Unit C: Cell Division, Genetics and Molecular Biology Chapter 17 Section 17.3-2 Gametogenesis

Gametogenesis Gametogenesis – the formation of gametes or sex cells in animals. Gametes are all created by the general process of meiosis. However, there are some differences in the formation of sperm and egg cells.

Spermatogenesis Formation of sperm. Very basic meiosis. The first meiotic division is the splitting of a spermatocyte (spermacytes = plural) (46 chromosomes) into two spermatids (23 chromosomes). The second meiotic division is the splitting of the spermatids into sperm cells. This split does not change the number of chromosomes.

Spermatogenesis spermatid

Oogenesis Formation of eggs. The difference in oogenesis is that the cytoplasm of the female gametes does not split evenly. In the first meiotic division, the primary oocyte (46 chromosomes) divides to form the secondary oocyte (23 chromosomes) and the first polar body. In humans, the first polar body does not undergo a second meiotic division because it dies off due to lack of cytoplasm.

Oogenesis Cont’d The second meiotic division of the secondary oocyte is quite different. It occurs in two steps. As we have learned, in order for the secondary oocyte to divide into the ovum and the second polar body, it must become fertilized. In the first step of meiosis II in oogenesis, the secondary oocyte pauses its division at metaphase II. While it is sitting in metaphase II, half-way through the second division, it is known as an ootid.

Oogenesis Cont’d An ootid is formally known as an unfertilized ovum, because it is halfway between the secondary oocyte and the fertilized ovum. Once sperm becomes present and the ootid is fertilized, the meiotic division will continue on to produce the fertilized ovum and the second polar body. Meiosis II is now complete.

Oogenesis 46 Primary Oocyte Secondary Oocyte 23 23 First Polar Body Ootid 23 pause at metaphase II Fertilized Ovum 46 23 Second Polar Body

INCORRECT TEXTBOOK DIAGRAM Can you see what is wrong with the diagram?

Comparing Plant and Human Life Cycles Certain plants that reproduce sexually have similar life cycles to humans. Both sexually producing plants and humans, have haploid and diploid stages of their life cycle.

Lodgepole Pine Life Cycle

Human Life Cycle