Please answer 67 Degradation of cyclins by the proteasome Gi

Degradation of cyclins by the proteasome Given the end results of the two types of cell division, i.e. mitosis and meiosis, why is it necessary for homologs to pair during meiosis and not desirable for them to pair in mitosis? During mitosis, sister chromatids are held together by cohesin. Cohcesin is eleaved by the protease separase to allow separation of the sister chromatids at anaphase. Securin is a protein that binds to separase and inhibits its activity. Some cancer cells overexpress securing (i.e. produce an excessive amount). These cells can undergo mitosis, but end up with abnormal numbers of chromosomes. Explain how overexpression of sccurin could lead to abnormal chromosome number. Briefly explain how the APC coordinates the timing of sister chromatid separation within the cell cycle.

Solution

6. The daughter cells produced during mitosis are identical and so there is no need for homologous pairing. But, the daughter cells produced in mitosis are different as they undergo crossing over. So pairing of homologous chromosomes takes place.

7. a) Securin inhibits the separation of sister chromatids during mitosis. The chromosomes generally move to the opposite poles to form two daughter cells containing identical number of chromosomes. If they don\'t move, daughter cells would contain unequal number of chromosomes.

b) For the activation of separase, the securin has to be eliminated. The purpose of APC is to ubiquitinate securin and bring about its degradation. After the degradation of securin, the active separase is released which can cleave Scc1 and allow the separation of sister chromatids.