Genetic Recombination and Crossing Over

Two large X structures on a grass field with birds flying between to represent X chromosomes and genes moving from to the other.

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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.

Recombination Versus Crossing Over

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.

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).

Chromosome Duplication

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. 

Crossing Over in Meiosis

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.

Crossing Over in Mitosis

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.

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, 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.

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Bailey, Regina. "Genetic Recombination and Crossing Over." ThoughtCo, Apr. 5, 2023, Bailey, Regina. (2023, April 5). Genetic Recombination and Crossing Over. Retrieved from Bailey, Regina. "Genetic Recombination and Crossing Over." ThoughtCo. (accessed June 3, 2023).