Question

Which two molecules, an regulatory protein and enzyme regulate cell division in mitosis?

How does the cell stop division?

What controls the breakdown of bonds between sister chromatids so they can seperate in anaphase?

Answer 1

1. The cell cycle regulators make sure that the cell cycle events occur in order and one phase triggers the onset of the next phase. Cell cycle is regulated by a protein called cyclins and an enzyme called Cdks (Cyclin dependent kinases). There are 4 types of cyclins in eukaryotes namely: G1 cyclins, G1/S cyclins, S cyclins and M cyclins. Each cyclin associated with a particular phase helps to drive the events of that phase. Cyclins drive the events of cell cycle by combining with Cyclin dependent kinases, which are initially inactive but binding of cyclins makes them active to modify target proteins.

2. Everytime a chromosome reproduce, the telomere gets shorter. As the length of the telomere diminishes, cell division stops altogether. This is due to cell senescence. During cell division, G1 checkpoint and G2 checkpoint screen the cells for cell size, nutrients, growth factors and damage, if the cell passes the checkpoint it progresses throgh the cell cycle if not it enters G0 phase and if the damage is irrepairable it is subjected to apoptosis.

3. Cohesin keeps the sister chromatids together. Separase (or Separin) is the enzyme that cleaves the cohesin to allow chromososme separation in anaphase. In humans, it is encoded by ESPL1 gene