Genetic recombination

Genetic recombination is a process that occurs when genetic material is exchanged between different chromosomes or between different sections of the same chromosome. Genetic recombination can occur in more advanced cells found in larger organisms and in simpler, single-celled organisms. In organisms born from sexual reproduction, the process increases genetic diversity, further allowing those organisms to pass down positive genetic traits to their own offspring. Genetic recombination can also be carried out in a laboratory setting, allowing scientists to manipulate DNA to develop medical and gene therapy treatments.

Background

Gregor Mendel, a Czech biologist, was the first to discover genetics in the 1850s and 1860s. Mendel experimented with pea plants and realized that traits such as height, flower color, and seed color could be passed down from parent plants to their offspring and reappear in distinct and predictable ways. In further experiments, he found that recessive traits, ones that were not apparent in the offspring of the original plants, could appear in some later generations.

About the same time, Swiss chemist Friedrich Miescher identified a substance called deoxyribonucleic acid, or DNA, in his study of white blood cells. Scientists did not know the significance of DNA until the mid-twentieth century, when a series of researchers uncovered its properties and shape. They found that DNA was the main substance that carried all the genetic instructions found in each cell of an organism.

DNA is found in the cell’s nucleus. It is a double helix, or coiled ladder, of molecules, with sugar and phosphate groups making up the sides of the ladder and a series of four chemical bases making up the rungs. These four substances—adenine (A), guanine (G), cytosine (C), and thymine (T)—act as a sort of chemical code that can be joined together in various combinations. Each of the DNA bases pairs up with a specific match. For example, adenine pairs with thymine and guanine pairs with cytosine.

Genes are sections of DNA that determine the traits of an organism. Some genes may carry more of a certain protein responsible for darker eye color, resulting in brown eyes. Others may not produce as much of the protein, leading to blue eyes. Chromosomes are long, thread-like strands of DNA that contain numerous genes. Each person usually has forty-six chromosomes, inheriting one set of twenty-three from their mother and another set of twenty-three from their father. These forty-six chromosomes contain about twenty thousand to twenty-five thousand genes, which are made up of about three billion base chemical pairs (AGCT).

Overview

Genetic recombination occurs when two chromosomes exchange genetic material or when the material is exchanged between different segments of the same chromosome. In essence, this creates a new molecule of DNA from an existing set of "parental" DNA. This process occurs in cells known as eukaryotes, cells with a defined nucleus, and prokaryotes, which are single-celled organisms that do not have a nucleus. The genetic material of a eukaryote cell, which is found in larger, multi-celled organisms, is contained within the nucleus. The genetic material in a prokaryote cell is free-floating inside the cell.

Genetic recombination happens in eukaryotic cells during meiosis, the process of dividing cells that happens during sexual reproduction and reduces the number of chromosomes in gametes and sex cells like the egg and sperm in humans. In humans, the body’s cells are classified as diploid, meaning they contain two sets of chromosomes, one from each parent. For successful fertilization to occur, the cells must be haploid, containing one set of twenty-three chromosomes. During meiosis, each of the diploid cells divides twice to produce four haploid gametes, each with twenty-three chromosomes.

At this point, genetic recombination occurs in a process called crossing over. One chromosome from the father and one from the mother line up and begin to overlap. Genetic material from one chromosome is then exchanged with material from the other. For example, a gene from the mother may replace a gene from the father, and vice versa. The recombination process is random, meaning the offspring produced by the genetic recombination are not exact replicas of either parent. This allows for greater genetic diversity within a species, which increases its ability to adapt to changing environments.

Genetic recombination can also occur in a process called mitosis, which is cellular division in non-sex cells in the body. Typically, mitosis produces two cells that are an exact copy of the original cell with identical chromosomes. However, on occasion, recombination can take place and change the genetic makeup of the original cell. This process is typically harmful and can result in the development of cancer.

Cells can also undergo recombination in the process of repairing damage, such as that brought about by exposure to harmful radiation. Left untended, a damaged strand of DNA could result in mutations that could lead to serious health complications. In this process, the cell replaces the damaged section of DNA with similar genetic material, repairing the damage to the DNA strand.

In the twenty-first century, scientists have also learned to use genetic recombination to alter the genetic code of certain genes that could pose health risks to patients. Recombinant DNA technology allows scientists to cut out selected genes and insert them into other DNA strands. For example, if a person is born with a genetic illness in which their body does not produce a necessary gene, scientists can remove that gene from a healthy person and reintroduce it into the body of the ill person. This can be done by combining that gene with the DNA of a virus and then allowing the virus to infect the person, spreading the needed gene throughout the body.

Types of Genetic Recombination

Scientists divide genetic recombination in nature into four main types. The first, and most common, is homologous recombination, which is the recombination of genetic material between DNA molecules with a similar or identical genetic sequence. This is the type of genetic recombination carried out during the process of meiosis. This type of recombination is also called general recombination.

Nonhomologous or illegitimate recombination occurs between DNA sequences that are not entirely similar. For example, sections of the DNA sequence may be deleted, or certain genes may be relocated. However, scientists who analyze regions where the genetic code is different will often find smaller sections that are similar. In some cases, the recombination process may remove the deleted section between the genes.

The third type, site-specific recombination, takes place between short genetic sequences that are similar, even though most of the molecule has a different genetic code. Finally, some DNA sequences use replicative recombination to replicate themselves and produce a new strand of DNA for recombination.

Recombinant DNA technology uses two additional types of genetic recombination. The first involves cutting away genetic material from one source and recombining it with another in a test tube or other laboratory setting. This type of recombination is more successful at transplanting DNA sequences to single-celled organisms such as bacteria or viruses. Recombination in larger organisms occurs less often and is more haphazard, resulting in a process known as random recombination.

Genetic recombination also occurs within prokaryote cells and takes three forms. The first process, called transformation, takes place when the single-celled organism combines with naked DNA in its environment. Naked DNA is genetic material that is free-floating and not protected by any molecules or proteins. This type of DNA usually comes from dead or burst cells that have released their genetic material into the environment.

Transduction is the transfer of genetic material by viruses that infect bacteria. These viruses are known as bacteriophages. Viruses are infectious agents that attack other cells, infect them, and use those cells to create more of the virus. When a bacteriophage attacks a bacterium, it can tear into it or chop it into smaller pieces. These pieces of genetic material from the original bacterium can then get stuck inside the newly born virus. When these bacteriophages infect a new host bacterium, they can carry the genetic pieces of the old host with them.

In the process of conjugation, genetic material is shared between two bacteria that meet. As a donor bacterium gets close to another bacterium, it extends a hairlike structure known as a pilus that facilitates the transfer of genetic material. When contact is lost, the transfer ends and the bacterium replaces existing genetic material in its chromosomes with the donated material.

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