Gametogenesis Made Easy
Easily Understand Gametogenesis
Gametes are cells specialized in sexual reproduction. They contain half of the maximum number of chromosomes of the species and unite with another gamete giving birth to a zygote with double of the number of chromosomes of the gametic cells.
2. What is the type of cell division that allows sexual reproduction? What is gametogenesis?
Meiosis is the type of cell division that allows sexual reproduction since it reduces to a half the number of chromosomes of the species making possible the combination of two gametes to form a new individual. (In some beings 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 separation of the homologous.)
Gametogenesis is the name given to the process of gamete production.
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 the germ cells as opposed to the somatic cells. The ploidy (number of chromosomes) of the germ cells is the same as the somatic cells (only during the formation of gametes meiosis occurs and the number of chromosomes is reduced to half).
4. What are gonads? What are the male and the female gonads in humans?
Gonads are the organs that produce gametes. They contain the germ cells that undergo division and generate gametes. In males the gonads are the testicles. In females the gonads are the ovaries.
5. Indicating the name and respective ploidy of each involved cell 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 spermatogonium (2n) that suffers mitosis and gives birth to the spermatocyte I (2n). The spermatocyte I undergoes meiosis I and generates two spermatocyte II (n) that then undergo meiosis II and produce four spermatids (n). Each spermatid undergoes a maturation process called spermiogenesis and four sperm cells appear.
6. What is the difference between spermatogonium and spermatocyte I?
The male germ cells are the spermatogonia (diploid cells, 2n) situated in the testicles. They mature and by means of mitosis give birth to spermatocytes I (2n) that will undergo meiosis.
7. What is the difference between spermatocyte I and spermatocyte II?
The spermatocyte I (2n) undergoes the first division of meiosis (meiosis I) originating two spermatocyte II (haploid, n).
8. What is the difference between spermatocyte II and spermatid?
The spermatids (n) are the products of the second division of meiosis (meiosis II) in the male gametogenesis. Each spermatocyte II originates two spermatids totaling four spermatids for each spermatocyte I that enter meiosis.
9. What is the difference between spermatids and sperm cells? What is the name of the transformation of spermatids into sperm cells?
Sperm cells (the male gametes) are matured spermatids that have already undergone differentiation (appearance of the flagellum, reduction of the cytoplasm, formation of the acrosome, 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 great number of digestive enzymes, it is located in the anterior end of the sperm cell and it is formed by 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 fecundation possible.
11. What is the function of the flagellum of the sperm cell? How is it formed?
The flagellum of the sperm cell is made by 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 reduced? Why do mitochondria of sperm cells concentrate in the base of the flagellum?
The reduced cytoplasm of sperm cells decreases the cell weight and provides a more hydrodynamic shape for the locomotion in fluids.
The high concentration of mitochondria at the base of the flagellum of the sperm cell is necessary for the energetic supply of the flagellum (for it to beat and move the sperm cell).
13. Concerning events during the periods of life how different is the gametogenesis in women and in men?
The formation of spermatogonia in men takes place during the embryonic period. The formation of sperm cells however is a continuous process that begins in puberty and goes on until old age and sometimes during all the remaining life of the man.
In women all oogonia are formed before birth. The oogonia turn into oocytes I that enter the first division of meiosis (meiosis I). This division however is interrupted at prophase and continues only in puberty. After the beginning of menses an egg cell is released during each period and, if fecundated, it finishes the meiotic division. The oogenesis stops after menopause (cessation of the menstrual activity) and the climacteric period of life begins.
14. Indicating the name and respective ploidy of each involved cell how can the formation of egg cells from germ cells be described?
The formation of egg cells begins with a germ cell called oogonium (2n) that undergoes mitosis and gives birth to the oocyte I (2n). The oocyte I undergoes meiosis I that however is interrupted at prophase. After puberty during each menstrual cycle an oocyte I finishes the meiosis I and generate one oocyte II (n) and the first polar body (n). With fecundation the oocyte II then undergoes meiosis II and produces the mature egg cell (n) and the second polar body (n).
15. What is the first polar body? How different is it from the oocyte II?
In oogenesis the oogonium differentiates into oocyte I (2n) and this cell enters 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 gets almost all the cytoplasm and the cytoplasmic structures of the oocyte I as a strategy for metabolite and nutrient storage. The oocyte II cell goes then to the second meiotic division. The first polar body is very small and almost lacks cytoplasm; it disintegrates or stays attached to the oocyte II.
16. What is the relation between fecundation and the end of the meiotic process during oogenesis?
The oocyte II only completes the second meiotic division (interrupted at metaphase) if fecundation by a male gamete occurs. (One can say therefore that in fact the female gamete is the oocyte II).
17. What is the second polar body?
After termination of the second meiotic division of the oocyte II two cells are generated: the egg cell proper and the second polar body. The second polar body is a very small cell that almost lacks cytoplasm and stays adnexal to the egg cell. The entire cytoplasmic content of the oocyte II passes 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 as the first day of the menstrual cycle the day when menses begins, the 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 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 carrying out fecundation.
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 (cortical reaction) make the zona pellucida unable to bind to other sperm cells (zonal reaction) and other male gametes cannot enter the egg cell.
20. What are the female pronucleus and the male pronucleus?
The female pronucleus is the proper haploid nucleus of the egg cell. Male pronucleus is the haploid nucleus of the sperm cell that has fecundated the egg cell. After fecundation both pronuclei fuse forming the nucleus of the diploid zygote.
21. Concerning their size and basic morphology how and why do the male and the female gametes differentiate from each other?
The female gametes are big cells full of vitellus (nutritive material). The male gametes are small, mobile and agile flagellate cells.
Those features are related to their respective biological functions. While the female gametes have the basic functions of receiving the sperm cell nucleus and of storing nutrients for the zygote, the male gametes have the function of active movement towards the egg cell.
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