Prophase
Prophase (from Ancient Greek προ- (pro-) 'before' and φάσις (phásis) 'appearance') is the first stage of cell division in both mitosis and meiosis.
[6] During both meiotic and mitotic prophase, giemsa staining can be applied to cells to elicit G-banding in chromosomes.
[11] The main events of prophase are: the condensation of chromosomes, the movement of the centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli break down.
[3] The movement of centrosomes to opposite poles is accompanied in animal cells by the organization of individual radial microtubule arrays (asters) by each centriole.
[13] In plant cells, microtubules gather at opposite poles and begin to form the spindle apparatus at locations called foci.
[10] The mitotic spindle is of great importance in the process of mitosis and will eventually segregate the sister chromatids in metaphase.
[3]: 98 In humans, decades can pass as oocytes remain arrested in prophase I only to quickly complete meiosis I prior to ovulation.
[2] In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived chromosomes have found their homologous partner.
[3]: 99 Visible junctions called chiasmata hold the homologous chromosomes together at locations where recombination occurred as the synaptonemal complex dissolves.
[3]: 99 In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four sister chromatids can be seen in bivalents with microscopy.
[12][10] In both animal and plant cells chromosomes may de-condense during telophase I requiring them to re-condense in prophase II.
[3]: 100 [10] If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the model organism Arabidopsis.
However, it has been proposed that the arrest of oocytes at the four genome copy stage may provide the informational redundancy needed to repair damage in the DNA of the germline.
[16] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.
[3] To ensure pairing of homologous chromosomes and recombination of genetic material occurs properly, there are cellular checkpoints in place.
The meiotic checkpoint network is a DNA damage response system that controls double strand break repair, chromatin structure, and the movement and pairing of chromosomes.
