Genetic Recombination and Crossing Over

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.