Anaphas

Anaphase is a stage of cell division in eukaryotic cells, occurring after metaphase and before telophase. It is characterized by the separation of sister chromatids (during mitosis) or homologous chromosomes (during meiosis I) and their movement to opposite poles of the cell.

Anaphase in Mitosis:

During mitotic anaphase, the centromeres that join sister chromatids together split, allowing the sister chromatids to separate. These separated sister chromatids are now considered individual chromosomes. Microtubules of the mitotic spindle shorten, pulling the chromosomes towards opposite poles of the cell. Kinetochore microtubules, which are attached to the kinetochores of the chromosomes, are primarily responsible for this movement. The polar microtubules lengthen, contributing to cell elongation. Anaphase ensures that each daughter cell receives a complete and identical set of chromosomes.

Anaphase in Meiosis I:

In meiosis I, anaphase I involves the separation of homologous chromosomes, not sister chromatids. The paired homologous chromosomes (tetrads) separate, with one chromosome from each pair moving to opposite poles of the cell. Sister chromatids remain attached at the centromere. This separation reduces the chromosome number from diploid to haploid.

Anaphase in Meiosis II:

Anaphase II in meiosis closely resembles mitotic anaphase. The centromeres of sister chromatids split, and the sister chromatids (now individual chromosomes) move to opposite poles of the cell.

Regulation of Anaphase:

The transition from metaphase to anaphase is tightly regulated by the Anaphase-Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase. The APC/C triggers the degradation of securin, a protein that inhibits separase. Separase is a protease that cleaves cohesin, the protein complex holding sister chromatids together. Once cohesin is cleaved, sister chromatids are free to separate. This process is crucial for proper chromosome segregation and preventing aneuploidy.

Anaphase A and Anaphase B:

Anaphase can be further divided into two sub-phases:

• Anaphase A: The movement of chromosomes towards the poles. This is primarily driven by the shortening of kinetochore microtubules.
• Anaphase B: The elongation of the cell. This is primarily driven by the lengthening of polar microtubules, which slide past each other, pushing the poles further apart.

Both Anaphase A and Anaphase B often occur concurrently, although they can also be asynchronous in some cell types.