An exam question asked what stage of meiosis corresponds with "first meiotic arrest of oogenesis". I can't work out the answer from the wiki page - can anyone explain which step this refers to?
When females are in their mother's womb all their Oogonium (plural Oogonia) are being made, they they undergo mitotic divisions to become a primary oocyte. Then, the primary oocytes start to undergo meiosis I - but meiosis I is arrested. This is the first meiotic arrest. Meiosis I continues when a female hits puberty, every month one or more primary oocytes undergo meiosis I and create secondary oocytes. These secondary oocytes will then start the second meiotic division, but this second meiotic division is arrested as well. It is arrested for about 24 hours, waiting for a sperm to fertilise it in the Fallopian tubes. The only time an egg (or in medical terms - secondary oocyte) will complete meiosis II is if a sperm penetrates the egg, and then it is called an ovum, which develops into a zygote and a then foetus.
Therefore, the answer to your question will be - the first meiotic arrest in oogenesis will occur from the time the primary oocytes are created in the womb, until a female reaches puberty. starting puberty until menopause every month one or more primary oocytes will mature and complete the first meiotic division.
Meiosis consists of two divisions. Both are somehow similar to "ordinary" type of cell division - mitosis, but there is no DNA replication between them. As mitosis, each of two meiosis divisions might be divided into 5 stages:
- Profaze (condensation of chromosomes, formation of microtubular spindle apparatus between two centrosomes)
- Prometaphase (the nuclear membrane desintegrate and chromosomes and microtubules attach to kinetochores of chromosomes)
- Metaphase (The chromosomes align at the metaphase plate)
- Anaphase (segregation of chromosomes)
- Telophase (restoration of nuclear membrane, cytokinesis)
The first division is crucial for reducing number of chromosomes. The prophase is subdivided to following stages:
- Leptotene (condensation of chromosomes)
- Zygotene (homologous chromosomes form tetrads )
- Pachytene (crossing over)
- Diplotene (chromosomes separate a little, sister chromosomes remain bounded in chiasmata)
- Diakinesis (futher condensation of chromosomes, the nuclear membrane desintegrates)
In animals the function of meiosis is producing gametes. In case of oogenesis (in human) this process starts during prenatal development. Between 12-th and 25-th week of female fetus development cells called oogonia become primary oocytes and enter meiosis, but become arrested in stage of dictyotene, which is prolonged diplotene of prophase of first division. Then they wait 12 up to 50 years (too long waiting increase risk of chromosomal disjunction disorders such as Down syndrome in children of old mothers), covered by a layer of cells - primordial follicle. At the beginning of each menstruation cycle one or a few of the follicles become growing (the follicle cells divide, but oocyte is still waiting). At the stage of Graafian follicle the follicle is about 75 times bigger and there is a cavity full of liquid between oocyte and surrounding cells. The hormone signal - LH from pituitary gland - brakes the meiosis block, the primary oocyte divide into secondary oocyte and small first polar body (which quickly degenerates). At this time the follicle breaks - its ovulation. The oocyte starts second division, but stops at the metaphase. This second arrest ends when sperm cell penetrates into oocyte. The second, small polar cell is released and haploid nuclei of ovum and spermatozoon fuse together.