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At the beginning of each menstrual cycle , FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia.
The process of meiosis in females occurs during oogenesis , and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes.
References and Recommended Reading
In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes , which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis.
Mitosis, Meiosis, and Inheritance
This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. In female mammals , meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo.
Some studies suggest that retinoic acid derived from the primitive kidney mesonephros stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid. The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction.
This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy.
Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis. Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic.
Medical conditions include but are not limited to:. The probability of nondisjunction in human oocytes increases with increasing maternal age,  presumably due to loss of cohesin over time. Alongside with the variations of meiosis related to the moment when meiosis occur in life cycles, resulting in post-zygotic, pre-gametic and intermediate meiosis see above , the number of nuclear divisions in meiosis is also variable.
The majority of eukaryotes have a two-divisional meiosis though sometimes achiasmatic , but a very rare form, one-divisional meiosis, occurs in some flagellates parabasalids and oxymonads from the gut of the wood-feeding cockroach Cryptocercus. In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis. From Wikipedia, the free encyclopedia. This is the latest accepted revision , reviewed on 26 August For the figure of speech, see Meiosis figure of speech. For the process whereby cell nuclei divide to produce two copies of themselves, see Mitosis.
For excessive constriction of the pupils, see Miosis. For the parasitic infestation, see Myiasis. For muscle inflammation, see Myositis. Play media. Main article: Origin and function of meiosis. Main article: Biological life cycle. Main article: Nondisjunction. Biological Science 6th ed. Hoboken, NY: Pearson. Retrieved 23 July Nature Reviews. Meiosis and mitosis: a terminological criticism.
CELL DIVISION: Meiosis
Ann Bot Rome — Retrieved The Journal of Heredity. Frontiers in Plant Science. Bibcode : Sci Principles of Genetics 5th ed.
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Differences Between Mitosis and Meiosis
Bibcode : PLoSO Europ J Protistol. Public Broadcasting Service. Retrieved 6 December Some structural and physiological aspects of meiosis in algae, fungi, and protozoa". Bacteriological Reviews. Sex portal Biology portal. Cell cycle proteins. G 1 phase S phase G 2 phase. Restriction point Spindle checkpoint Postreplication checkpoint.
Apoptosis G 0 phase Meiosis.
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