The Reproductive System Intro # Overview of Sexual Reproduction

Overview of Sexual Reproduction Animals expend a considerable amount of energy and time on reproduction. After all, reproduction ensures that the animal's genes are passed on to the next generation. The life cycle of any particular animal comes to an end, but its genes can be perpetuated as long as reproduction has taken place. Usually animals practice sexual reproduction, which involves sex cells (gametes). The gametes may be specialized as eggs (oocytes) or sperm (spermatocytes), and even if egg and sperm are produced by the same individual, animals usually practice cross-fertilization. When animals reproduce sexually, an offspring inherits half its genes from one parent and the other half from the other parent. Therefore, an offspring has a different combination of genes than either parent. In this way, variation is introduced and maintained. Such variation is an advantage to the species if the environment is changing, because an offspring might be better adapted to the new environment than is either parent. Animals usually produce gametes in specialized organs called gonads. The gonads are testes for the production of sperm and ovaries for the production of eggs. These gametes are produced by germ cells, cells set aside early in development for this specific purpose. The other cells in a gonad support and nourish the developing gametes or produce hormones necessary to the reproductive process. Copulation is sexual union to facilitate the reception of sperm by a female. In terrestrial animals, males typically have a penis for depositing sperm into the vagina of females. When gametes mature the cell divides two times. The first division of the gametes provides two new cells that have the whole set of chromosomes. This is known as mitosis. The second division is known as meiosis. Meiosis only occurs in reproductive cells, unlike mitosis which occurs in all cells. The second time the gametes divide they form four gametes with only half of the chromosomes that a regular cell contains. Human cells typically contain 46 chromosomes; however, in gametes there are only 23 chromosomes. Then when a sperm gamete and an egg gamete combine to form the zygote, or fertilized egg, it has a total of 46 chromosomes. After fertilization, the zygote continues to divide by mitosis developing into a human baby. These 46 chromosomes determine the characteristics of the offspring such as eye color and skin color. Gender is also determined by the chromosomes. Males have one X chromosome and one Y chromosome; whereas, females have two X chromosomes. Therefore the gender of a couple’s offspring is determined by the father’s sperm cell (whether the offspring receives an X or Y chromosome from the sperm cell). Problem Set 1: 1. What is the general term used to describe animal sex cells? What are the specific male and female sex cells called? 2. How does it happen that an offspring has a different combination of genes than either parent? 3. What advantage is produced by an offspring having a different combination of genes than either parent? 4. What is the general term used to describe the organs in which gametes are produced? What are the names of the specific male and female gamete producing organs? ...........................................continued.............................................