Genetic Recombination

Genetic Recombination

Crossing Over
Artwork of two chromosomes swapping genetic material prior to meiosis. This is a natural genetic process, known as crossing over, that occurs between homologous chromosomes. The recombination of DNA increases the genetic variation in a population. FRANCIS LEROY, BIOCOSMOS/Science Photo Library/Getty Images

Genetic Recombination

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 homologous 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 diversity within organisms is attained through genetic recombination.

Chromosome Structure

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 at 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. In meiosis, genetic recombination occurs as a result of the random segregation of these homologous chromosomes into different cells or from the process of transferring genes between homologous chromosomes, known as crossing over.

Crossing Over

Genetic recombination that involves crossing over occurs during prophase I of meiosis in sex cell production. Crossing over occurs as follows:

  • 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.
  • The crossed chromatids form what is 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.

Crossing Over in Mitosis

In eukaryotic cells, 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 cross over that occurs between homologous chromosomes in mitosis does not produce a new combination of genes.

Crossing Over in Non-Homologous Chromosomes

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.

Recombination In Prokaryotic Cells

Prokaryotic cells 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.