Mitosis/Meiosis Simulation Activities

Transcription

1 Mitosis/Meiosis Simulation Activities In this simulation, you will demonstrate an understanding of mitosis, meiosis, segregation, independent assortment, and crossing over, all processes involved with chromosomes. Materials 40 red beads 40 yellow beads 2 red centromeres 2 yellow centromeres 4 clear plastic centrioles Activity #1 Mitosis Read Text Reference: pgs A. Interphase After mitosis (cell division) takes place, the cell enters the longest stage of the cell cycle. This is called interphase. Interphase is not mitosis. During this stage, the cell is performing its normal cellular functions in G1, S, and G2 phase and will eventually prepare for the next mitotic division (M phase). Distinct chromosomes are not visible. DNA exists in an uncoiled state and the chromosome material appears as granular matter, called chromatin, within the nucleus. 1. Connect 7 red beads on one end of a red centromere and 7 red beads on the other end. Repeat using yellow beads and a yellow centromere. These will represent a homologous pair of chromosomes. One chromosome from dad and one from mom. 2. Clear of your entire table. Draw an imaginary circular boundary in the center of your table to represent the nuclear membrane. Place the two chromosomes in the center of your nucleus. The rest of your desk represents the cytoplasm of the cell. 3. DNA replication occurs in the S phase, producing a duplicate of each chromosome. Make another red and yellow chromosome. Each of the four strands in your nucleus are now called chromatids. Join the red chromatids together to form a pair of sister chromatids. Do the same for the yellow chromosome. 4. Place 2 plastic centrioles at right angles just outside the imaginary nucleus. During G2 of Interphase the cell will duplicate its centrioles, so place another pair beside your centriole in the cell. B. Prophase This is the beginning of mitosis. Chromatin condenses within the nucleus and chromosomes become visible. Cohesins keep the chromosomes joined together. Nucleoli disappear. Centrioles migrate to opposite poles of the cell and microtubule spindle fibres begin to emerge from the centrioles. Draw some early spindle fibres and Asters. 1. Move your two pairs of centrioles to opposite poles (sides) of the cell (desk). Keep each pair at right angles to each other. C. Prometaphase and Metaphase As the spindle fibres appear, the nuclear membrane begins to break apart. The spindle fibres attach to the centromere region (kinetochore) of each chromosome. The centromeres of each sister chromatid are attached, by kinetochore microtubules coming from the centriole to the kinetochore in each centromere. Draw some nonkinetichore microtubules. The chromosomes line up in the middle of the nucleus along the metaphase plate.

2 D. Anaphase The chromatids of each chromosome separate at the centromeres when the cohesion proteins are cleaved. The chromosomes move to opposite poles of the cell, forming daughter chromosomes. Nonkinetichore microtubules push against each other causing the cell to elongate. 1. As you separate and move the centromeres of each chromosome toward opposite poles, notice how the strands of each chromosome trail behind the centromeres to the poles. E. Telophase and Cytokinesis The spindle apparatus disappears. Nuclear membranes begin to reappear, forming two separate nuclei; one for each daughter cell. The chromosomes uncoil and become diffuse chromatin. Cytokinesis begins and separates the cytoplasm into two discrete daughter cells. 1. Move one red strand and one yellow strand to the centrioles it was heading toward during anaphase. Imagine a cleavage furrow developing between nuclei and separating the cell into two daughter cells. 2. Note how each cell now contains one red and one yellow chromosome, as well as one pair of centrioles, exactly like the cell with which you began. Activity #2 Mitosis with Two Pairs of Homologous Chromosomes Now that you are familiar with the process of mitosis, work with another group of 2 students and repeat the procedure with two pairs of chromosomes. One of the groups will shorten both strands of one red chromosome to six beads per side and do the same for their yellow chromosome. Repeat all of the stages of mitosis. (remember, mitosis starts at prophase). After mitosis, you should have two identical daughter cells, each with one long red and yellow chromosome and one short red and yellow chromosome.

3 Activity #3 Meiosis I and II Meiosis I Interphase I When DNA replication occurs, resulting in the formation of paired chromatids. Centrioles, and other organelles, replicate as well. 1. Just like in mitosis, connect 7 red beads on one end of a red centromere and 7 red beads on the other end. Repeat using yellow beads and a yellow centromere. 2. Draw a circular chalk boundary in the center of your desk to represent the nuclear membrane. Place the chromosomes in the center of the imaginary nucleus. 3. DNA replication occurs, producing a duplicate of each chromosome. Repeat step one again and make another red and yellow chromosome identical to the first two. Each half of the duplicated chromosome is called a chromatid. Join both red chromatids at the centromere to form a pair of sister chromatids. Repeat this for the yellow chromosome. Show someone the following: a pair of sister chromatids, homologous chromosomes, and a locus on a chromosome. 4. Place a pair of clear plastic centrioles, at right angles, just outside of your nuclear membrane. The centrioles also replicate during interphase, so place another pair next to the first pair. Prophase I A process called synapsis occurs in which homologous chromosomes move close together and pair up along their entire length by cohesions. A tetrad, consisting of four chromatids is formed. Centrioles move to opposite poles of the cell and the nuclear membrane begins to break down. 1. Align your homologous chromosomes and entwine them (lay the yellow and red beads on each other) in the center of the nucleus. 2. Move your centrioles to opposite poles of the cell. Metaphase I Homologous chromosomes become aligned in the center of the cell (yellow on one side of the nucleus, red on the other) in homologous pairs. The kinetochores face the centrioles. 1. Move the homologous chromosomes side-by-side in the center of the cell called the Metaphase plate. Anaphase I The homologous chromosomes separate and are drawn to opposite sides of the cell by spindle fibres. The sister chromatids are still attached at the centromere. 1. Move each homologous chromosome pair toward its respective centrioles. Move the chromosome pairs by the centromere, noting how the chromosome strands trail the centromere.

4 Telophase I During Telophase I, a cleavage furrow occurs in animal cells, and cell plate occurs in plant cells resulting in two daughter cells. Each cell has a complete haploid set of duplicated chromosomes consisting of two sister chromatids. Meiosis II Interphase II DNA replication occurs in interphase I but it DOES NOT occur during interphase II stage of meiosis. Prophase II With the daughter cells the way they are after Telophase I, a spindle apparatus forms. The chromosomes begin to move toward the metaphase plate. 1. Move the centrioles to opposite poles of each of the two daughter cells that was produced in telophase I. Place the centromeres of the paired strands in the center of each daughter cell. Metaphase II Move the centromeres to opposite poles. All of the chromosomes line up at the metaphase plate just like mitosis. Their kinetochores face the centrioles. Anaphase II The sister chromatids of each chromosome finally separate and are drawn to opposite poles of the cell. Each chromatid, with a well-defined centromere, is now called a chromosome. 1. Separate each paired strand at its centromere. Move each strand toward its respective pair of centrioles, noting how the chromosome strands trail the centromere as it moves toward each pole. Do this for both daughter cells that were produced from meiosis I. Telophase II and Cytokinesis Cell division is completed and four haploid daughter cells are formed. Each contains half the chromosome number of the original parent cell. A nuclear membrane forms around each cell s chromosomes and the daughter cells from meiosis I finish dividing completely. Centrioles remain outside the nuclear membrane of each of the four daughter cells.

5 Activity #4 The Segregation of Alleles, Independent Assortment, and Crossing Over A. Segregation of Alleles Homologous chromosomes in diploid organisms ensure that there is a pair of genes for each trait. These genes are found in the same locus (gene position) of each chromosome. Each of these versions on gene is called an allele. If the alleles for a specific trait are identical on both chromosomes, the organism has a homozygous genotype. If the organism has two different alleles for the same gene on both chromosomes then the organism has a heterozygous genotype. In meiosis, the alleles on homologous chromosomes separate during the process and the alleles are said to be segregated. This is Mendel s first law: alleles segregate during meiosis. 1. Start with four centromeres, two red and two yellow. Each centromere has two strands of seven beads. (just like you ended Interphase I). Exchange one yellow bead for one red bead at a locus on each sister chromatid on your yellow chromosome. This will represent allele R. Replace a red bead with a yellow bead at the same locus (position) on one of your red chromosome strands. This will represent allele r. 2. Beginning at prophase, follow the procedure for meiosis you used previously. Notice how these alleles segregate. B. Independent Assortment Mendel s second law states that alleles on separate chromosomes assort independently of each other. If alleles for a particular trait, trait R, are found on one chromosome and alleles for a different trait, trait B, are found on another chromosome, then the alleles for traits R and B will arrange (assort) independently during meiosis. Heterozygous parents, with the genes RrBb, can produce gametes with the genotypes RB, Rb, rb, and rb, depending on how the chromosomes arrange during metaphase I. Independent assortment means that these four genotype combinations are all the possible combinations that could occur if the alleles randomly assemble together. 1. Form a team with another group. Return all your chromosome beads to their original color. You will be near the end of interphase, your chromosomes have already duplicated so your nucleus should have four homologous chromosomes in it. (two yellow, two red). Remove three

6 beads from one of the homologous yellow strands (six in total). When you are done, the homologous pairs of yellow chromosomes will have strands of unequal length on either side of the centromere (one chromosome has two strands of seven beads the other has two strands of four beads) while the pairs of red homologous chromosomes will have strands of equal length (all with seven) on either side of the centromere. 2. Replace one yellow bead with a red bead on the yellow homologous chromosome with equal strand length. This will be allele R. Create allele r by replacing a red bead with a yellow bead at the same locus on the red homologous chromosome with strands of equal length. 3. You will now create a different trait on a different chromosome. Replace one yellow bead with a blue bead on the longer strand of a yellow chromosome with strands unequal in length. This will be allele B. You will represent allele b with a red bead at the same locus on the red chromosome with strands of unequal length. 4. Starting at Prophase I, follow the procedure form meiosis, using all four of the chromosomes you have constructed. Notice that there are two ways that the chromosomes can align in Metaphase I, depending on which side you lined up the red and yellow chromosomes. Maintain this orientation throughout the rest of the process. C. Crossing Over Crossing over occurs when strands of homologous chromosomes make contact and exchange portions between each other. During prophase I, the chromosomes align in the center of the nucleus and the strands of the chromosome often tangle up. In some cases, the strands break off and reattach to the chromosome with which they tangled up to. This process results in a redistribution of genetic material following meiosis. 1. Build chromosomes in a nucleus just like the meiosis I procedure. During prophase I, when the homologous chromosomes align in the center of the nucleus with their strands tangled, snap three red beads off of one red chromatid and exchange them with three yellow beads with on one yellow chromatid. Place the three yellow beads on the altered strand of the red chromatid. Continue the meiosis process and notice how the four gametes are all different from each other. Cell Cycle Assessment 1. Explain the difference between the G 1, G 2, and S phases of the cell cycle. What is the G 0 phase? How does a cell determine if it is time to move to a new phase in the cell cycle? 2. Differentiate between a chromosome, chromatin, and chromatid. 3. What is the mitotic spindle made of? What is the role of nonkinetochore microtubules? 4. At which end of the cell do kinetochore microtubules shorten during anaphase? How do chromosomes move along the spindle? 5. How does a cleavage furrow differ from a cell plate? Explain fully. 6. How is binary fission in Escherichia coli, different from cell division in your somatic cells?