Genetic recombination refers to the rearrangement of DNA sequences by the breakage and rejoining of chromosomes or chromosome segments. It also describes the consequences of such rearrangements, that is, the inheritance of novel combinations of alleles in the offspring that carry recombinant chromosomes.
Recombination also serves as a mechanism to repair some types of potentially lethal damage to chromosomes. Genetic recombination is often used as a general term that includes many types of DNA rearrangements and underlying molecular processes.
It can be observed in both eukaryotes (like animals and plants) and prokaryotes (like archaea and bacteria). In most cases, for an exchange to occur, the sequences containing the swapped regions have to be homologous, or similar, to some degree.
The process occurs naturally and can also be carried out in the lab. Recombination increases the genetic diversity in sexually reproducing organisms and can allow an organism to function in new ways.
Read Also: Major Genetic Differences between Tropical and Temperate Livestock Breeds
Recombination and Genetic Variation in Eukaryotic Organisms
Genetic recombination is of fundamental importance for a wide variety of biological processes in eukaryotic cells. One of the major questions in recombination relates to the mechanism by which the exchange of genetic information is initiated.
In recent years, DNA double-strand breaks (DSBs) have emerged as an important lesson that can initiate and stimulate meiotic and mitotic homologous recombination.
In this regard, it can be viewed as the models by which DSBs induce recombination, describe the types of recombination events that DSBs stimulate, and compare the genetic control of DSB-induced mitotic recombination in budding and fission yeasts.
The result of recombination is a novel genetic entity that carries genetic information in non-parental combinations. Biochemically, recombination is a process of combining or substituting portions of nucleic acid molecules.
Recombination has been recognised as an important process leading to the genetic diversity of viral genomes upon which natural selection can function.
Examples of Genetic Recombination in Living Organisms
Genetic recombination occurs naturally in meiosis. Meiosis is the process of cell division that occurs in eukaryotes, such as humans and other mammals, to produce offspring.
In this case, it involves crossing-over. What happens is that two chromosomes, one from each parent, pair up with each other. Next, a segment from one crosses over or overlaps, a segment of the other. This allows for the swapping of some of their material.
What results is a new combination of genes that didn’t exist before and is not identical to either parent’s genetic information. Recombination is also observed in mitosis, but it doesn’t occur as often in mitosis as it does in meiosis.
1. Recombination for Natural Self-Healing
The cell can also undergo recombinational repair, for example, if it notices that there is a harmful break in the DNA: the kind of break that occurs in both strands. An exchange occurs between the broken DNA and a homologous region of DNA that fills the gaps. There are also other ways that recombination is used to repair DNA.
2. Functions of Genetic Recombination in Agricultural Science
Recombinant DNA Technology – This is a relatively new technology that is allowing scientists to change genes and organisms by manipulating DNA.
What makes this important is the fact that it has improved the understanding of diseases and, consequently, has expanded the ways of fighting them. DNA segments are joined together in this technology.
For example, a gene can be cut out from a human and introduced into the DNA of a bacterium. The bacterium will then be able to produce human protein that is otherwise only made by humans.
The same process is used in gene therapy. For example, when a person is born without a particular essential gene and suffers from an illness due to its absence, the missing gene can be introduced into that person’s genome by using a virus that infects humans.
First, the needed gene is joined with the virus’s DNA and then the person is exposed to that virus. Since all viruses blend their DNA with their host’s DNA, the gene added becomes part of the person’s genome.
Read Also: Crossbreeding as Tool for Tropical Livestock Improvement
Types of Genetic Recombination Observed in Nature
At least four types of naturally occurring recombination have been identified in living organisms:
1. General or homologous recombination: occurs between DNA molecules of very similar sequences, such as homologous chromosomes in diploid organisms. It can occur throughout the genome, using one or a small number of common enzymatic pathways.
2. Illegitimate or nonhomologous recombination: occurs in regions where no large-scale sequence similarity is apparent, such as translocations between different chromosomes or deletions that remove several genes along a chromosome. Short regions of sequence similarity may still be found at breakpoints.
3. Site-specific recombination: occurs between particular short sequences (about 12 to 24 bp) on otherwise dissimilar parental molecules. It requires special enzymatic machinery for each specific site.
Examples include the integration of some bacteriophages, such as ƛ, into a bacterial chromosome and the rearrangement of immunoglobulin genes in vertebrates.
4. Replicative recombination – generates a new copy of a segment of DNA. Many transposable elements use this type of recombination to replicate at a new location.
Use of Recombinant DNA Technology in Agricultural Research
Recombinant DNA technology uses two other types of recombination. The directed cutting and rejoining of different DNA molecules in vitro using restriction endonucleases and DNA ligases is well-known.
Once created, these recombinant DNA molecules are introduced into a host organism, often a bacterium. If the recombinant DNA is a plasmid, phage, or another molecule capable of replicating in the host, it remains extrachromosomal.
However, recombinant DNA can also be introduced into a host in which it cannot replicate, such as a plant, an animal cell in culture, or a fertilised mouse egg.
For stable transformation, the introduced DNA must be integrated into the host chromosome. In bacteria and yeast, this can occur by homologous recombination at a reasonably high frequency.
Advantages of Genetic Recombination in Agricultural Genetics
Not only is recombination essential for homologous pairing during meiosis, but it also has at least two additional benefits for sexual species. It creates new combinations of alleles along chromosomes and limits the effects of mutations mainly to the region around a gene.
Since each chromosome undergoes at least one recombination event during meiosis, new allele combinations are generated. The arrangement of alleles inherited from each parent is not preserved, but instead, new germ cells carry chromosomes with new allele combinations. This remixing is a rich source of diversity in a population.
Over time, recombination separates alleles at one locus from alleles at a linked locus. A chromosome through generations becomes “fluid,” having many different allele combinations.
This helps clear nonfunctional or less functional alleles from a population. Without recombination, one harmful mutant allele could cause the entire chromosome to be eliminated.
Mechanisms of Homologous Recombination in Plants and Animals
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA.
It is widely used by cells to repair harmful breaks on both DNA strands, known as double-strand breaks (DSBs). DNA integration by homologous recombination provides a way to introduce mutations to the mouse genome at specific loci, known as gene targeting.
There are two modes of DNA integration by homologous recombination. In the insertion mode, foreign DNA is added without loss of preexisting chromosomal DNA. In the replacement mode, foreign DNA replaces part of the chromosomal DNA.
Gene knockout (KO) is the most commonly used strategy of homologous recombination by replacement. This method involves creating a DNA construct containing a drug resistance gene in place of the target gene.
The construct also includes at least 2 kb of homologous sequence flanking the target gene.
Homologous recombination (HR) occurs spontaneously in plants. HR can happen between two short identical DNA repeats, resulting in the deletion of the DNA sequence flanked by those repeats. Compared with site-specific recombination, HR does not require recombinase to induce selectable marker gene (SMG) removal, making it a simpler strategy.
This has been implemented to delete SMG in genetically modified organisms (GMO). For example, a vector carrying the trait gene uidA and the two SMGs aadA and bar, flanked by three 418 bp direct repeats, was constructed.
HR is an outcome, rather than a process a detectable result with possibly multiple mechanisms leading to the same recombinant chromosome.
Recombination in DNA and RNA Viruses
Genetic recombination is observed in many DNA and RNA viruses. Natural sequence rearrangements and experimental data show that recombination provides genetic diversity during virus infections. The molecular mechanisms involved depend on the virus class.
DNA viruses use homologous (general) recombination mechanisms from host cells, though some encode their own recombination proteins. Site-specific (nonhomologous) recombination events have also been found in certain DNA virus classes.
Do you have any questions, suggestions, or contributions? If so, please feel free to use the comment box below to share your thoughts. We also encourage you to kindly share this information with others who might benefit from it. Since we can’t reach everyone at once, we truly appreciate your help in spreading the word. Thank you so much for your support and for sharing!
Frequently Asked Questions
We will update this section soon.