Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. During the formation of egg and sperm cells (meiosis), paired chromosomes from the male and female parents align so that similar DNA sequences can cross over, or be exchanged, from one chromosome to the other. This exchanging of DNA is an important source of the genomic variation seen among offspring.
Homologous Recombination. Homologous recombination is a miraculous yet rather random process of gene shuffling that makes us who we are. Otherwise, we would all be like our parents. This is a type of genetic recombination that occurs during the formation of the egg and sperm cells. During this process, meiosis, what happens is that paired chromosomes from the male and female parent align so that similar DNA sequences from the paired chromosomes have an opportunity to flip-flop, or cross over one another. This crossing results in the shuffling of genetic materials, just like some gentle shuffling of a deck of cards. And it's important because it's one of the sources of genetic variation that we see among offspring of a set of parents. Thus, offspring receive bits of DNA from each of their parents, but also from their grandparents and ancestors.
Ellen Sidransky, M.D. Chief & Senior Investigator Medical Genetics Branch
Genetic recombination refers to the process of recombining genes to produce new gene combinations that differ from those of either parent. Genetic recombination produces genetic variation in organisms that reproduce sexually. Genetic recombination happens as a result of the separation of genes that occurs during gamete formation in meiosis, the random uniting of these genes at fertilization, and the transfer of genes that takes place between chromosome pairs in a process known as crossing over. Crossing over allows alleles on DNA molecules to change positions from one homologous chromosome segment to another. Genetic recombination is responsible for genetic diversity in a species or population. For an example of crossing over, you can think of two pieces of foot-long rope lying on a table, lined up next to each other. Each piece of rope represents a chromosome. One is red. One is blue. Now, cross one piece over the other to form an "X." While the ropes are crossed, something interesting happens: a one-inch segment from one end of the red rope breaks off. It switches places with a one-inch segment parallel to it on the blue rope. So, now, it appears as if one long strand of red rope has a one-inch segment of blue on its end, and likewise, the blue rope has a one-inch segment of red on its end. Chromosomes are located within the nucleus of our cells and are formed from chromatin (mass of genetic material consisting of DNA that is tightly coiled around proteins called histones). A chromosome is typically single-stranded and consists of a centromere region that connects a long arm region (q arm) with a short arm region (p arm). When a cell enters the cell cycle, its chromosomes duplicate via DNA replication in preparation for cell division. Each duplicated chromosome is comprised of two identical chromosomes called sister chromatids that are connected to the centromere region. During cell division, chromosomes form paired sets consisting of one chromosome from each parent. These chromosomes, known as homologous chromosomes, are similar in length, gene position, and centromere location.
Genetic recombination that involves crossing over occurs during prophase I of meiosis in sex cell production. The duplicated pairs of chromosomes (sister chromatids) donated from each parent line up closely together forming what is called a tetrad. A tetrad is composed of four chromatids. As the two sister chromatids are aligned in close proximity to one another, one chromatid from the maternal chromosome can cross positions with a chromatid from the paternal chromosome. These crossed chromatids are called a chiasma. Crossing over occurs when the chiasma breaks and the broken chromosome segments get switched onto homologous chromosomes. The broken chromosome segment from the maternal chromosome gets joined to its homologous paternal chromosome, and vice-versa. At the end of meiosis, each resulting haploid cell will contain one of four chromosomes. Two of the four cells will contain one recombinant chromosome.
In eukaryotic cells (those with a defined nucleus), crossing over can also occur during mitosis. Somatic cells (non-sex cells) undergo mitosis to produce two distinct cells with identical genetic material. As such, any crossover that occurs between homologous chromosomes in mitosis does not produce a new combination of genes.
Crossing over that occurs in non-homologous chromosomes can produce a type of chromosome mutation known as a translocation. A translocation happens when a chromosome segment detaches from one chromosome and moves to a new position on another non-homologous chromosome. This type of mutation can be dangerous as it often leads to the development of cancer cells.
Prokaryotic cells, like bacteria which are unicellular with no nucleus, also undergo genetic recombination. Although bacteria most commonly reproduce by binary fission, this mode of reproduction does not produce genetic variation. In bacterial recombination, genes from one bacterium are incorporated into the genome of another bacterium through crossing over. Bacterial recombination is accomplished by the processes of conjugation, transformation, or transduction. In conjugation, one bacterium connects itself to another through a protein tube structure called a pilus. Genes are transferred from one bacterium to the other through this tube. In transformation, bacteria take up DNA from their environment. The DNA remnants in the environment most commonly originate from dead bacterial cells. In transduction, bacterial DNA is exchanged through a virus that infects bacteria known as a bacteriophage. Once the foreign DNA is internalized by a bacterium via conjugation, transformation, or transduction, the bacterium can insert segments of the DNA into its own DNA. This DNA transfer is accomplished via crossing over and results in the creation of a recombinant bacterial cell.
Genetic recombination occurs when genetic material is exchanged between two different chromosomes or between different regions within the same chromosome. We can observe it in both eukaryotes (like animals and plants) and prokaryotes (like archaea and bacteria). Keep in mind that in most cases, in order 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. 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, as you can see in the illustration below. What we end up with is a new combination of genes that didn’t exist before and is not identical to either parent’s genetic information. Note that recombination is also observed in mitosis, but it doesn’t occur as often in mitosis as it does in meiosis. Natural Self-HealingThe cell also 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. What we observe is an exchange between the broken DNA and a homologous region of DNA that will fill the gaps. There are also other ways that recombination is used to repair DNA. We’ve already covered some the consequences of genetic recombination, but in this section we will discuss Recombinant DNA Technology. This is a relatively new technology that is allowing scientists to change genes and organisms by manipulating DNA. What makes this so important is the fact that it has improved our understanding of diseases and, consequently, has expanded our ways of fighting them. As you might expect, 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 thing is done in gene therapy. Let’s assume a person is born without a particular essential gene, and is suffering from an illness due to the absence of that gene. Scientists can now introduce the missing gene into that person’s genome by using a virus that infects humans. First, they join the needed gene with the virus’s DNA and then they expose the person to that virus. Since all viruses blend their DNA with their host’s DNA, the gene that is added by the scientists ends up being part of the person’s genome. Scientists have observed the following types of recombination in nature:
Prokaryotic cells can undergo recombination through one of these three processes:
1. Recombination decreases the genetic diversity within a species. False. Genetic recombination increases genetic diversity by producing new combinations of genes. 2. Genetic recombination in the process of meiosis involves: B is correct. Crossing over is what results in the exchange of DNA between chromosomes during meiosis. 3. Which of the following is not a form of genetic recombination that occurs in prokaryotes? B is correct. Transformation, conjugation, and transduction are the forms of recombination that occur in prokaryotes.
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When does genetic recombination occur
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