Meiosis Read the lesson title aloud to students.

Learning Objectives Contrast the number of chromosomes in body cells and in gametes. Summarize the events of meiosis. Contrast meiosis and mitosis. Describe how alleles from different genes can be inherited together. Click to reveal each learning objective. Read the objectives aloud or ask a volunteer to do so.

Chromosome Number Homologous: chromosomes with the same genes, one originally from each of the organism’s parents Diploid: containing both sets of homologous chromosomes; 2N Explain that chromosomes—those strands of DNA and protein inside the cell nucleus—are the carriers of genes. The genes are located in specific positions on chromosomes. Remind students that almost every cell in their bodies has two copies of each chromosome, one from their fathers and one from their mothers. These chromosomes have the same genes; they are homologous. Humans have 23 pairs of chromosomes, or 46 total. The human chromosome number is 46. Ask students to identify the chromosome number for the organism this cell is from. Click to reveal the chromosome number. Discuss terminology from diploid/haploid. Ask: Are most cells in our body haploid or diploid? Answer: diploid Ask: Before mitosis occurs, is the parent cell haploid or diploid? At the end of mitosis, are the daughter cells haploid or diploid? Answer: diploid; diploid Ask for a student to identify a single chromosome in the picture. Then ask the student to identify a pair of homologous chromosomes. The student should identify both large or both small chromosomes as a pair. Ask: Why is each chromosome here duplicated? Answer: The cell has just replicated its DNA so it can enter the process of cell division. Haploid: containing only a single set of chromosomes; 1N chromosome number = 4

Reviewing Mitosis Prophase Metaphase Anaphase Telophase Ask a volunteer to come to the board to record the phases of mitosis. Click to reveal the labels. To review, ask students to describe what happens at each stage Ask: What is the goal of mitosis? What is the end product? Answer: To create nuclei for new cells; cell division. The end product is two genetically identical daughter cells. Telophase

Meiosis Lead a discussion about the significance of a cell being haploid or diploid. Ask students to consider what kind of cells would need to be haploid and why. Guide them to conclude that gametes (eggs and sperm) need to be haploid so that, when fertilization occurs, the offspring do not end up with two full sets of chromosomes from each parent. Point out that the process that produces sex cells (meiosis), therefore, needs to divide nuclei and shuffle chromosomes so that resulting cells have half the normal chromosome number. Emphasize that before meiosis begins every chromosome is copied, so the cell has four copies of each chromosome. Have students identify those structures. Then use the figure to walk students through a brief overview of meiosis I. Emphasize here that the stages of meiosis are divided into two segments: meiosis I and meiosis II. Encourage students to think, as they look more closely at each stage of meiosis, about how they are similar to and different from the stages of mitosis. Remind them to use their lesson worksheets to jot down ideas.

Prophase I Centrioles Spindle forming Chromosomes Nuclear envelope Centromere Review the structures shown in the Prophase I cell and in the close-up of crossing-over. Have volunteers come to the board to point out the major structures of a nucleus in prophase in this order: centrioles, spindle forming, nuclear envelope, chromosomes, centromere. Click to reveal the labeled structures one by one. Explain that in prophase I of meiosis each replicated chromosome pairs with its corresponding homologous chromosome. This pairing forms a structure called a tetrad, which contains four chromatids. Have a volunteer come to the board to point out a tetrad on the diagram. Click to reveal a labeled tetrad. Explain that, as the homologous chromosomes form tetrads, they undergo a process called crossing-over. First, the chromatids of the homologous chromosomes cross over one another. Then, the crossed sections of the chromatids—which contain alleles—are exchanged. Crossing-over, therefore, produces new combinations of alleles in the cell. Ask: How does crossing-over affect the alleles of a chromosome? Answer: During crossing-over, the alleles can be exchanged between chromatids of homologous chromosomes to produce new combinations of alleles. Tetrad

Pair of homologous chromosomes Metaphase I Explain that during metaphase I of meiosis paired homologous chromosomes line up across the center of the cell. Ask for a volunteer to go to the board to point out a pair of homologous chromosomes. Click to reveal label for one pair of homologous chromosomes. Ask: How is metaphase I of meiosis similar to metaphase I of mitosis? Answer: Chromosomes line up along the center of the cell. Ask: What do the pink and yellow chromosomes and pieces of chromosomes represent in the diagram? Answer: The yellow pieces are originally from one of the organism’s parents, and the red pieces are originally from the other parent. Ask: How many chromatids are in this cell? Are the sister chromatids identical? Answer: 8; they are not identical any longer, because crossing over exchanged alleles. Pair of homologous chromosomes

Anaphase I Sister chromatids Explain that during anaphase I spindle fibers pull each homologous chromosome pair toward opposite ends of the cell. Ask for a volunteer to go to the board to point out one pair of sister chromatids. Click to reveal labels for one pair of sister chromatids. Ask: What effect does anaphase I have on chromosome number of the cells that will ultimately result? Answer: It cuts the chromosome number in half. Sister chromatids

Telophase I Nuclear envelopes reforming The next phase is telophase I, in which a nuclear membrane forms around each cluster of chromosomes. Cytokinesis follows telophase I, forming two new cells. Ask for a volunteer to come to the board to point out the new nuclear envelopes. Click to reveal the labels. Ask: How do anaphase I and telophase I of meiosis differ from anaphase and telophase in mitosis? Answer: The chromosomes are still doubled at the end of meiosis I. The number of chromosomes is halved. Ask: What is the end produce of meiosis I? Answer: Two daughter cells Ask: Are the daughter cells haploid or diploid? Answer: Haploid; each cell has only one set of chromosomes, even though the each chromosome exists as sister chromatids. Be sure students understand that the two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Nuclear envelopes reforming

Prophase II Draw students’ attention to the two cells at the top of the figure. Reinforce that the cells have 2 (1 N) chromosomes, but 4 chromatids. Then have volunteers use their own words to describe what occurs during each step of meiosis II. Point out that, unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

Metaphase II and Anaphase II Explain that during metaphase of meiosis II chromosomes line up in the center of each cell. And as the cell enters anaphase, the paired chromatids separate. The final four phases of meiosis II are similar to those in meiosis I. However, the result is four haploid daughter cells. Ask: How does anaphase II differ from anaphase of mitosis? How are they similar? Answer: In anaphase of mitosis and in anaphase II, sister chromatids separate. But in anaphase II, there are no homologous chromosomes; the cell is haploid, not diploid. Anaphase II

Telophase II and Cytokinesis four The end product of meiosis is daughter cells that have the normal chromosome number. half Ask: What is the chromosome number of each cell? Answer: 2 Ask: What was the chromosome number of the original parent cell? Answer: 4 Ask: How many haploid (1N) daughter cells are produced at the end of meiosis II? Ask students what terms complete the sentence. Click to reveal the correct answer. Guide students to see that meiosis resulted in cells with half the normal chromosome number. Ask: What are some differences between meiosis I and meiosis II? Sample Answer: Homologous chromosomes separate during meiosis I but not during meiosis II. The centromeres and sister chromatids separate during meiosis II.

Summary of Meiosis Use this slide to summarize the stages of meiosis. Have volunteers describe what happens at each stage. Encourage students in their descriptions to use the terms chromatids, homologous chromosomes, haploid, and diploid as appropriate.

Comparing Mitosis and Meiosis prophase (I) metaphase (I) anaphase (I) telophase (I) Have students compare and contrast mitosis and meiosis using the diagram. Draw particular attention to phases in meiosis where genetic recombination occurs. For example, in prophase in mitosis, the replicated chromosomes do not pair up, whereas in prophase I in meiosis, the replicated chromosomes pair up with their homologues and the process of crossing-over occurs. As you walk students through the diagram, have them note differences in the lining up of chromosomes, the number of chromosomes each cell contains, and how chromosomes separate into new cells. Have volunteers identify the chromosome number of resulting cells from each process. Click to reveal the correct answers. Emphasize that in mitosis, when the two sets of genetic material separate, each daughter cell receives one complete set of chromosomes. In meiosis, homologous chromosomes line up and then move to separate daughter cells. Mitosis does not normally change the chromosome number of the original cell. This is not the case, though, for meiosis, which reduces the chromosome number by half. Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells. Guide students to conclude ultimately that mitosis and meiosis both ensure that cells inherit genetic information. Both processes begin after interphase, when chromosome replication occurs. However, the two processes differ in the separation of chromosomes, the number of cells produced, and the number of chromosomes each cell contains. N = 2 2N = 4

Gene Linkage Alleles of different genes tend to be inherited together when those genes are located on the same chromosome. Ask for a volunteer to state Mendel’s principle of independent assortment. Mendel found that genes that are located on different chromosomes assort independently. But what about genes that are located on the same chromosome? Explain that researcher Thomas Hunt Morgan, back in 1910, considered the question of whether genes on the same chromosome are generally inherited together. He studied the genes of fruit flies and found that many genes were “linked” in a way that seemed to violate independent assortment. Use the diagram to point out that during crossing-over chunks of the chromosomes, not individual genes, are exchanged between chromosomes. This means that some genes are likely to travel together; that is, they are linked. Explain that this is what Morgan found in his fruit fly experiments: that chunks of genes sorted independently, but genes within certain “linkage groups” were inherited together.

Gene Maps Explain that one of Morgan’s students, Alfred Sturtevant, wondered if the frequency of crossing-over between genes during meiosis might be a clue to the genes’ locations. Sturtevant reasoned that the farther apart two genes were on a chromosome, the more likely it would be that crossing-over would occur between them. Ask: Do you think crossovers are more common between genes that are far apart or genes that are close together on a chromosome? Answer: Crossovers are more common between genes that are farther apart. Sturtevant reasoned that he could use the frequency of crossing-over between genes to determine their distances from each other. Using this logic, he was able to create a diagram like this one called a gene map, which shows the location of a variety of genes on chromosome 2 of the fruit fly. The genes are named after the problems that abnormal alleles cause, not after the normal structures. Ask: Where on the chromosome is the “purple eye” gene located? Answer: at 54.5 Ask: Which genes are most likely inherited together? Why? Sample Answer: Star eye and dumpy wing; because these genes are so close together on the chromosome, the chance that crossing-over would separate them is smaller.