Question

6. Explain the frog oocyte experiment and MPF.

7. Explain the yeast cdc mutants.

Note: if you're doing only 1 please choose 6.

Answer 1

Ans 6.

The Identi?cation of the factor that induces mitosis came from studies of oocyte maturation in the frog Xenopus laevis. During oocytes development in the ovary, oocytes In the Xenopus laevis arrest in the G2 stage of the cell for up to 8 months, and are then induced by the steroid hormone progesterone to enter mitosis I. The oocytes then undergo a second arrest at the metaphase of meiosis II, where they remain until fertilization.

In 1971. Yoshio Masui and Clement Markert found that oocytes in the frog Xenopus laevis arrested in G2 phase could be induced to enter meiosis I by microinjection of cytoplasm from oocytes that had been stimulated with hormone progesterone. It thus appeared that a cytoplasmic factor present in hormone-treated oocytes was suf?cient to trigger the transition from G2 to meiosis I in oocytes that had not been exposed to hormone. Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. Following its discovery in frog oocytes, MPF was also found to be present in somatic cells, where it induces the G2 to M transition of mitosis. Thus, MPF appeared to be a general regulator of entry into the M phase of both mitotic and meiotic cell cycles. The eventual purification of MPF from frog oocytes was found to be a dimer of cyclin and CDK. The discovery of MPF m frog oocytes thus paved the way to understanding a cell cycle regulatory apparatus that is conserved throughout all eukaryotes.