Crossing Over

In sexually reproducing organisms, such as humans, offspring inherit one set of chromosomes from each parent.

During the formation of germ cells, such as sperm or eggs, homologous chromosomes inherited from each parent exchange DNA segments. This process is called crossing over.

Before cell division, in the S phase of the cell cycle, the cell copies its DNA. As meiosis I begins, during the leptotene stage of prophase I, the duplicated chromosomes condense.

During the zygotene stage of prophase I, homologous chromosomes pair closely along their lengths. This process is called synapsis. A protein structure called the synaptonemal complex forms between them, creating paired structures called tetrads.

During the pachytene stage of prophase I, non-sister chromatids of homologous chromosomes exchange matching DNA segments. This creates new combinations of genes through genetic recombination.

After crossing over, the crossover sites become visible as chiasmata. At these sites, homologous chromosomes remain connected.

As the synaptonemal complex breaks down during the diplotene stage of prophase I, the chiasmata remain visible. Together with cohesin proteins, which hold sister chromatids together, chiasmata help keep homologous chromosomes paired.

This genetic recombination through crossing over increases genetic variation in gametes and helps produce genetically unique offspring.