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.

Unlike mitosis, meiosis aims for genetic diversity in its creation of haploid gametes. Dividing germ cells first begin this process in prophase I, where each chromosome—replicated in S phase—is now composed of two sister chromatids (identical copies) joined centrally.

The homologous pairs of sister chromosomes—one from the maternal and one from the paternal genome—then begin to align alongside each other lengthwise, matching corresponding DNA positions in a process called synapsis.

In order to hold the homologs together, a protein complex—the synaptonemal complex—forms. The synaptonemal complex facilitates the exchange of corresponding random pieces of DNA between non-sister chromatids, yielding new combinations of alleles via homologous recombination.

As the synaptonemal complex begins to dissolve, X-shaped structures hold the homologous chromosomes together until recombination is completed. The structures—called chiasmata—mark the areas where crossover of genetic information occurred.

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19.5 : Crossing over