Module 5 Cell Division

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In order to consider common chromosome disorders in people, we will first look at the formation of human gametes.

Spermatogenesis

Recall from your studies in unit B that the male gamete is created in the testicles. Here, in the seminiferous tubules, diploid germ cells known as spermatogonia can either divide by mitosis for growth, repair, and replacement, or they divide by meiosis to create four haploid sperm cells (each with only 23 chromosomes). The process is outlined well in your text on page 569. As you read, you should make summary notes, a flow chart, 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, and it occurs in males at around the rate of tens, or hundreds of millions of sperm produced per day once puberty is reached.

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Oogenesis

Gamete creation in females is similar to that in males in regards to its location, the female gonad, which is the ovary, and in the reduction of the chromosomal number. However, it also differs in several key 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 three small polar bodies, which are also haploid.

Another key difference is timing. In males, spermatogenesis starts at puberty, and the process is very rapid. In females, oogensis starts before birth, but freezes or stops at prophase 1. Oogenesis remains at this stage until puberty. At the onset of puberty, one egg or primary oocyte will continue through meiosis 1, 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 2, and the final reduced nucleus will fuse with the sperm nucleus to start a new life. The whole process is summarized in figure 16.16 on page 569 in your text. You should summarize this information in a chart, a flow chart, a labeled diagram or into summary notes for future reference when you are studying. Store your work in your course folder.

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Now that we’ve reviewed meiosis and gamete creation, you should better 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. Examine figure 16.15 in your text on p. 567. It compares nondisjunction in meiosis I to nondisjunction in meiosis II. Which one leads to more gametes with an abnormal number of chromosomes?

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Recall from Lesson 1 that chromosome counts are done by creating a karyotype of cells undergoing mitosis. From this picture of a cells 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 non-disjunction are Down syndrome (trisomy 21) or Edward syndrome (trisomy 18). Trisomy means three, so in each of these cases, there is an extra chromosome present. Syndromes are conditions with a multitude of specific characteristics that can vary widely in severity.

For practice in analyzing karyotypes and in diagnosing the syndrome present, complete Lab 16.A. on page 554 of your text. Ask your instructor 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 instructor.