LAB EXERCISE: Mitosis and Meiosis Laboratory Objectives After completing this lab topic, you should be able to: 1. Describe the activities of chromosomes and microtubules in the cell cycle, including all phases of mitosis and meiosis. 2. Identify the phases of mitosis in root tip and whitefish blastula cells. 4. Describe differences in mitosis and cytokinesis in plant and animal cells. 5. Describe differences in mitosis and meiosis. Introduction The nuclei in cells of eukaryotic organisms contain chromosomes with clusters of genes, discrete units of hereditary information consisting of double-stranded deoxyribonucleic acid (DNA), Structural proteins in the chromosomes organize the DNA and participate in DNA folding and condensation. When cells divide, chromosomes and genes are duplicated and passed on to daughter cells. Single-celled organisms divide for reproduction. Multicellular organisms have reproductive cells (eggs or sperm), but they also have somatic (body) cells that divide for growth or replacement. In somatic cells and single-celled organisms, the nucleus divides by mitosis into two daughter nuclei, which have the same number of chromosomes and the same genes as the parent cell. In multicellular organisms, in preparation for sexual reproduction, a type of nuclear division called meiosis takes place. In meiosis, nuclei of certain cells in ovaries or testes (or sporangia in plants) divide twice, but the chromosomes replicate only once. This process results in four daughter nuclei with differing alleles on the chromosomes. Eggs or sperm (or spores in plants) are eventually formed. Generally, in both mitosis and meiosis, after nuclear division the cytoplasm divides, a process called cytokinesis. Events from the beginning of one cell division to the beginning of the next are collectively called the cell cycle. The cell cycle is divided into two major phases; interphase and mitotic phase (M). The M phase represents the division of the nucleus and cytoplasm. Part A: Mitosis I. Definitions. Using your textbook, define each of the following cellular stages of interphase, mitosis, and cytokinesis, and describe the appearance of the cell and what is happening to the cellular DNA at each stage. I. = II. Mitosis = A. Prophase = B. Metaphase = C. Anaphase = 1
D. Telophase = III. = II. Modeling the Cell Cycle and Mitosis In the model of mitosis that you will build, your cell will be a diploid cell (2n) with four chromosomes. This means that you will have two homologous pairs of chromosomes. One pair will be long chromosomes, the other pair, short chromosomes. (Haploid cells have only one of each homologous pair of chromosomes, denoted n.) Procedure 1. Build a homologous pair of single-stranded chromosomes using 10 beads of one color for one member of the long pair and 10 beads of the other color for the other member of the pair. Place the centromere at any position in the chromosome, but note that it must be in the same position on homologous chromosomes. Build the short pair in the same manner, but use fewer beads. You should have enough beads left over to duplicate each chromosome. 2. Using the chromosomes you have built, model replication during the S phase of and Prophase, Metaphase, Anaphase and Telophase of Mitosis. Be sure to draw out each step you have modeled. 2
III. Observing Mitosis and in Plant Cells The root tip of plants is an area of rapid cell division. Using the microscopes available in the lab, examine an Allium (Onion) Root Tip slide to see cells undergoing mitosis. You will probably want to start by examining your slide at 40x magnification, then 100x, and 400x. Find cells that are in each of the following cellular stages and sketch what they look like. You will probably want to examine cells at 400x magnification for this exercise. 3
IV. Observing Chromosomes, Mitosis, and in Animal Cells In this exercise, you will observe chromosomes and the stages of mitotic division in whitefish blastula cells. You will also compare these chromosomes with the plant chromosomes studied in Exercise III. Chromosome structure in animals and plants is basically the same in that both have centromeres and arms. However, plant chromosomes are generally larger than animal chromosomes. 4
Part B: Meiosis I. Modeling Meiosis Meiosis is the duplication of chromosomes and the separation of homologous chromosomes and sister chromosomes to form haploid gametes. Model this process using the beads provided. Procedure 1. Build the premeiotic interphase nucleus much as you did the mitotic interphase nucleus. Have two pairs of chromosomes (2n = 4) of distinctly different sizes and different centromere positions. Have one member of each pair of homologues be one color, the other, a different color. 2. Using the chromosomes you have built, model replication during the S phase of and ALL phases of Meiosis. Be sure to draw out each step you have modeled. Meiosis I: Prophase I: Each chromosome is duplicated forming a tetrad. Crossing over occurs. Metaphase I: The tetrad aligns in the center of the cell. Anaphase I: The homologous chromosomes split up. Telophase I: Sister chromatids reach separate poles. : Cell division forms two cells. Meiosis II: Prophase II: Chromosomes move to middle of the cell. Metaphase II: The sister chromatids align in the center of the cell. Anaphase II: The sister chromatids are split up. Telophase II: One chromatic reaches each cellular pole. : Cell division of each of the two cells from Meiosis I results in four total haploid cells. 5