Lab 7: Mitosis and Meiosis
- To name and describe the stages of mitosis and meiosis
- To simulate mitosis and meiosis using pop-beads as representative chromosomes
- To simulate crossing over and independent assortment during meiosis
- To contrast the behaviour of chromosomes during mitosis and meiosis
- a kit of pop-beads (supplied to you)
- Mitosis and Meiosis worksheets (located at the end of this lab)
- colored pencils or pens
Description of the problem
Mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm) are the processes of cell division, which results in two identical daughter cells — identical to each other and identical to the parent cell from which they came. Mitosis is needed for growth, maintenance, and repair throughout an individual’s life.
Meiosis is the process that reduces a diploid parent cell to four non-identical haploid daughter cells. Thus, sexual reproduction relies on meiosis to form gametes (eggs and sperm). Spermatogenesis results in four haploid sperm; however, oogenesis results in only one viable egg cell (the other three small nonfunctional cells are called polar bodies).
In this lab, you will simulate mitosis and meiosis using pop-beads supplied to you. You may find it helpful to refer to the figures in Ch. 19 from your book.
The four stages of mitosis are prophase, metaphase, anaphase, and telophase. Keep in mind that prior to mitosis, during interphase of the cell cycle (specifically the S, or synthesis, phase), DNA in the nucleus replicates. Thus, during the beginning of mitosis, all chromosomes have been replicated. Replicated chromosomes are held together by a centromere, and each strand in a duplicated chromosome is called a chromatid (sister chromatids make up a duplicated chromosome).
Building chromosomes to simulate mitosis
1. Build 4 chromosomes using the kit of pop-beads. Use Figure 1 below as a model (16 red beads for chromosome #1, 16 yellow beads for chromosome #2, 8 red beads for chromosome #3, and 8 yellow beads for chromosome #4). Your cell has a total of 4 chromosomes, so its diploid number is 2n = 4.
2. Before mitosis begins, chromosomes must replicate. Using your kit of pop-beads, replicate the chromosomes (use Figure 2 below as a model). There are still 4 chromosomes in the cell, but each has been replicated in preparation for mitosis.
3. Simulate the process of mitosis by aligning the chromosomes on the cell’s equator and splitting them apart to form two new daughter cells. Notice how both daughter cells have the same number of chromosomes as the original parent cell (2n = 4).
1. Using coloured pencils and the Mitosis worksheet at the end of this lab, draw what you just simulated with the pop-beads.
Meiosis differs from mitosis in that there are two rounds of cell division resulting in four non-identical haploid daughter cells. Both meiosis I and meiosis II can be split into four stages: prophase, metaphase, anaphase, and telophase.
Building chromosomes to simulate meiosis
1. Use the same 4 chromosomes that you built for mitosis. Recall that chromosomes come in homologous pairs, with one member of each pair inherited from mom and the other from dad. Thus, your cell has a total of 4 chromosomes, or 2 pairs of chromosomes, with the diploid number being 2n = 4. In Figure 1 above, chromosomes #1 and 2 are one homologous pair, and #3 and 4 are the other homologous pair (i.e. the “red” chromosomes were inherited from one parent, whereas the “yellow” chromosomes were inherited from the other parent).
2. Before meiosis begins (meiosis only occurs in the testes and ovaries), chromosomes must replicate. Using your kit of pop-beads, replicate the chromosomes (use Figure 2 above as a model).
3. Meiosis has two rounds of cell division rather than just one. Simulate meiosis I. Start with the process of synapsis, where homologous pairs of chromosomes come together (below).
4. After homologous pairs synapse, crossing over occurs. Crossing over entails an exchange of DNA between the homologous pairs, resulting in non-identical sister chromatids (below). Crossing over can occur anywhere on the chromosome, so you don’t have to copy the model below exactly. You may choose to have different sites where crossing over occurs.
5. Simulate the rest of meiosis I by aligning the homologous pairs of chromosomes on the cell’s equator and splitting them apart. At the end of meiosis I, two haploid daughter cells result (to better understand this process, you might want to simulate meiosis I without crossing over, then repeat the process with crossing over). At the end of meiosis I, each of the two haploid cells contains chromosomes composed of non-identical sister chromatids.
6. Simulate meiosis II, which is the process of separating the sister chromatids. The end result is four haploid cells (n = 2 chromosomes). The parent cell initially had 4 chromosomes, but through the process of meiosis I and II, the four daughter cells each have 2 chromosomes (one member of each homologous pair). Notice, too, that the daughter cells aren’t identical to one another.
2. Using coloured pencils and the Meiosis worksheet at the end of this lab, draw what you just simulated with the pop-beads.
|3. State the differences between mitosis and meiosis.|
|Number of rounds of cell divisions|
|Chromosome number in daughter cells|
|Number of daughter cells|
|4. State how each stage of meiosis I differs from the corresponding stage of mitosis.|
|Prophase: no pairing of chromosomes||Prophase I:|
|Metaphase: duplicated chromosomes align along the cell’s equator||Metaphase I:|
|Anaphase: sister chromatids separate||Anaphase I:|
|Telophase: chromosomes have one chromatid||Telophase I:|
|5. State how each stage of meiosis II differs from (or in some cases is similar to) the corresponding stage of mitosis.|
|Prophase: no pairing of chromosomes||Prophase II:|
|Metaphase: duplicated chromosomes align along the cell’s equator||Metaphase II:|
|Anaphase: sister chromatids separate||Anaphase II:|
|Telophase: two diploid daughter cells||Telophase II:|