Module 5
1. Module 5
1.22. Page 2
Module 5—Cell Division: The Processes of Mitosis and Meiosis
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To understand common chromosome disorders, you will review the normal formation of human gametes.
Spermatogenesis
In Unit B you examined how the male gamete is created in the seminiferous tubules of the testicles. Diploid germ cells known as spermatogonia can either divide by mitosis for growth, repair, and replacement, or divide by meiosis to create four haploid sperm cells (each with only 23 chromosomes). Read from page 568 to the end of “Spermatogenesis” on page 569 of your textbook. You should make summary notes, a flow chart similar to “Figure 16.16,” a diagram, or any other form you choose to summarize this information for your course folder.
The process of creating sperm is known as spermatogenesis. Once puberty is reached, tens to hundreds of millions of sperm are produced per day.
Watch and Listen
Review the meiotic process of sperm formation in the “Spermatogenesis” animation. Note the difference between the spermatogonium, the spermatocyte, the spermatids, and the sperm.
spermatogonium: the initial diploid germ cell in spermatogenesis; undergoes mitosis, forming many primary spermatocytes that each carry out meiosis
spermatocyte: cells undergoing spermatogenesis; primary spermatocyte undergoes first meiotic division, forming two secondary spermatocytes; each secondary spermatocyte undergoes second meiotic division, forming two spermatids; four spermatids formed from one primary spermatocyte
spermatids: haploid cells that result from meiosis; swim to epididymis for maturation to a spermatazoan and storage until ejaculation
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Oogenesis
Gamete creation in either sex occurs in the gonads and results in the reduction of the chromosomal number. However, gamete creation in the female gonads, the ovaries, differs in two important ways. First, the two meiotic divisions have unequal cytokinesis, resulting in one larger cell and three small cells that deteriorate and are reabsorbed. The small cells are called polar bodies. Since meiosis involves two chromosome separation events, oogenesis results in one large haploid (23 chromosomes) egg cell and two or three smaller haploid polar bodies, depending on whether or not the first polar body undergoes another mitotic division. Polar bodies are not viable gametes; therefore, they degenerate and are reabsorbed by the body.
The second key difference is timing. In males, spermatogenesis starts at puberty, and the process is very rapid. In females, oogenesis starts before birth, but freezes or stops at prophase I. Oogenesis remains at this stage until puberty. At the onset of puberty, one egg or primary oocyte will continue through meiosis I, and then is released into the fallopian tubes for fertilization. This is the egg cell or secondary oocyte that is matured each month as part of the menstrual cycle you studied in Unit B.
If the egg is fertilized, then it will complete meiosis II, and the final reduced nucleus will fuse with the sperm nucleus to start a new life. Read “Oogenesis” on pages 569 and 570 of your textbook. You should summarize this information in a chart, a flow chart similar to “Figure 16.16” on page 569, a labelled diagram, or into summary notes. Store your work in your course folder.
polar body: a tiny cell that results from each division of oogenesis
Meiosis I results in large secondary oocyte and a tiny polar body that is reabsorbed. Meiosis I results in a huge ootid and a tiny second polar body that is reabsorbed. Polar bodies are “garbage cans” that extra nuclear material is dumped into to fulfill the need for one large haploid ovum.
oocyte: a cell undergoing oogenesis
The primary oocyte undergoes meiosis I, producing a large secondary oocyte and the first polar body, which is reabsorbed. The secondary oocyte undergoes meiosis II, forming a large ootid and a second polar body, which is reabsorbed.
Watch and Listen
The video “Meiosis and Gamete Formation: Creating New Genetic Combinations” is an excellent review of the concepts of this module; however, concentrate on the sections “Spermatogenesis and Oogenesis” and “Gametogenesis.” Notice how both result in the reduced chromosome number needed for fertilization, yet the timing and process of oogenesis is more complex and is characterized by long periods of rest or waiting.
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Your understanding of meiosis and gamete creation will help you understand how errors could occur in the process. Sometimes homologous chromosomes or sister chromatids fail to separate when forming haploid gametes. This is known as nondisjunction. Nondisjunction leads to gametes with either too many or two few chromosomes. Read pages 567 to 568 and examine “Figure 16.15” in your textbook. It compares nondisjunction in meiosis I to nondisjunction in meiosis II. Which one leads to more gametes with an abnormal number of chromosomes?
Try This
Recall from Lesson 1 that chromosome counts are done by creating a karyotype of cells undergoing mitosis. From this picture of cell chromosomes, you can determine how many of each type of chromosome there are and the gender of the person being tested. Some common disorders that result from nondisjunction are Down syndrome (trisomy 21) or Edward syndrome (trisomy 18). Trisomy means “three.” In each of these examples, there is an extra chromosome present. Syndromes are conditions with specific characteristics that can vary widely in severity.
TR 1. For practice analyzing karyotypes and diagnosing the syndrome present, complete “Lab 16.A: Modelling a Karyotype” on page 554 of your textbook. Ask your teacher for a sample karyotype. After you have completed the “Analysis” questions and "Conclusion," compare your results with a fellow student or discuss your work with your teacher.