Meiotic Cell Cycle Control by Mos in Ascidian Oocytes

Abstract

Mitotic cell division cycle is regulated by several control mechanisms generally known as checkpoints, whose main function is to ensure that critical events in the cell division such as DNA replication and chromosome segregation occur with high fidelity and in the correct order and time. A recent view of cell cycle regulation, indicate that checkpoints work as signal transduction pathways with their initiating signals, sensors, transducers, and effectors (Elledge, 1996). Similarly, meiotic cell cycle regulation have specific checkpoints and two of them have been recently characterized at molecular level. The first ensures the completion of recombination before the formation of meiosis I spindle; the second blocks anaphase I until all the paired chromosomes are correctly attached to the spindle (Page and Orr-Weaver, 1997). However, the most intriguing difference between mitosis and meiosis respect to cell cycle regulation, is the ability of animal oocytes to arrest at specific stage during maturation, and maintain this block for extremely long time: from years (frog) to decades (human) (Sagata, 1996a,b). It is universally accepted that the meiotic block is due to the activity of a cytostatic factor (CSF) primarily identified by Masui (Masui and Markert, 1971).