Lab #11: Mitosis & Meiosis Simulation Objectives. Materials. Procedure

Transcription

1 Lab #11: Mitosis & Meiosis Simulation Objectives Understand the cell cycle and process of cell division Demonstrate mitosis and meiosis using pop bead models Simulate segregation of alleles, independent assortment, and crossing over during cell division Materials MATERIALS NEEDED PER GROUP 40 Red pop beads 40 Yellow pop beads 2 Red centromeres 2 Yellow centromeres 8 Plastic tubular centrioles In the time it takes to read this sentence, approximately 50,000 cells will die and be replaced with new ones. Procedure Activity #1: Mitosis Use Analysis Sheet #1: Mitosis to diagram each stage of mitosis as you simulate in this exercise. A. Interphase After cell division takes place, the cell enters the longest stage of the cell cycle. This is called interphase. During this stage, the cell is preparing for the next division. 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. Construct two strands of seven red pop beads and attach them to a red centromere. Repeat with two strands of seven yellow pop beads and a yellow centromere. These will represent a homologous pair of chromosomes.

2 2. Picture an imaginary boundary in the center of your desk. This boundary will represent the nuclear membrane. Place the chromosomes in the center of the imaginary nucleus. 3. DNA replication occurs, producing a duplicate of each chromosome. Construct two chromosomes identical to the ones you made previously. 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 for the yellow chromosome. 4. Place a pair of plastic centrioles, at ninety degree angles, just outside of your nuclear membrane. The centrioles also replicate during interphase so place another pair next to them in your cell. B. Prophase Chromatin condenses within the nucleus and chromosomes become visible. Centrioles migrate to opposite poles (sides) of the cell and spindle fibers begin to form. As the spindle fivers appear, the nuclear membrane disappears. The spindle fibers attach to the centromere region of each chromosome. Scientist have discovered checkpoint mechanisms that ensure each step in the mitotic process is properly executed before the cell moves onto the next phase. 1. Move your two pairs of centrioles to opposite poles (sides) of the cell (your desk). C. Metaphase The chromosomes line up in the middle of the nucleus along the metaphase plate. The centromeres of each sister chromatid are attached, by spindle fibers, to the centrioles at opposite poles of the cell. 1. Center your chromosomes along an imaginary metaphase plate with the centrioles still at opposite poles of the cell.

3 D. Anaphase Many human cancers are likely to stem from defective mitotic checkpoints. The chromatids of each chromosome separate at the centromeres and move to opposite poles of the cell, forming daughter chromosomes. 1. Separate and move the centromeres of each chromosome toward opposite poles of the cell. Notice how the arms of each chromosome trail 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 each 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: Meiosis I & II Use Analysis Sheet #2: Meiosis to diagram each stage of meiosis as you simulate it in this exercise. Meiosis I The segregation of alleles in a diploid A. Interphase I organism ensures that offspring receive genes from both parents, increasing the genetic variability of the offspring. DNA replication occurs, resulting in the formation of paired chromatids. Centrioles, and other cell organelles, replicate as well.

4 1. Construct two strands of seven red pop beads and attach each strand to a red centromere. Repeat with two strands of seven yellow pop beads and a yellow centromere. 2. Picture an imaginary boundary in the center of your desk. This boundary will represent the nuclear membrane. Place the chromosomes in the center of the imaginary nucleus. 3. DNA replication occurs, producing a duplicate of each chromosome. Construct two strands identical to the ones you made previously. 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 for the yellow chromosome. 4. Place a pair of plastic centrioles, at ninety degree angles, just outside of the nuclear membrane. The centrioles also replicate during interphase so place another pair next to them in your cell. B. Prophase I A process called synapsing occurs in which homologous chromosomes move close together and pair up along their entire length. A tetrad, consisting of four chromatids is formed. Centrioles move to opposite poles of the cell and the nuclear membrane begins to break down. Crossing over serves as a mechanism for further genetic variation by re- organizing the chromosomes New combinations of genes can be created, often affecting the phenotype of the offspring. 1. Align your homologous chromosomes and entwine them in the center of the nucleus. 2. Move your centrioles to opposite poles of the cell. 3. Snap three beads off of one red chromatid and exchange them with three beads on a yellow chromatid. This simulates crossing over. C. Metaphase I Chromosomes disentangle and become aligned in the center of the cell in homologous pairs. 1. Disentangle the entwined homologous chromosomes and place them side-by-side in the center of the cell.

5 D. Anaphase I The homologous chromosomes separate and are drawn to opposite sides of the cell by spindle fibers. 1. Move each homologous pair toward its respective centrioles. Move the chromosome pairs by the centromere, noting how the chromosome arms trail the centromere as movement occurs. E. Telophase I During meiosis, cell division occurs and centrioles replicate, resulting in two daughter cells still containing paired chromatids. 1. Keeping each paired strand near its respective centrioles, create an imaginary line around each paired strand, representing two new daughter cells. Duplicate the centrioles for both daughter cells and place them next to the original centrioles. Meiosis II F. Interphase II DNA replication DOES NOT occur during the second interphase stage of meiosis. 1. Leave the daughter cells as they were after telophase I. G. Prophase II The centrioles move to opposite poles of both daughter cells. The chromosomes move toward the center of the daughter cells.

6 To Simulate: 1. Move the centrioles to opposite poles of each daughter cell. Place the centromeres of the paired strands in the center of each daughter cell. H. Metaphase II All of the chromosomes line up, single file, in the center of the cell. 1. Center the paired strands along an imaginary line across the center of the cell. Line up the strands so they are centered in each daughter cell. I. Anaphase II The chromatids of each paired strand separate and are drawn to opposite poles of the cell. Each chromatid, with a well-defined centromere, is now a chromosome. 1. Separate each paired strand at its centromere. Move each strand toward its respective centrioles, noting how the chromosome arms trail the centromere as it moves towards each pole. Repeat this procedure for both daughter cells from meiosis I. J. Telophase II and Cytokinesis Cell division is completed and four daughter cells are formed. Each contains half of 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. 1. Place each chromosome strand near its respective centrioles. Imagine a nuclear membrane around each chromosome and a complete division in the daughter cells from meiosis I, resulting in the four daughter cells.