Gamete formation

This is all about how a gamete is formed including the 2 Classifications of Gametes.

1. Gamete Formation

2. Gamete A gamete is a sex cell; a haploid egg or sperm. It is one of the differentiated cells that is a product of meiosis. The union of two gametes of the opposite sex (fertilization) produce a zygote.  

3. Gamete Formation Gametogenesis is a  biological  process by which  diploid  or  haploid precursor cells  undergo  cell division  and differentiation to form mature haploid  gametes . Depending on the biological life cycle of the organism, gametogenesis occurs by  meiotic  division of diploid gametocytes into various gametes, or by  mitotic  division of haploid gametogenous cells. For example,  plants  produce gametes through mitosis in gametophytes . The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as  alternation of generations .

4. A.  Gonial cells are the designation given to germ cells before they enter meiosis. They replicate mitotically; hence they are genetically identical to the original zygote. When they begin meiosis, they are called spermatocytes and  oöcytes . 1. In mammals, female gonial cells ( oögonia ) expand in number during embryonic development. At approximately 3 months gestation, all the oögonia become oöcytes , indicating the beginning of the meiotic cycle.2. In mammalian males, the gonial cells expand in number during the prepubertal period. Beginning at puberty, the  spermatogonia  behave as stem cells, i.e. at the end of each mitosis, one product becomes a spermatocyte and the other remains a spermatogonial cell, capable of additional mitoses. 3. Mature haploid germ cells, either male or female, are called gametes.

5. B.  Oögenesis (mammals) 1. Meiosis begins simultaneously in all primary oöcytes  in late embryonic/early fetal period, proceeding to late prophase I.2. All oöcytes then go into a resting phase until an ovarian follicle develops further. 3. At that point, meiosis resumes. The mature ovum that is released is a secondary oöcyte . 4. In telophase I, one of the meiotic products becomes a polar body, with very little cytoplasm; the other product receives virtually all the cytoplasm. 5. If fertilization occurs, MII (meiosis II) is completed, with formation of a second polar body. 6. Only one of the four haploid products is functional. 7. In female embryos, there are several million ovarian follicles; at birth, only 2 million; only 400 mature during lifetime.

6. C. Spermatogenesis 1. Meiosis in males is a continual process from puberty until death.2. When spermatogonia reproduce, one daughter cell remains a stem cell and the other becomes a spermatocyte, ready to undergo meiosis. 3. At the completion of meiosis, there are four haploid spermatids, which resemble other cells in having cytoplasm and nucleus. 4. By the process of  spermiogenesis , the spermatids change their physical structure to become mature spermatozoa (sperm). This involves getting rid of cytoplasm and developing an  acrosomal body and tail. The time required is about 50 days from primary spermatocyte to spermatozoon. 5. All four haploid products become functional sperm. 6. Ca. 200-500 million sperm/ejaculate several times per week; ca. 2-5 trillion per lifetime.

7. D.  Fertilization -the joining of a sperm and an ovum to form a diploid zygote, will be covered in a later lecture.