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Gametogenesis

Gametogenesis is a chemical process of formatting the mature germ cells in reproductive organs, in this process immature germs cells are converted into mature gametes (sperm or oocyte), the final gametes that appear are used for fertilization.
A special type of division appears in gametogenesis which is the meiosis in which chromosomes’ number decreases, it is reduced from diploid to haploid number.
The gametogenesis process:
Basically, when in males gametogenesis consists of two processes, spermatogenesis in males and oogenesis in females. In spermatogenesis, spermatogonia (2n), which are the primordial germ cells go through meiosis and finally produce 4 sperms.
In females, there is the oogenesis, the primordial germ cells are the oogonia (2n) that undergoes meiosis (oogenesis) in order to produce 1 ovum and 3 polar bodies, however, the produced polar bodies are not able to be fertilized.
Once both processes finish, fertilization happen between 1 sperm and 1 ovum and they form a zygote and the zygote eventually develops into an embryo.
Cells of both genders would die if they didn’t get the opportunity to fertilize.

Spermatogenesis
Spermatogenesis is a process in which spermatogonia (2n) the primordial germ cells is used to produce sperms.
In the first step of the spermatogenesis process, the primordial germ cell which is spermatogonia (2n) undergoes mitotic divisions, in result of that division, two types of daughter cells are produced, type A and type B spermatogonium.
* Type A (2n) remains out of blood-testis barrier and will produce daughter cells until it dies.
Type B (2n) differentiates into primary spermatocyte which undergoes meiosis I and divides into two secondary spermatocytes (1n).
Secondary spermatocytes then undergo meiosis II and produce four spermatids (1n) in total.
Finally, spermatid undergoes spermiogenesis which is the transformation into spermatozoon. The final result of the whole process ends with four mature cells (1n) that consist of an acrosome and a nucleus within the head, a middle piece and a tail.

Oogenesis
Oogenesis is the process of formation and maturation of the egg which is frequently called the ovum, it occurs in the female gonads or ovaries. Oogenesis begins before birth, is completed after puberty and continues up to menopause.
There are two million primary oocyte (ovums) at birth, only 40,000 of them remain during adolescence and 40 of them get ovulated and become secondary oocytes.
The first step of the oogenesis process is the mitosis of oogonia ?(2n) diploid ovarian stem cells in order to produce more oogonia that would differentiate into primary oocytes.
In the second step, primary oocytes start to duplicate the DNA in all of the chromosomes (46) and remain together as a replicated chromosome that pairs up and crosses over in order to exchange DNA.
After the DNA duplication, actions are stopped and primary follicles stay inactive until primary oocyte completes meiosis I and becomes a secondary oocyte (haploid set 1n).
The secondary oocyte completes meiosis II when it gets fertilized by a sperm (1n). The final product of meiosis II is the ovum and the second polar body.
Then, the nucleus of one daughter cell fuses with one sperm nucleus to produce the zygote (2n) and the second daughter cell becomes the second polar body which degenerates eventually.

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