Genetic Recombination

Genetic recombination definition

Genetic recombination is a molecular event involving an exchange of material between two double-stranded DNA molecules. It generally occurs between corresponding sequences in order to ensure the exact conservation of the genetic material in the recombinant filaments. Its frequency is affected by both global and local effects within the genome and, therefore, is not constant.

Genetic recombination process in eukaryotes

Before we proceed further, let's do a quick refresher as to what eukaryotes are.

Eukaryotes examples are plants, animals, and fungi! Human cells are eukaryotic cells! So how do these cells recombine their genes?

The exchange of genes between alleles typically occurs in the course of sexual reproduction. Therefore, one can assume that the resulting genotype significantly extends the genetic variability of a population.

Recombination is a relatively random process. Genetic stability, i.e. the preservation of DNA sequences over generations, is crucial for the survival of individuals and species. It is however also crucial for the DNA to undergo reassortments that can switch up the combination of genes in every single genome.

Why is that though? Well, think about it. We are all different compared to one another (genetically speaking of course) and this process requires that our bodies switch up our genes. Therefore, genetic diversity requires recombination. Furthermore, it guarantees that children inherit both maternal and paternal genes from both sets of grandparents. It guarantees both genetic diversity and continuity.

Genetic exchange occurs between pairs of homologous DNA sequences. The recombination events are represented by the breaking of two double-stranded DNA molecules at sites exactly corresponding to the homologous chromosomes. Here, the exchange of both strands and the reconstruction of the double helices by welding the free ends occurs. The whole process is so thorough that the number of nucleotides in the recombined strands remains the same. Amongst sexually reproducing organisms, genetic recombination occurs during the meiotic process by crossing over between homologous chromosomes.

Types of recombination in genetics

As we learned above, genetic recombination involves the physical exchange of genetic material between genetic elements. We have several types of recombination:

• Recombination involving a reaction between homologous DNA sequences is called homologous or generalized recombination (Figure 1), and in eukaryotes, it usually occurs during meiosis in both males (spermatogenesis) and females (oogenesis).

• Non-homologous recombination: Such interactions take place between DNA molecules that aren't always related. The exchange generally occurs at the level of short specific nucleotide sequences recognized by a series of enzymes. This type of recombination, therefore, alters the relative positions of the nucleotide sequences of the genome.

• Site-specific recombination promotes recombination between specific pairs of sequences and was first characterized in bacteria. This one is responsible for integrating phage genomes into the bacterial chromosome.

• A further type of recombination is used by viruses, in which the polymerase passes from one template to another while it is synthesizing RNA. As a result, the newly synthesized molecule merges sequence information that comes from two different parents. This type of mechanism is called copy choice.

Figure 1. This diagram shows the process of genetic recombination in eukaryotes.

Genetic recombination in bacteria

The increase in genetic variability derived from the mixing of genes is important for the survival of species such as bacteria and viruses.

In prokaryotes however, the process is so distinct that sometimes the recombination does not even require the participation of two whole cells. Although they reproduce asexually, prokaryotes have several ways to recombine their genes. Let's address them!

• Transformation: the recombination that occurs when a bacterium acquires free DNA from the environment. This phenomenon occurs naturally in some species of bacteria, when cells die and their DNA escapes. Once the transforming DNA is found in the host cell, the host cell's chromosome can incorporate new genes (Figure 2).

Figure 2. This illustration shows how transformation typically occurs in bacteria.

• Transduction: A mechanism for the transfer of DNA from one bacterium to another by a virus. When phages undergo a lytic cycle, they pack their DNA in the capsid. Capsids generally form before the DNA enters them, so sometimes a fragment of bacterial DNA can be introduced into an empty capsid; when the new virion infects another bacterial cell, the foreign DNA fragment will be injected into it and can recombine with the host chromosome, leading to the replacement of some host genes with bacterial genes from the cell that previously hosted the virus (Figure 3).
  • This type of step is called generalized transduction because it can randomly transfer any piece of DNA from one bacterium to another.
  • There is also a specialized transduction mechanism which involves prophages: when the prophage detaches from the chromosome that hosts it, it carries with it a contiguous fragment of the bacterial DNA.

Figure 3. Bacterial transduction refers to the transfer of DNA from one bacterium to another by a virus.

• Conjugation: This is a process in which two bacteria come into direct contact and the copy of a part of the DNA passes from a donor bacterium to a recipient bacterium.
  • The process begins with the appearance of one or more small growths, called sexual pili on the donor bacterium. Once the sexual pili have brought the two bacteria into close contact, a thin cytoplasmic bridge is produced between the two cells called the conjugation tube. Only one of the two DNA strands of the donor molecule passes through this bridge, while the remaining strand acts as a template to reconstruct the entire molecule.
  • The contact between the two cells is short, and generally does not last long enough to allow the passage of the entire genome of the donor into the recipient cell.
  • Once inside the recipient cell, the DNA fragment from the donor cell can recombine with the genome of the recipient cell. The donor's DNA is placed side by side with the homologous genes of the recipient's DNA allowing crossing-over to occur.
  • The conjugation mechanism (Figure 4).
    • Formation of the F+ / F- conjugative couple.
    • DNA mobilization.
    • Separation of conjugating cells.

Figure 4. During bacterial conjugation, bacteria come together and the copy of a DNA segment is transferred from the donor bacterium to the recipient bacterium.

Genetic recombination examples

Genetic recombination is a part of us. One of the most important examples of recombination in sexually reproducing organisms occurs during the meiotic process by crossing over between homologous chromosomes. Although meiosis occurs only in eukaryotes, the increase in genetic variability derived from the mixing of genes is so important for the survival of species that recombination processes are also common in bacteria and viruses.

Another example is when one's cell heals itself in a natural manner: if for instance, the cell notices that there is a break in your DNA, what will happen is that an exchange between a homologous stretch of DNA will occur, filling in the gaps of the broken DNA.

Importance of genetic recombination

Why is genetic recombination so important? Well, genetic differences between individuals are the basis on which natural selection operates on to implement the evolution and adaptation of species to environmental conditions. Genetic recombination is critical to this process as it allows the genes to be "scrambled" around in different combinations and reorganizing specific DNA sequences.

Most of the alterations to one's genetic system through genetic recombination are mainly borne by those involved in DNA damage repair. In fact, on the other side of the spectrum, the fundamental role of genetic recombination in such processes is evidenced by the severity of the genetic diseases associated with them. Often these diseases are also characterized by an increased incidence of tumors, due to the role played by genetic instability in their onset (e.g., xeroderma pigmentosum).