Genetic Recombination

The history of genetics can be divided into two phases: one before and the other after 1953, the year in which the structure of DNA, the molecule of life, was discovered. In the first part of the century the foundations of classical genetics were laid. In the second part, genetics became molecular, often achieving unexpected results, culminating in the anatomical knowledge of the genome of our species. 

Genetic Recombination Genetic Recombination

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    Most of the time, geneticists have concentrated on identifying single genes and characterizing their functions. For about twenty years, however, the belief has been strengthened that there are very few genes that act alone and that genes and proteins continually interact with each other. The biological destiny of every organism in fact depends on complex networks of interactions between molecules. In the following, we are going to learn about the process of genetic recombination, different types, and the manipulation of genetics. Let's get started!

    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 is the physical exchange of genetic material between genetic elements.

    Genetic recombination process in eukaryotes

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

    Eukaryote is a type of cell whose nucleus and organelles are characterized by the presence of a membrane. The chromosomes of this type of cells are composed of DNA that is combined with histone proteins.

    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.

    On the other hand, site-specific recombination can also take place between non-homologous DNA strands and 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. The genetic recombination events are not always clear, although injuries caused by ultraviolet radiation or chemical mutagens are known to increase the frequency with which recombination occurs.

    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.

    Genetic Recombination Types of recombination in genetics Genetic Recombination in Eukaryotes | StudySmarterFigure 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).

    The first experimental example of transformation dates back more than 75 years ago, thanks to Frederick Griffith. Today we are able to explain the results obtained by Griffith: DNA leaked from the dead cells of the pathogenic pneumococci, which was then taken as free DNA by the live non-virulent pneumococci and made them virulent.

    Genetic Recombination Genetic recombination in bacteria Bacterial Transformation | StudySmarterFigure 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.

    Genetic Recombination Genetic recombination in bacteria Bacterial Transduction | StudySmarterFigure 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.

    Not all bacteria have the ability to build the sexual pili and conjugation tube. In order to do this, they must have specific genes that are not normally present in the bacterial chromosome. These genes are found on a small DNA molecule called plasmid F, which is present in donor bacteria.

    Genetic Recombination Genetic recombination in bacteria Bacterial Conjugation | StudySmarterFigure 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).

    Genetic recombination can also occur artificially. This is done through biotechnology. Biotechnology is a technique that makes it possible to produce both substances and services (for example, analysis, water purification, etc.) with the use of living organisms, cells, and their constituents or through their genetic manipulation.

    The techniques that allow to alter the genetic heritage of organisms go by the name of genetic engineering. Unlike natural recombination, genetic engineering alters the genome of an organism in a targeted way, isolating some of its genes and inserting them into cells of another species, where their proteins multiply and synthesize. The host cell (into which to introduce the genes) is normally a bacterium. The advantages in the use of bacteria are many: simplicity of the cell, knowledge of natural recombination mechanisms and growth rate.

    One of the most important methodologies of genetic engineering uses recombinant DNA. This technique uses restriction enzymes, capable of cutting bacterial and human DNA at precise sequences, so that the ends of the two chains are complementary and can pair. The recombinant DNA molecule is formed when DNA-ligase molecules firmly bond the ends of human DNA to those of bacterial DNA. The recombinant DNA is then introduced into a bacterial cell, where it produces the protein corresponding to the inserted human gene. As it reproduces by cell division, a clone of genetically identical cells is formed with recombinant DNA.

    Genetic Recombination - Key takeaways

    • Genetic recombination is the physical exchange of genetic material between genetic elements. Recombination is a relatively random process.
    • Genetic exchange occurs between pairs of homologous DNA sequences.
    • Transformation is the recombination that occurs when a bacterium acquires free DNA from the environment.
    • Transduction is a mechanism for the transfer of DNA from one bacterium to another by a virus.
    • Conjugation 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.
    Frequently Asked Questions about Genetic Recombination

    which process of genetic recombination involves genes from both parents?

    Fertilization 

    which three processes are methods of genetic recombination?

    1. Recombination involving a reaction between homologous DNA sequences is called homologous or generalized recombination, and in eukaryotes it usually occurs during meiosis in both males (spermatogenesis) and females (oogenesis).
    2. 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.
    3. Site-specific recombination is another type of event 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.

    what is genetic recombination?

    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. The frequency of it is affected by both global and local effects within the genome and, therefore, is not constant.

    when does genetic recombination occur?

    Prophase of meiosis I in Eukaryotes

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