Frog oocyte and egg have been widely used for the study of shift from mitotic cell division to meiotic cell division. The major events from immature oocyte (primary oocyte) to the early mitotic cell divisions in the zygote are shown in Figure 8.8. The cell cycle remains arrested at G2 in interphase before meiotic division and secretion of progesterone (extracellular) by the follicle cells induces the oocyte to proceed rapidly through meiosis I and prophase of meiosis II, before arresting at metaphase of meiosis II. The immature eggs were also shown to produce MPF described earlier for mitotic cell division, so that MPF
activity (discovered initially in egg for meiosis) is needed for mitosis as well as meiosis, and is associated with metaphase state. It was also shown that the rise in MPF activity was dependent on protein synthesis, although it was not clear whether the newly synthesized component was MPF itself or its activity.
Biochemistry of chromosome pairing in meiosis
Fig. 8.8. Early frog life cycle; the appearance of frog egg and oocyte and the level of MPF activity are shown during meiosis, fertilization and early development of zygote. Points of cell cycle arrest and the stimuli that release these arrests are also indicated.
the previous topic, we discussed the structure and function of synaptonemal complex and recombination nodules associated with chromosome pairing and crossing over respectively. These structures are morphological manifestations of the mechanism that may be controlling these phenomena, but the study of biochemistry associated with these morphological structures may be more difficult to study. Chromosome pairing, however, is the major distinction between mitosis and meiosis and one would like to know the mechanism responsible for making this distinction. Despite concerted efforts in this direction very little is known about the biochemical basis of chromosome pairing. Further, although we know many mutations (asynapsis, desynapsis, diploidizing genes, etc.) which regulate chromosome pairing, but none of these genes could be isolated or cloned, nor could we isolate the product of any of these genes.
During the last two decades, in Lilium,
however, studies have been conducted on DNA synthesis during the different phases of meiotic cell cycle. It has now been established that a group of single or low copy number DNA sequences (3 x 107
bp) that constitute 0.1 to 0.2% of the genome do not replicate during the premeiotic S-phase. This small fraction of DNA replicates instead at zygotene in co-ordination with chromosome pairing. This is called zyg DNA (or zygotene DNA) and occurs in segments averaging about 4-5 kb (range 2.5-10 kb) in length and is broadly distributed among all the chromosomes. Zyg DNA
replication occurs after the meiotic cells have entered prophase, but is not completed at zygotene or soon after. Single strand gaps remain at the ends of the otherwise replicated zyg DNA segments until sometime after pachytene, when the synaptonemal complex begins to disassemble. Zyg DNA is transcribed into poly (A)+
RNA at a time, when the chromosomes initiate pairing process. This 'zyg RNA' has not been detected in nonmeiotic tissues. Even within the meiocytes, zyg RNA is not detectable prior to leptotene or beyond mid-pachytene. In mouse spermatocytes also, the profile of zyg RNA formation and disappearance is very similar to that of zyg RNA in lily meiocytes. Further, no zyg RNA has been detected in mouse somatic tissues.
In contrast to zyg DNA, there is another group of DNA sequences called pachytene DNA,
which undergoes a programmed repair replication, once pairing is complete.