Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Reading: The Cells of Genetic Continuity Revised by IHS What, exactly, do we get from our parents? And how does it get from them to us? The legacy given to us by our parents is their chromosomes. Their chromosomes come to us packaged in their gametes. To understand sexual reproduction, we need to explore where gametes originate, how they develop, and what their purpose is. During development of an organism, cells take on special functions in addition to their “housekeeping” functions of metabolism. Muscle cells make proteins that enable the cells to expand and contract. Brain cells make proteins that enable them to communicate by chemical and electrical interactions. Similarly, certain cells in the reproductive organs (ovaries or testes) become sex cells and develop the capacity to carry out the reproductive functions. Males develop sperm cells; females develop egg cells. Within these cells is all the information needed to create the next generation. complete set of unpaired chromosomes. The cytoplasm of the oocyte is distributed unequally. This results in one large cell and one smaller cell. In Meiosis II, the two chromatids of each chromosome separate and cell division takes place. This results in three small cells and one large cell; each cell contains one set of unpaired chromosomes. The large cell matures to become the ovum or egg. It contains one copy of each chromosome of the woman and is ready to be fertilized by the sperm. Thus, during maturation of the egg the chromosome number is reduced from 46 to 23. This is the haploid number in humans. What do these sperm and egg cells look like? How do they form? All human body cells, except sex cells, contain 46 chromosomes. Meiosis is the process that produces sex cells. This process reduces the chromosome number to half. The female reproductive cell (ovum or egg) develops by a process called oogenesis. At birth, a human female contains about 400,000 primary oocytes in her ovary. These oocytes contain 46 chromosomes (the diploid or full chromosome number) or 23 pairs. This is the same number of chromosomes as every other cell in the body. Maturation of the primary oocyte involves several steps as illustrated in Figure 1. Before meiosis, the cell contains a complete set of paired or homologous chromosomes. Then, the chromosomes replicate; they form two chromatids in each chromosome. In Meiosis I, the homologous chromosomes separate and the oocyte divides. This results in two cells; each cell contains one At monthly intervals after puberty, one egg is released from the ovary (ovulation) and makes its way down the Fallopian tube to the uterus (see Figure 2). If the egg is fertilized, it will attach to the uterine wall and develop. Otherwise, it will travel down the vaginal canal and be released in the monthly menses (menstruation). During a woman’s reproductive life, approximately 400 eggs will make this journey. Fig. 3. Male reproductive organs, viewed in mid-sagittal section. In other words, we see the left leg only. Fig. 2. Female reproductive organs, viewed in mid-sagittal section. In other words, we see the left leg only, as well as the left ovary and left Fallopian tube only. A similar process of chromosome reduction or meiosis takes place during sperm development or spermatogenesis. The primary spermatocyte also has 46 chromosomes. Development takes place in the male sex organ, the testis, and involves several steps as shown in Figure 1 (on front side of handout). The cell contains a complete set of paired or homologous chromosomes. Prior to meiosis, the chromosomes replicate; they form two chromatids in each chromosome. In Meiosis I, the homologous chromosomes separate, and the spermatocyte divides. This results in two cells; each cell contains one complete set of unpaired chromosomes. In Meiosis II, the two chromatids of each chromosome separate and cell division takes place. This results in four cells; each cell contains one set of unpaired chromosomes. Each of these cells then matures to become sperm. Each sperm contains a complete set of unpaired chromosomes (23) of the man, a compacted head, and a long powerful flagellum (tail) that enables the sperm to move. In the female, only one mature egg is produced for each oocyte. But a primary spermatocyte will produce four mature sperm. After maturation is complete, sperm leave the testes and travel through a system of ducts or tubules. These ducts produce fluids to help move the sperm to the opening of the penis where they are released (see Figure 3). After puberty, males continually produce vast numbers of sperm— approximately 200 million per day. When egg and sperm meet and fuse in the process called fertilization, a new kind of cell is formed. This cell is called a zygote. The zygote contains a full complement of 46 chromosomes. It is the starting place for the development of a complex, multicellular organism.