Crossing-over

Humans produce genetically distinct egg and sperm cells, and thus unique offspring, as a result of the meiotic process of crossing over.

In such organs, crossing over occurs within the nuclei of diploid precursor cells during the first stage of Meiosis I called Prophase I. Previously, all of the cell's chromosomes replicated and condensed, yielding X-shaped structures.

Two sets of Xs are visible in a cell, one maternally derived and the other, paternal. Importantly, each arm of an X is a copy of the same parental chromosome and such duplicate pairs are termed sister chromatids.

Maternal and paternal versions of the same chromosome then begin to pair up and become linked as a protein framework manifests between them called the synaptonemal complex.

The result is connected pairs of homologous chromosomes, aligned so that the same maternal and paternal genes match up that begin to intertwine. The genetic material at the sites where non-sister chromatids intersect breaks off and the disconnected segments reattach to opposite chromosomes.

After this crossing over, the synaptonemal complex dissipates, but the homologous pairs stay fastened at points of genetic transfer, individually called chiasma, during most of Meiosis I, thus crossing over ends in chromatids with new, unique blends of parental information and as a result, is an example of genetic recombination.