Which Method of Genetic Recombination is Illustrated in the Diagram

Production of offspring with combinations of traits that differ from those institute in either parent

Genetic recombination
(also known equally
genetic reshuffling) is the exchange of genetic cloth betwixt dissimilar organisms which leads to production of offspring with combinations of traits that differ from those plant in either parent. In eukaryotes, genetic recombination during meiosis can atomic number 82 to a novel set of genetic information that can exist further passed on from parents to offspring. Most recombination occurs naturally and tin can be classified into two types: (1)
interchromosomal
recombination, occurring through independent assortment of alleles whose loci are on dissimilar just homologous chromosomes (random orientation of pairs of homologous chromosomes in meiosis I); & (2) intrachromosomal
recombination, occurring through crossing over.^[ane]

During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed past data transfer between the chromosomes. The information transfer may occur without physical commutation (a section of genetic material is copied from one chromosome to another, without the donating chromosome being inverse) (see SDSA pathway in Effigy); or by the breaking and rejoining of DNA strands, which forms new molecules of DNA (encounter DHJ pathway in Figure).

Recombination may also occur during mitosis in eukaryotes where it ordinarily involves the 2 sis chromosomes formed after chromosomal replication. In this case, new combinations of alleles are not produced since the sister chromosomes are ordinarily identical. In meiosis and mitosis, recombination occurs between similar molecules of DNA (homologous sequences). In meiosis, not-sis homologous chromosomes pair with each other and so that recombination characteristically occurs betwixt not-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes is a mutual machinery used in Deoxyribonucleic acid repair.

Gene conversion – the procedure during which homologous sequences are made identical also falls under genetic recombination.

Genetic recombination and recombinational DNA repair too occurs in leaner and archaea, which use asexual reproduction.

Recombination can exist artificially induced in laboratory (in vitro) settings, producing recombinant Deoxyribonucleic acid for purposes including vaccine evolution.

V(D)J recombination in organisms with an adaptive allowed organisation is a type of site-specific genetic recombination that helps allowed cells rapidly diversify to recognize and adapt to new pathogens.

Synapsis

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During meiosis, synapsis (the pairing of homologous chromosomes) ordinarily precedes genetic recombination.

Mechanism

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Genetic recombination is catalyzed by many dissimilar enzymes. Recombinases are key enzymes that catalyse the strand transfer step during recombination. RecA, the chief recombinase found in
Escherichia coli, is responsible for the repair of Deoxyribonucleic acid double strand breaks (DSBs). In yeast and other eukaryotic organisms there are two recombinases required for repairing DSBs. The RAD51 protein is required for mitotic and meiotic recombination, whereas the Dna repair protein, DMC1, is specific to meiotic recombination. In the archaea, the ortholog of the bacterial RecA protein is RadA.

Bacterial recombination

In Bacteria there are:

• regular bacterial recombination, as well equally noneffective transfer of genetic material, expressed equally
• unsuccessful transfer or abortive transfer which is whatever bacterial Deoxyribonucleic acid transfer of the donor cell to recipients who have set the incoming DNA as part of the genetic material of the recipient. Abortive transfer was registered in the following transduction and conjugation. In all cases, the transmitted fragment is diluted past the culture growth.^{[2]
  [iii]
  [4]}

Chromosomal crossover

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In eukaryotes, recombination during meiosis is facilitated by chromosomal crossover. The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric alleles. The shuffling of genes brought about by genetic recombination produces increased genetic variation. It likewise allows sexually reproducing organisms to avert Muller’due south ratchet, in which the genomes of an asexual population tend to accumulate more deleterious mutations over fourth dimension than other types of benign or reversing mutations.

Chromosomal crossover involves recombination between the paired chromosomes inherited from each of 1’s parents, generally occurring during meiosis. During prophase I (pachytene stage) the iv available chromatids are in tight formation with one another. While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information.^[5]

Because recombination tin occur with small probability at whatever location along chromosome, the frequency of recombination between 2 locations depends on the altitude separating them. Therefore, for genes sufficiently distant on the aforementioned chromosome, the corporeality of crossover is high enough to destroy the correlation betwixt alleles.

Tracking the movement of genes resulting from crossovers has proven quite useful to geneticists. Considering two genes that are shut together are less likely to become separated than genes that are farther apart, geneticists tin can deduce roughly how far autonomously ii genes are on a chromosome if they know the frequency of the crossovers. Geneticists can also utilise this method to infer the presence of sure genes. Genes that typically stay together during recombination are said to be linked. One cistron in a linked pair can sometimes be used as a marking to deduce the presence of another cistron. This is typically used in club to notice the presence of a disease-causing gene.^[six]

The recombination frequency between ii loci observed is the
crossing-over value. It is the frequency of crossing over betwixt two linked cistron loci (markers), and depends on the common distance of the genetic loci observed. For any fixed set up of genetic and environmental atmospheric condition, recombination in a item region of a linkage structure (chromosome) tends to be constant, and the same is and then true for the crossing-over value which is used in the production of genetic maps.^[ii]
[vii]

Gene conversion

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In gene conversion, a section of genetic material is copied from 1 chromosome to another, without the donating chromosome existence inverse. Gene conversion occurs at high frequency at the bodily site of the recombination upshot during meiosis. It is a process by which a Deoxyribonucleic acid sequence is copied from ane DNA helix (which remains unchanged) to another Dna helix, whose sequence is altered. Factor conversion has often been studied in fungal crosses^[eight]
where the 4 products of individual meioses can exist conveniently observed. Gene conversion events can exist distinguished equally deviations in an individual meiosis from the normal 2:ii segregation blueprint (due east.one thousand. a 3:1 pattern).

Nonhomologous recombination

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Recombination can occur betwixt Deoxyribonucleic acid sequences that contain no sequence homology. This can crusade chromosomal translocations, sometimes leading to cancer.

In B cells

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B cells of the immune system perform genetic recombination, called immunoglobulin class switching. It is a biological machinery that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG.

Genetic engineering

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In genetic technology, recombination can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what is called recombinant DNA. A prime example of such a utilise of genetic recombination is gene targeting, which tin exist used to add, delete or otherwise change an organism’s genes. This technique is of import to biomedical researchers equally it allows them to written report the furnishings of specific genes. Techniques based on genetic recombination are also applied in protein engineering to develop new proteins of biological interest.

Recombinational repair

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Dna damages caused by a multifariousness of exogenous agents (due east.grand. UV light, X-rays, chemic cross-linking agents) can be repaired by homologous recombinational repair (HRR).^[ix]
[10]
These findings suggest that DNA damages arising from natural processes, such equally exposure to reactive oxygen species that are byproducts of normal metabolism, are also repaired by HRR. In humans, deficiencies in the gene products necessary for HRR during meiosis probable cause infertility^[11]
In humans, deficiencies in gene products necessary for HRR, such every bit BRCA1 and BRCA2, increase the gamble of cancer (meet Dna repair-deficiency disorder).

In bacteria, transformation is a process of gene transfer that ordinarily occurs between individual cells of the same bacterial species. Transformation involves integration of donor Dna into the recipient chromosome by recombination. This procedure appears to be an adaptation for repairing DNA damages in the recipient chromosome by HRR.^[12]
Transformation may provide a benefit to pathogenic leaner past allowing repair of DNA damage, particularly damages that occur in the inflammatory, oxidizing environment associated with infection of a host.

When two or more viruses, each containing lethal genomic damages, infect the aforementioned host cell, the virus genomes can often pair with each other and undergo HRR to produce feasible progeny. This process, referred to as multiplicity reactivation, has been studied in lambda and T4 bacteriophages,^[xiii]
as well every bit in several pathogenic viruses. In the example of pathogenic viruses, multiplicity reactivation may be an adaptive benefit to the virus since information technology allows the repair of DNA damages caused by exposure to the oxidizing surround produced during host infection.^[12]
See besides reassortment.

Meiotic recombination

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Molecular models of meiotic recombination take evolved over the years equally relevant evidence accumulated. A major incentive for developing a fundamental understanding of the machinery of meiotic recombination is that such understanding is crucial for solving the problem of the adaptive function of sex, a major unresolved issue in biology. A recent model that reflects electric current agreement was presented by Anderson and Sekelsky,^[14]
and is outlined in the first figure in this article. The figure shows that two of the four chromatids nowadays early on in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this version of the model, is initiated past a double-strand pause (or gap) shown in the DNA molecule (chromatid) at the top of the first figure in this article. Withal, other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (acquired by exposure to a cross-linking agent such equally mitomycin C) can be repaired by HRR.

Every bit indicated in the offset figure, in a higher place, 2 types of recombinant product are produced. Indicated on the right side is a “crossover” (CO) blazon, where the flanking regions of the chromosomes are exchanged, and on the left side, a “not-crossover” (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two “Holliday junctions” indicated in the lower right of the effigy by two Ten shaped structures in each of which in that location is an exchange of single strands between the two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway.

The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to equally “synthesis dependent strand annealing” (SDSA). Recombination events of the NCO/SDSA type appear to be more mutual than the CO/DHJ blazon.^[15]
The NCO/SDSA pathway contributes little to genetic variation, since the arms of the chromosomes flanking the recombination consequence remain in the parental configuration. Thus, explanations for the adaptive office of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events.

Achiasmy and heterochiasmy

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Achiasmy
is the phenomenon where autosomal recombination is completely absent in 1 sex of a species. Achiasmatic chromosomal segregation is well documented in male
Drosophila melanogaster.
Heterochiasmy
occurs when recombination rates differ betwixt the sexes of a species.^[16]
This sexual dimorphic pattern in recombination rate has been observed in many species. In mammals, females most often have higher rates of recombination. The
“Haldane-Huxley rule”
states that achiasmy usually occurs in the heterogametic sex activity.^[16]

RNA virus recombination

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Numerous RNA viruses are capable of genetic recombination when at to the lowest degree two viral genomes are present in the aforementioned host cell.^[17]
[xviii]
Recombination is largely responsible for RNA virus diverseness and immune evasion.^[19]
RNA recombination appears to exist a major driving force in determining genome architecture and the grade of viral evolution amidst picornaviridae ((+)ssRNA) (due east.yard. poliovirus).^[20]
In the retroviridae ((+)ssRNA)(e.grand. HIV), damage in the RNA genome appears to exist avoided during reverse transcription by strand switching, a grade of recombination.^[21]
[22]

Recombination as well occurs in the reoviridae (dsRNA)(e.g. reovirus), orthomyxoviridae ((-)ssRNA)(e.g. influenza virus)^[22]
and coronaviridae ((+)ssRNA) (e.g. SARS).^[23]
[24]

Recombination in RNA viruses appears to be an adaptation for coping with genome damage.^[17]
Switching betwixt template strands during genome replication, referred to every bit re-create-choice recombination, was originally proposed to explain the positive correlation of recombination events over brusque distances in organisms with a DNA genome (see outset Effigy, SDSA pathway).^[25]

Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans.^[23]

Particularly in coronaviruses, recombination may also occur fifty-fifty amid distantly related evolutionary groups (subgenera), due to their characteristic transcription mechanism, that involves subgenomic mRNAs that are formed by template switching.^[26]
[24]

When replicating its (+)ssRNA genome, the poliovirus RNA-dependent RNA polymerase (RdRp) is able to carry out recombination. Recombination appears to occur by a copy choice mechanism in which the RdRp switches (+)ssRNA templates during negative strand synthesis.^[27]
Recombination by RdRp strand switching likewise occurs in the (+)ssRNA plant carmoviruses and tombusviruses.^[28]

Recombination appears to exist a major driving forcefulness in determining genetic variability within coronaviruses, as well every bit the ability of coronavirus species to jump from ane host to another and, infrequently, for the emergence of novel species, although the machinery of recombination in is unclear.^[23]
During the first months of the COVID-19 pandemic, such a recombination event was suggested to accept been a critical step in the development of SARS-CoV-2’s ability to infect humans.^[29]
SARS-CoV-ii’s unabridged receptor binding motif appeared, based on preliminary observations, to accept been introduced through recombination from coronaviruses of pangolins.^[30]
However, more than comprehensive analyses later refuted this suggestion and showed that SARS-CoV-2 likely evolved solely within bats and with little or no recombination.^[31]
[32]

Role of recombination in the origin of life

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Nowak and Ohtsuki^[33]
noted that the origin of life (abiogenesis) is also the origin of biological development. They pointed out that all known life on earth is based on biopolymers and proposed that any theory for the origin of life must involve biological polymers that act as data carriers and catalysts. Lehman^[34]
argued that recombination was an evolutionary development as aboriginal every bit the origins of life. Smail et al.^[35]
proposed that in the primordial World, recombination played a fundamental office in the expansion of the initially short informational polymers (presumed to be RNA) that were the precursors to life.

See also

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• Eukaryote hybrid genome
• Four-gamete test
• Homologous recombination
• Contained assortment
• Recombination frequency
• Recombination hotspot
• Site-specific recombinase technology
• Site-specific recombination
• Reassortment
• 5(D)J recombination

References

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External links

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• Animations – homologous recombination: Animations showing several models of homologous recombination
• The Holliday Model of Genetic Recombination
• Genetic+recombination at the US National Library of Medicine Medical Subject Headings (MeSH)
• Animated guide to homologous recombination.

 This article incorporates public domain material from the NCBI certificate:
“Scientific discipline Primer”.