Deoxyribonucleic acid (DNA) is a polymer composed of two polynucleotide chains that coil around one another to form a double helix. DNA is the only known source of genetic material required for the development, function, growth, and reproduction of all known animals, including many viruses. DNA and ribonucleic acid are examples of nucleic acids (RNA). Along with proteins, lipids, and complex carbohydrates, nucleic acids are one of the four primary types of macromolecules required for all known forms of life (polysaccharides).
The ability of organisms to adapt to their surroundings and to keep their DNA sequence largely unchanged in their cells from generation to generation are their two most crucial characteristics. Genetic variety is a crucial component of organisms’ ability to adapt to a changing environment, and it is essential for their long-term survival. The ability of DNA to go through genetic rearrangements, which results in a slight alteration in gene combination, is what causes this variation among the creatures. Genetic recombination is the process that causes DNA rearrangement.
Recombination is the process of fusing genetic material from two different species to create a new recombinant chromosome. The novel recombinants exhibit modifications in their phenotypic traits. The majority of eukaryotes exhibit a full sexual life cycle, which includes meiosis, a crucial process that leads to novel allelic combinations through recombination. Chromosome exchange, which results from crossing over between two homologous chromosomes with similar gene sequences, enables it.
Mechanism of Recombination
There are basically three ideas that describe the mechanism of recombination: breakage and reunion, breakage and copying, and complete copy choice.
- Breakage and Reunion: The gene loci a and b, as well as a+ and b+, are separated by two homologous duplexes of chromosomes that are laid out in paired fashion. Recombinants with the broken segments reconnect crosswise and contain the a and b+ segments as well as the a+ and b segments. This kind of recombination doesn’t call for the creation of fresh DNA. Genetic recombination has been explained using this idea.
- Breakage and Copying: Between a and b, a paired homologous chromosome (ab and a+ b+) helix breaks. A newly synthesized segment that was cloned from segment b+ and joined to a section takes the place of segment b. In consequence, the recombinants have ab+ and a+ b+.
- Complete Copy Choice: Belling put forth this notion for chromosomal recombination in higher animals in 1931. However, numerous employees have questioned it. It therefore only has historical significance. This idea states that a piece of a homologous chromosome’s parental strand serves as a template for the creation of a duplicate of its DNA molecule. The copying process switches to the other parental strand. As a result, the recombinants have a combination of genetic material from both of the parental strands.
Types of Recombination
These can mainly be divided into the following three categories:
- General recombination,
- Non-reciprocal recombination,
- Site-specific recombination.
Only between the complementary strands of two homologous DNA molecules does general recombination take place. Prokaryotes’ homologous recombination was discussed by Smith. Base pairing interactions between two homologous DNA molecules’ complementary strands control general recombination in E. coli.
The two broken ends of a double helix formed by two DNA molecules combine with their opposing partners to form a new double helix. The location of the exchange can be anywhere along the homologous nucleotide sequence where one DNA strand pairs with another to form a heteroduplex sandwiched between two double helices.
Due to cleavage and rejoining events, no nucleotide sequences are altered at the site of exchange in the heteroduplex. However, there may be a few mismatched base pairs in heteroduplex joints.
Due to the requirement of homologous chromosomes, general recombination is often referred to as homologous recombination. The RecA protein and other rec gene products are responsible for carrying out generic recombination in bacteria and viruses. Recombination is recA reliant because the RecA protein is crucial for DNA repair.
The two partners both contribute an equal number of genes to the offspring, according to the fundamental law of genetics. It means that the children receive a set of genes that is divided equally between the male and the female. Meiosis causes one diploid cell to divide into four haploid cells, resulting in a halving of the quantity of male- and female-contributed genes. It is impossible to analyze these genes in higher animals like a man using just one cell. However, it is possible to recover and analyze all four daughter cells that were formed from a single cell through meiosis in some organisms, such as fungi.
Meiosis can occasionally produce three copies of the mother allele and just one copy of the paternal allele. This shows that the maternal allele has undergone changes to one of the two copies of the parental alleles. Gene conversion is the term used to describe this non-reciprocal type of gene modification. Gene conversion, which results from the mechanism of universal recombination and DNA repair, is regarded to be a significant event in the evolution of some genes.
The mechanism of homologous recombination and gene conversion has been covered by Kobayashi (1992).
When one of the strands is nicked, this process begins. The initial copy is now replaced as a single strand by an additional copy that DNA polymerase creates. With the same homologous area as in the bottom duplex of the DNA molecule, this single strand begins pairing. When the transfer of the nucleotide sequence is finished, the short unpaired strand created in the step is broken down. When DNA replication has separated the two mismatching strands, the consequences are seen (in the following cycle).
The relative location of nucleotide sequences in chromosomes is changed by site-specific recombination. Protein-mediated recognition of the two DNA sequences that will join is necessary for the base pairing event to occur. There is no need for a very long homologous sequence.
Site-specific recombination, as opposed to generic recombination, is controlled by an enzyme that recognizes particular nucleotide sequences on one of the DNA molecules that are recombining. Although there is no base pairing involved, if it does, the heteroduplex joint is only a few base pairs long.
It was first found in phage λ, which is how its genome enters and exits the chromosome of E. coli. An enzyme called lambda integrase, which facilitates recombination, was encoded by the penetration phage. Each chromosome has a unique attachment place where lambda integrase can bind.
A short homologous DNA sequence is chopped and broken. The partner strands are switched by the integrase, which then rejoins them to create a heteroduplex junction that is 7 bp long. In reassembling the previously broken strands, the integrase is similar to a DNA topoisomerase.
The two types of site-specific recombination are as follows:
- Conservative site-specific recombination: Conservative site-specific recombination is the process of creating a very short heteroduplex from two DNA molecules that share a portion of their DNA sequence.
- Trans-positional site-specific recombination: Trans-positional site-specific (TSS) recombination is a different form of recombination system. There are no specific sequences needed on the biggest DNA segment for the TSS recombination, which does not result in heteroduplex. Numerous viruses and transposable elements are among the mobile DNA sequences that can encode and integrate. By using a different mechanism than phages λ, the enzyme integrates by inserting its DNA into a chromosome. Each enzyme in the integration process recognizes a unique DNA sequence, such as the phage.
During meiosis, a particular kind of genetic recombination called homologous recombination takes occurs. During homologous recombination, the paired chromosomes from the male and female parents align, allowing similar DNA sequences from the paired chromosomes to pass over. Stain invasion is the term for this. Genetic material is moved about as a result of these crossovers, which results in genetic variation among offspring. Through a procedure known as homologous recombination repair, homologous recombination is mostly employed to fix damaging DNA breaks. A non-crossover product of this type of DNA repair usually restores the damaged DNA molecule to its pre-double-strand break state.
When horizontal gene transfer occurs, different strains of bacteria and viruses exchange genetic material by homologous recombination. All domains and DNA and RNA viruses use homologous recombination, which is a universal process. Homologous recombination is hence almost a ubiquitous biological process. Increased vulnerability to cancer, gene targeting, and gene therapy are all highly correlated with this. In eukaryotes, cell division requires it. Ionizing radiation and toxic chemicals can damage DNA, yet homologous recombination can repair those damages. Through meiotic cell division to become specialized gamete cells, it contributes to genetic variety production in addition to DNA repair.
DNA double-strand breaks can be repaired through a process called non-homologous recombination. Since there is no requirement for a homologous template because the break terminates directly ligate, it is referred to as non-homologous. Microhomologies, which are short DNA sequences, are typically what directs this pathway. On the extremities of DNA double-strand breaks, they can be found as single-stranded overhangs.
When these overhangs are perfectly compatible, non-homologous recombination correctly fixes the break. Translocation and telomere fusion occurs in tumor cells as a result of inappropriate non-homologous recombination. Nearly all biological systems have a non-homologous recombination mechanism, which is also the main double-strand break repair pathway in mammals. The double-strand breaks are repaired by a more error-prone mechanism when this pathway is deactivated. Through this route, DNA sequences between microhomologies are deleted during repairs. Bacteria and archaea don’t have a non-homologous route. Eukaryotes, in contrast, use a variety of proteins during the non-homologous recombination mechanism. This happens during processes like ligation, end processing, and end binding and tethering.
Occurrence of Recombination
- In the pachytene phase of the prophase of meiosis-I, genetic recombination takes place.
- Because recombination takes place during the protracted prophase of meiosis I, prophase-I is the longest and maybe the most significant period of meiosis.
- The eukaryotes and the prokaryotes both exhibit this process.
- It broadens the genetic diversity of organisms that reproduce sexually.
Importance of Recombination
In sexual organisms, genetic recombination is a crucially important planned component of meiosis. It is employed to appropriately partition the chromosomes. It offers the fundamental component for genomic mapping. Recombination rates are roughly inversely correlated with the physical separation between markers.
Recombination of genetic material had an impact on adaptability as well, although it had the reverse result. It is employed to break them as well as to combine the advantageous gene combinations. Recombination has the power to both aid in the rescue and prevents it. Because it creates genetic variation, genetic recombination is significant. Recombination of genes is crucial for maintaining species variety and population size.
The genetic integrity between the cells and tissues is provided by this recombination. The range of environmental stimuli, such as temperature and condition, will likewise vary as a result of genetic recombination. The process of swapping genetic components during allelic development is known as recombination.
Genetic recombination aids in the filling of gaps during DNA replication and prevents the replication fork from stalling. It is also utilized in DNA repair. It is crucial for a number of biological processes that take place in eukaryotic cells. Diversity in living things is produced by genetic recombination.
FAQs on Recombination
Question 1: What is the purpose of DNA recombination?
DNA fragments are broken and recombined during the recombination process to create novel allele combinations. The genetic variation that results from this recombination process at the gene level reflects variations in the DNA sequences of various species.
Question 2: What is the mechanism of DNA recombination?
Only if two DNA double helices include a significant stretch of sequence similarity will they perform an exchange reaction, according to the process of general recombination (homology).
Question 3: Which processes result in recombination?
Genetic recombination results from gene separation that takes place during gamete production in meiosis, random gene pairing that occurs during fertilization, and gene crossing over, which is the transfer of genes between chromosome pairs.
Question 4: Where does recombination occur?
Recombination happens when two DNA molecules trade genetic material with one another. When homologous chromosomes align in pairs and exchange DNA segments during prophase I of meiosis, one of the most notable instances of recombination occurs.
Question 5: What is the benefit of recombination?
Recombination is necessary for homologous pairing during meiosis in addition to having at least two other advantages for sexual organisms. It creates novel allele combinations along chromosomes and limits the consequences of mutations to the area surrounding a gene rather than the entire chromosome.
Question 6: What affects recombination frequency?
Recombination rates are heritable, supported by particular genetic loci, and responsive to selection. They can also be modified by environmental and demographic factors. As a result, they could change depending on the evolutionary or selective settings.
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