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Lab 3- Meiosis and Trait Inheritance Overview This lab will introduce students to the purpose of meiosis. Students will also learn how chromosomes segregate according to Mendelian laws. Learning objectives By the end of this lab, students will be able to: State the purpose of meiosis Differentiate mitosis and meiosis Explain the difference between haploid and diploid cells Describe key events that occur in the stages of meiosis I and meiosis II Learn to compute a Punnett square ● Exercise 1: Fertilization A. Comparing Mitosis and Meiosis The diagrams in Figure 1 show mitosis and meiosis after DNA has been replicated and wound tightly into sister chromatids. The dotted lines represent cytokinesis. Figure 1. Mitosis and meiosis Pa A Pril Sex B sex D Activity A questions 1. In Figure 1, which column shows meiosis and which column shows mitosis. Meiosis Click/tap to enter A or B Mitosis Click/tap to enter A or B 2. What are some similarities between cell division by mitosis and cell division by meiosis? Click or tap here to enter text. 3. Complete Table 1 to describe some important differences between mitosis and meiosis. Table 1. Differences between mitosis and meiosis Characteristic Type of cells produced Number of daughter cells Click or tap here to enter Click or tap here to enter text. text. Genetic differences or similarities between daughter cells Mitosis Number of cell divisions Meiosis Click or tap here to enter Click or tap here to enter text. text. Click or tap here to enter Click or tap here to enter text. text. Click or tap here to enter Click or tap here to enter text. text. B. Meiosis - Cell Divisions to Produce Haploid Gametes Before meiosis, the cell makes a copy of the DNA in each chromosome. Then, during meiosis there are two cell divisions, Meiosis I and Meiosis II. These two cell divisions produce four haploid daughter cells. Meiosis I is different from mitosis because each pair of homologous chromosome lines up next to each other and then the two homologous chromosomes separate. Figure 2 shows Meiosis I for a cell with a single pair of homologous chromosomes. The stripes on the chromatids of one of the chromosomes indicates that this chromosome has different alleles than the other homologous chromosome. Meiosis I will produce daughter cells with half as many chromosomes as the parent cell, so the daughter cells are haploid. Each daughter cell has a different chromosome from the original pair of homologous chromosomes. Activity B questions Examine Figure 2 which shows a diploid cell (2n) with a pair of homologous chromosomes. After completing meiosis I, the resulting cells each contain one chromosome and are now considered haploid (n). Figure 2. Meiosis I X xx X 1. Do the chromosomes in the two daughter cells produced by Meiosis I have the same alleles for each gene? Explain your reasoning. Click or tap here to enter text. Meiosis II is like mitosis since the sister chromatids of each chromosome are separated. As a result, each daughter cell gets one copy of one chromosome from the pair of homologous chromosomes that was in the original cell. These haploid daughter cells are the gametes. 2. Use asterisks (*) to indicate the cells in Figure 3 that represent sperm produced by meiosis. Figure 3. Meiosis II Ⓒ O C. Meiosis and fertilization Almost all the cells in your body were produced by mitosis. The only exceptions are the gametes - sperm or eggs - which are produced by a different type of cell division called meiosis. During fertilization the sperm and egg unite to form a single cell called the zygote which contains all the chromosomes from both the sperm and the egg. The zygote divides into two cells by mitosis, then these cells each divide by mitosis, and mitosis is repeated many times to produce the cells in an embryo which develops into a baby (Figure 4). Figure 4. Meiosis and fertilization mother's cells meiosis↓↓ egg father's cells meiosis↓↓ fertilization zygote repeated mitosis↓↓ embryo's cells repeated mitosis ↓ baby's cells sperm Activity C questions 1. Each cell in a normal human embryo has 23 pairs of homologous chromosomes, for a total of 46 chromosomes per cell. How many chromosomes are in a normal human zygote? Explain your reasoning. Click or tap here to enter text. 2. What would happen if human sperm and eggs were produced by mitosis? Explain why this would result in an embryo, which had double the normal number of chromosomes in each cell. Click or tap here to enter text. A human embryo with that many chromosomes in each cell would be abnormal and would die before it could develop into a baby. So, gametes cannot be made by mitosis. For questions 3 and 4, click or tap number to enter the correct number. 3. Each human sperm and egg should have number_chromosomes, so fertilization will produce a zygote with number_chromosomes. This zygote will develop into a healthy embryo with number chromosomes in each cell. 4. Each sperm and each egg produced by meiosis has only one chromosome from each pair of homologous chromosomes. When a sperm and egg unite during fertilization, the resulting zygote has number pairs of homologous chromosomes. For each pair of homologous chromosomes in a zygote, one chromosome came from the egg and the other chromosome came from the Click or tap to enter text.