36.5:
Crossing Over
In humans, male and female germ cells undergo crossing over or genetic recombination to ensure a distinct offspring.
Crossing over occurs during the prophase I of meiosis I between homologous chromosome pairs inherited from each parent.
Before meiosis I starts, the loosely arranged chromatin inside the nucleus duplicates itself during the S phase of the cell cycle. As the cells enter prophase I, the duplicated chromatin condenses to form sets of X-shaped chromosomes.
Each arm of an X is a copy of the same parental chromosome called sister chromatids. In contrast, chromatids of a homologous chromosome pair are called non-sister chromatids.
As the homologous chromosomes pair up, a protein framework called the synaptonemal complex links them into bivalents.
As the same genes of the bivalents align, they begin to intertwine. The intersecting segment breaks off and reattaches to opposite chromosomes, resulting in genetic recombination. These points of genetic transfer between the homologous pairs are called chiasma.
As the synaptonemal complex disassembles, the chiasma, together with cohesins that tightly hold the sister chromatids, keep the homologs attached until they are correctly segregated into two daughter cells.
36.5:
Crossing Over
Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I, duplicated chromosomes condense and form two sister chromatids (identical copies of the original chromosome) joined by the centromere. Next, the homologous chromosomes pair up and align the same gene segment from the maternal and paternal chromosomes, forming a synapse. A protein complex called the synaptonemal complex is formed that holds these homologs together. As crossing over proceeds, random pieces of DNA are swapped between the homologs, producing new combinations of alleles via homologous recombination. The 'chiasmata' mark the areas where the crossover of genetic information has occurred. As the synaptonemal complex begins to dissolve, the chiasma holds the homologous chromosomes together until recombination is completed and chromosomes are segregated correctly into the daughter cells.
Suggested Reading
- Székvölgyi, L., Ohta, K., & Nicolas, A. (2015). Initiation of meiotic homologous recombination: flexibility, impact of histone modifications, and chromatin remodeling. Cold Spring Harbor Perspectives in Biology, 7(5), a016527.
- Hunter, N. (2015). Meiotic recombination: the essence of heredity. Cold Spring Harbor perspectives in biology, 7(12), a016618.