2. What type of cell division permits sexual reproduction? What is gametogenesis?
Meiosis is the type of cell division that allows sexual reproduction, since it reduces the number of chromosomes of the species to a half, making the combination of two gametes to form a new individual possible. (In some organisms, meiosis creates haploid gametophytes that by means of mitosis generate gametes. Even in this case, the function of meiosis is the same: to provide cells with half of the number of chromosomes of the species, with the separation of homologous.)
Gametogenesis is the name given to the process of gamete production.
- Gametogenesis Review - Image Diversity: meiosis
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Gonads and Germ Cells
3. What is the name of the cells capable of making gametes? What is the ploidy of these gamete-forming cells?
The cells that form gametes are germ cells, as opposed to somatic cells. The ploidy (number of chromosomes) of germ cells is the same as somatic cells (only during the formation of gametes does meiosis occur, reducing the number of chromosomes to half).
- Gametogenesis Review - Image Diversity: germ cells
4. What are gonads? What are the male and the female gonads in humans?
Gonads are the organs that produce gametes. They contain germ cells that undergo division and generate gametes. In males, the gonads are the testicles. In females, the gonads are the ovaries.
- Gametogenesis Review - Image Diversity: gonads
5. Indicating the name and respective ploidy of each cell involved, how can the formation of sperm cells from germ cells be described?
The formation of sperm cells, or spermatogenesis, begins with a germ cell called the spermatogonium (2n), which undergoes mitosis and gives birth to the spermatocyte I (2n). The spermatocyte I undergoes meiosis I and generates two spermatocyte II (n) cells, which then undergo meiosis II and produce four spermatids (n). Each spermatid undergoes a maturation process called spermiogenesis and four sperm cells are produced.
6. What is the difference between spermatogonium and spermatocyte I cells?
The male germ cells are spermatogonia (diploid cells, 2n),which are located in the testicles. They mature and by means of mitosis give birth to spermatocytes I (2n), which will undergo meiosis.
7. What is the difference between spermatocyte I and spermatocyte II cells?
The spermatocyte I (2n) undergoes the first division of meiosis (meiosis I), producing two spermatocyte II (haploid, n) cells.
8. What is the difference between spermatocyte II and spermatid cells?
Spermatids (n) are the products of the second division of meiosis (meiosis II) during male gametogenesis. Each spermatocyte II produces two spermatids, totaling four spermatids for each spermatocyte I that undergoes meiosis.
9. What is the difference between spermatids and sperm cells? What is the name given to the transformation of spermatids into sperm cells?
Sperm cells (male gametes) are mature spermatids that have already undergone differentiation (the appearance of the flagellum, the reduction of the cytoplasm, the formation of the acrosome, the increase in the number of mitochondria). This differentiation process is called spermiogenesis.
10. What is the acrosome of the sperm cell? How is it formed?
The acrosome is a structure that contains a large number of digestive enzymes. It is located at the anterior end of the sperm cell and is formed through the union of Golgi apparatus vesicles. The function of the acrosome is to release its enzymes when the sperm cell meets the egg cell to break the external covering of the female gamete, thus making fertilization possible.
11. What is the function of the flagellum of the sperm cell? How is it formed?
The flagellum of the sperm cell is formed of the centrioles that migrate to the region posterior to the nucleus. Its function is to promote locomotion towards the egg cell.
12. Why is the cytoplasm of sperm cells very small? Why do the mitochondria of sperm cells concentrate at the base of the flagellum?
The reduced cytoplasm of sperm cells decreases the cell weight and provides a more hydrodynamic shape for its locomotion in fluids.
The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for supplying energy to the flagellum (for it to vibrate and move the sperm cell).
13. Concerning events during the periods of life, how different is gametogenesis in women and in men?
The formation of spermatogonia in men takes place during the embryonic period. However, the formation of sperm cells is a continuous process that begins in puberty and goes on until old age, and sometimes during the whole life of the man.
In women, all oogonia are formed before birth. The oogonia turn into oocytes I, which enter the first division of meiosis (meiosis I). However, this division is interrupted at prophase and continues only in puberty. After the beginning of menses, an egg cell is released during each period and, if fertilized, it finishes its meiotic division. Oogenesis stops after menopause (cessation of menstrual activity) and the climacteric period of life begins.
14. Indicating the name and respective ploidy of each cell involved, how can the formation of egg cells from germ cells be described?
The formation of egg cells begins with a germ cell called an oogonium (2n), which undergoes mitosis and gives birth to the oocyte I (2n). The oocyte I undergoes meiosis I, but this is interrupted at prophase. After puberty, during each menstrual cycle, an oocyte I finishes meiosis I and generates one oocyte II (n) and the first polar body (n). With fertilization, the oocyte II then undergoes meiosis II and produces the mature egg cell (n) and the second polar body (n).
- Gametogenesis Review - Image Diversity: oogenesis
15. What is the first polar body? How different is it from an oocyte II?
In oogenesis, the oogonium differentiates into an oocyte I (2n) and this cell undergo meiosis. After finishing the first meiotic division (meiosis I), the oocyte I forms two cells: the oocyte II (n) and the first polar body. The oocyte II is bigger because it receives almost all the cytoplasm and the cytoplasmic structures of the oocyte I as a strategy for metabolite and nutrient storage. The oocyte II cell then undergoes the second meiotic division. The first polar body is very small and has almost no cytoplasm; it either disintegrates or stays attached to the oocyte II.
16. What is the relationship between fertilization and the end of the meiotic process during oogenesis?
The oocyte II only completes the second meiotic division (interrupted at metaphase) if fertilization by a male gamete occurs. (Therefore, it can be said that the female gamete is the oocyte II).
17. What is the second polar body?
After the end of the second meiotic division of the oocyte II, two cells are generated: the egg cell and the second polar body. The second polar body is a very small cell that has almost no cytoplasm and which stays adnexal to the egg cell. The entire cytoplasmic content of the oocyte II passes on to the egg cell.
18. What is the relationship between the menstrual cycle and ovulation?
Ovulation is the releasing of the female gamete from the ovary. Ovulation is a periodical event that occurs during each menstrual cycle. Considering the day when menses begins the first day of the menstrual cycle , ovulation occurs around the 14th day, when the concentrations of the hormones LH and FSH reach high levels.
19. How does the male gamete penetrate the egg cell? How does the female gamete protect itself from the entrance of more gametes after the entrance of the first sperm cell?
The sperm cell that reaches the egg cell first triggers the acrosome reaction, a process in which hydrolytic enzymes of the acrosome are released on the external surface of the zona pellucida (the protective layer that surrounds the egg cell). A portion of this layer is digested by the acrosomal enzymes, allowing the sperm cell to reach the plasma membrane of the egg cell, thus fertilizing it.
At the moment that the sperm cell makes contact with the egg cell membrane, a chemical alteration of this membrane occurs. Enzymes secreted by exocytosis (a cortical reaction) make it impossible for the zona pellucida to bind to other sperm cells (zonal reaction) and, as a result, other male gametes cannot enter the egg cell.
20. What are the female pronucleus and the male pronucleus?
The female pronucleus is the haploid nucleus of the egg cell. the male pronucleus is the haploid nucleus of the sperm cell that has fertilized the egg cell. After fertilization, both pronuclei fuse, forming the nucleus of the diploid zygote.
21. Concerning their size and basic morphology, how and why are male and female gametes different from each other?
Female gametes are large cells full of vitellus (nutritional materials). Male gametes are small, mobile and agile flagellate cells.
These features are related to their respective biological functions. While female gametes have the basic functions of receiving the sperm cell nucleus and storing nutrients for the zygote, male gametes have the function of active movement towards the egg cell.