found at Escherichia coli , is responsible for the repair of double-stranded DNA (DSBs). In yeast and other eukaryotic organisms, there are two recombinants needed to repair the DSB. RAD51 proteins are required for the recombination of mitosis and meiosis, whereas DNA repair proteins, DMC1, specifically for meiotic recombination. In archaea, the ortholog of the bacterial RecA protein is RadA.
- Recombination of bacteria
In Bacteria there are:
- recombination of common bacteria, as well as ineffective transfer of genetic material, expressed as
- unsuccessful or failed transfer which is a bacterial DNA transfer from a donor cell receiver that has regulated incoming DNA as part of the recipient's genetic material. Failed transfers are listed in the following transduction and conjugation. In all cases, the transmitted fragments are attenuated by culture growth.
Chromosomal crossover
In eukaryotes, recombination during meiosis is facilitated by crossover chromosomes. The crosses process leads to offspring that have different gene combinations of the parent gene, and can sometimes produce new chimeric alleles. Gene randomization brought by genetic recombination results in an increase in genetic variation. It also allows the organism to reproduce sexually to avoid ratchet Muller, where the genome of the asexual population accumulates genetic deletion in an irreversible manner.
Chromosomal crossover involves recombination between the couple chromosomes inherited from each parent of a person, commonly occurring during meiosis. During prophase I (pachytene stage), the four available chromatids are in tight formation with each other. While in this formation, homologous sites on two chromatids can be closely paired with each other, and can exchange genetic information.
Because recombination can occur with a small probability at each location along the chromosome, the recombination frequency between two locations depends on the distance that separates them. Therefore, for a gene far enough on the same chromosome, the number of crossovers is high enough to destroy the correlation between alleles.
Tracking the movement of genes resulting from crossovers has proven to be very useful for geneticists. Because the two adjacent genes are less likely to separate from a further apart gene, the geneticist can infer approximately how far two genes are on the chromosome if they know the frequency of the crossover. Geneticists can also use this method to infer the existence of certain genes. The genes that usually remain together during recombination are said to be related. One gene in a connected pair can sometimes be used as a marker to infer the presence of other genes. This is usually used to detect the presence of disease-causing genes.
The recombination frequency between the two loci observed is the value of crossing-over . This is the frequency of crossings between two connected gene loci (markers), and depends on the mutual distance of the observed genetic locus. For fixed sets of genetic and environmental conditions, recombination in certain areas of the relationship structure (chromosome) tends to be constant, and the same goes for the crossing-over value used in the production of the genetic map.
Gene conversion
In gene conversion, the genetic material part is copied from one chromosome to another, without the transformed donor chromosome. Gene conversion occurs at high frequencies on the actual site of the recombination event during meiosis. This is the process by which the DNA sequence is copied from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. Gene conversion is often studied at fungus crosses where 4 products from individual meioses can be easily observed. Genetic conversion events can be distinguished as irregularities in individual meiosis of a normal 2: 2 segregation pattern (eg pattern 3: 1).
Nonhomologous recombination
Recombination may occur between DNA sequences that do not contain sequence homology. This can cause chromosomal translocation, sometimes causing cancer.
In cell B
B cells from the immune system perform a genetic recombination, called immunoglobulin-class transfer. It is a biological mechanism that converts antibodies from one class to another, for example, from an isotype called IgM to an isotype called IgG.
Genetic engineering
In genetic engineering, recombination can also refer to artificial and intentional recombination of different pieces of DNA, often from different organisms, creating so-called recombinant DNA. A prime example of the use of such genetic recombination is gene targeting, which can be used to add, remove or alter organism genes. This technique is important for biomedical researchers because it allows them to study the effects of certain genes. Techniques based on genetic recombination are also applied in protein engineering to develop novel proteins that attract biologics.
Improved Recombination
During mitosis and meiosis, DNA damage caused by various exogenous agents (eg UV rays, X-rays, chemical crosslinking agents) can be improved by improving homologous recombination (HRR). These findings suggest that DNA damage arising from natural processes, such as exposure to reactive oxygen species that are a byproduct of normal metabolism, is also improved by HRR. In humans and rodents, a deficiency in gene products required for HRR during meiosis leads to infertility. In humans, deficiencies in the gene products needed for HRR, such as BRCA1 and BRCA2, increase the risk of cancer (see DNA repair-deficiency disorder).
In bacteria, transformation is a process of gene transfer that usually occurs between individual cells of the same bacterial species. Transformation involves the integration of donor DNA into the receiving chromosome through recombination. This process appears to be an adaptation to repair DNA damage to the recipient's chromosomes by HRR. Transformation can provide benefits for pathogenic bacteria by allowing repair of DNA damage, especially damage occurring in the inflammatory environment, oxidation associated with host infection.
When two or more viruses, each containing deadly genomic damage, infect the same host cell, viral genomes can often pair with each other and undergo HRR to produce a living progeny. This process, referred to as multiplicity reactivation, has been studied in lambda and T4 bacteriophages, as well as some pathogenic viruses. In the case of a pathogen virus, multiplicity reactivation may be an adaptive benefit to the virus as it allows repair of DNA damage caused by exposure to the oxidation environment generated during host infection.
Mayotik Recombination
Molecular model of meiotic recombination has evolved over the years as the relevant evidence accumulated. A major incentive to develop a fundamental understanding of the meiotic recombination mechanism is that it is essential to solve the problem of adaptation of sex, a major unresolved problem in biology. The latest models that reflect current understanding are presented by Anderson and Sekelsky, and are described in the first picture in this article. The image shows that two of the four chromatids present at the beginning of meiosis (profase I) are paired with each other and are able to interact. Recombination, in this model version, is initiated by a double-strand break (or gap) shown in a DNA molecule (chromatid) at the top of the first image in this article. However, other types of DNA damage can also initiate recombination. For example, inter-strand cross relations (caused by exposure to crosslinking agents such as mitomycin C) may be corrected by HRR.
As shown in the first figure, above, two types of recombinant products are generated. Indicated on the right side is a type "crossover" (CO), where the chromosome flanking area is interchangeable, and on the left side, the "non-crossover" type (NCO) where flanking is not interchangable. The CO type of recombination involves the formation between two "Holliday connections" shown at the lower right of the image by two X-shaped structures in each of which there is a single-strand exchange between two participating chromatids. This line is labeled as DHJ (double-holliday junction) lines.
NCO recombinants (illustrated to the left of the drawing) are generated by a process known as "annaaling dependent strand synthesis" (SDSA). NCO/SDSA recombination events appear to be more common than CO/DHJ types. The NCO/SDSA pathway contributes little to the genetic variation, since the chromosome arm flanking the recombination event remains in the parent configuration. Thus, the explanation for a meiotic adaptive function that focuses exclusively on cross-over is insufficient to account for most recombination events.
Achinamy and heterochiasmy
Achiasmy is a phenomenon in which autosomal recombination is completely absent in one sex of the species. Achiasmatic chromosome segregation is well documented in Drosophila melanogaster men. Heterochiasmy is the term used to describe different recombination rates between the sexes of a species. This sexual dimorphic pattern in the recombination rate has been observed in many species. In mammals, females most often have higher recombination rates. The "Haldane-Huxley rule" states that achiasmy usually occurs in heterogametic sex.
See also
- Four-gamete test
- Independent collection
- recombination frequency
- Recombination hotspot
- Location-specific recombbinase technology
- Custom site reshuffling
References
External links
- Homologous recombination: Animations that show some homologous recombination models
- Holliday Genetic Recombination Model
- Genetic recombination at the US National Library of Medicine's Medical Subject Headings (MeSH)
- An animation guide for homologous recombination.
This article incorporates public domain material from the NCBI document "Science Primer".
Source of the article : Wikipedia
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