Interest Grabber Section 11-4
How Many Chromosomes? Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. Working with a partner, discuss and answer the questions that follow.
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Interest Grabber continued Section 11-4
1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?
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Section Outline Section 11-4
11–4
Meiosis A. Chromosome Number B. Phases of Meiosis 1. Meiosis I 2. Meiosis II C. Gamete Formation D. Comparing Mitosis and Meiosis
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Chromosome Number •Organisms get two sets of chromosomes, one from the father, one from the mother. •These two sets of chromosomes are called homologous. •A cell that contains both sets of chromosomes is called diploid, or 2N. •A cell that contains a single set is called haploid, or N. •Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Go to Section:
Steps of Meiosis •Meiosis I – Interphase – Prophase I – Metaphase I – Anaphase I – Telophase I
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•Meiosis II – Prophase II – Metaphase II – Anaphase II – Telophase II – Cytokinesis
Interphase •Cell grows in size. •Organelles double in number. •DNA replicates.
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Prophase I
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• DNA forms chromosomes. • Nuclear membrane and nucleolus disappear. • Chromosomes line up next to its homologue. • The homologues twist around each other to form a tetrad.
Prophase I •Once twisted around, the chromosomes can exchange parts of the chromotids. •This process is called crossing-over.
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Crossing-Over Section 11-4
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Crossing-Over Section 11-4
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Crossing-Over Section 11-4
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Metaphase I •The tetrads move to the center of the cell by the spindle fibers.
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Anaphase I •The homologue pairs of chromosomes separated by the contraction of the spindle fibers.
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Telophase I •Cytoplasm divides forming two daughter cells.
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Meiosis II •Each new daughter cell goes through the process of meiosis II. •In meiosis II, the cells do not undergo an interphase.
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Prophase II Nuclear membrane disappears Spindle fibers begin to develop.
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Metaphase II
•Chromosomes line up on the middle of the cell.
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Anaphase II •The centromere divides in half and the two chromatids separate. •They move toward opposite sides of the cell.
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Telephase II •Spindle fibers disappear. •Nuclear membrane reappears.
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Cytokinesis •The cytoplasm divides equally between the two cells. •The final result of meiosis is four new cells with exactly one half of the number of chromosomes.
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Figure 11-15 Meiosis Section 11-4
Meiosis I
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Figure 11-15 Meiosis Section 11-4
Meiosis I
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Figure 11-15 Meiosis Section 11-4
Meiosis I
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Figure 11-15 Meiosis Section 11-4
Meiosis I
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Figure 11-15 Meiosis Section 11-4
Meiosis I
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Figure 11-17 Meiosis II Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two The chromosomes line up in a The sister chromatids haploid (N) daughter cells, similar way to the metaphase separate and move toward each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original.
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Telophase II Meiosis II results in four haploid (N) daughter cells.
Figure 11-17 Meiosis II Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two The chromosomes line up in a The sister chromatids haploid (N) daughter cells, similar way to the metaphase separate and move toward each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original.
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Telophase II Meiosis II results in four haploid (N) daughter cells.
Figure 11-17 Meiosis II Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two The chromosomes line up in a The sister chromatids haploid (N) daughter cells, similar way to the metaphase separate and move toward each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original.
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Telophase II Meiosis II results in four haploid (N) daughter cells.
Figure 11-17 Meiosis II Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two The chromosomes line up in a The sister chromatids haploid (N) daughter cells, similar way to the metaphase separate and move toward each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original.
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Telophase II Meiosis II results in four haploid (N) daughter cells.
Figure 11-17 Meiosis II Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two The chromosomes line up in a The sister chromatids haploid (N) daughter cells, similar way to the metaphase separate and move toward each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original.
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Telophase II Meiosis II results in four haploid (N) daughter cells.
Gamete Formation •In males, each adult cell produces 4 living sperm cells. •In females, each adult cell produces 3 polar bodies that will eventually die and 1 fertile egg.
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Videos
Click a hyperlink to choose a video. Meiosis Overview Animal Cell Meiosis, Part 1 Animal Cell Meiosis, Part 2 Segregation of Chromosomes Crossing Over
Video Contents
Video 1
Meiosis Overview
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Video 1
Video 2
Animal Cell Meiosis, Part 1
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Video 2
Video 3
Animal Cell Meiosis, Part 2
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Video 3
Video 4
Segregation of Chromosomes
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Video 4
Video 5
Crossing Over
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Video 5
Go Online
The latest discoveries in genetics Interactive test Articles on genetics For links on Punnett squares, go to www.SciLinks.org and enter the Web Code as follows: cbn-4112. For links on Mendelian genetics, go to www.SciLinks.org and enter the Web Code as follows: cbn-4113. For links on meiosis, go to www.SciLinks.org and enter the Web Code as follows: cbn-4114.
Internet
Interest Grabber Answers
1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics? All offspring had the same characteristic, which was like one of the parents’. The other characteristic seemed to have disappeared. 2. How do the characteristics of the second generation compare to the characteristics of the first generation? Both characteristics appeared in this generation. The characteristic that had “disappeared” in the first generation did not appear as often as the other characteristic. (It appears about 25 percent of the time.)
Section 1 Answers
Interest Grabber Answers
1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected? Results will vary, but should be close to 5 heads and 5 tails. 2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected? The results for 20 tosses may be closer to the predicted 10 heads and 10 tails. 3. If you compiled the results for the whole class, what results would you expect? The results for the entire class should be even closer to the number predicted by the rules of probability. 4. How do the expected results differ from the observed results? The observed results are usually slightly different from the expected results.
Section 2 Answers
Interest Grabber Answers
1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches. Check students’ answers to make sure they are realistic. 2. What can you observe about the heights of the ten people? Students should notice that there is a range of heights in humans. 3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer. No, height does not seem to be controlled by two alleles, as it is in pea plants. Height in humans can vary greatly and is not just found in tall and short phenotypes.
Section 3 Answers
Interest Grabber Answers
1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? 46 chromosomes 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? 46 + 46 = 92; a developing embryo would not survive if it contained 92 chromosomes. 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have? Sperm and egg should each have 23 chromosomes.
Section 4 Answers
Interest Grabber Answers
1. In how many places can crossing over result in genes A and b being on the same chromosome? One (between A and B) 2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e? Two (between A and B and A and C); Four (between A and B, A and C, A and D, and A and E) 3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes? The farther apart the genes are, the more likely they are to be recombined through crossing over.
Section 5 Answers
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