Comparing Mitosis and Meiosis

In both meiosis and mitosis, the original parent cell is a diploid cell. Remember, this means the cell has two copies of each chromosome. Meiosis and mitosis are both nuclear divisions that result in new daughter cells. However, the two processes have significant differences. Fill out the following chart comparing the two forms of nuclear division.

Part 1: Meiosis Bead Simulation

Materials

• 12 magnets (=centromeres)
• 48 beads of one color
• 48 beads of another color

Procedure

1. Set up half of the beads exactly as follows, representing genes on the chromosome of a hypothetical critter. We will assume that the critter is diploid (2N) and has three different chromosomes. Because the critter has two copies of each of the three chromosomes, the diploid number is 6 (2 × 3 = 6).
   

   This is what your critter’s chromosomes look like in the unreplicated form. Note that there are six chromosomes here consisting of three homologous pairs. Each chromosome pair consists of a maternal and paternal version of the chromosome. The maternal and paternal versions are represented by the respective bead color.

2. Replicate your chromosomes! Make enough copies of each chromosome to represent both paternal and maternal chromosomes in a replicated form, as shown below. Note that the sister chromatids are identical in color. Be sure you can identify the sister chromatids, chromosomes, and the difference between a replicated and non-replicated form.
   
3. Using your maternal and paternal sets of replicated chromosomes and this lab (or the text) as a reference, practice the process of meiosis until you are very comfortable with it. Each person in the group should practice the entire process.
   

Do NOT proceed until you are comfortable with this! Don’t forget crossing over.

Part 2: Independent Assortment

There are two possible ways pairs of homologs (also known as tetrads) can line up on the metaphase plate during Metaphase I. This possible number of alignments equals 2_n, where n is the number of chromosomes per set. In humans, n=23, so there are 2₂₃ possible ways the homologous pairs can line up on the metaphase plate!

Procedure

1. Use the beads from the last simulation. This time, demonstrate the principle of independent assortment by determining how many different gametes you can form with three homologous pairs.
2. Use the chromosomes to demonstrate all the different ways they can line up on the metaphase plate.
3. Draw a picture of each possible way of lining up.
4. Then draw a picture of each possible gamete formed when the chromosomes line up like that.
   

Lab Questions

1. How many possible gametes can be formed following meiosis (excluding crossing over events) from an original cell that contains a diploid number of six (2_n = 6)? [The number of possible gametes = 2_n where n is the number of chromosomes per set.]
2. How many possible gametes can be formed following meiosis (excluding crossing over events) from an original cell that contains a diploid number of 46 (2_n = 46)?
3. How many possible gamete types can be generated through the process of crossing over alone?
4. Based upon the processes of independent assortment, crossing over, and random fertilization, what important differences would you expect to see between a sexually reproducing population of organisms and an asexually reproducing population of organisms?

Part 3: Mammalian Gametogenesis

The formation of gametes, or gametogenesis, is the first stage in sexual reproduction. In single-celled organisms, e.g., many Protista, the vegetative cell can simply act as a gamete. In more complex organisms specialized regions within the organism take on the role of gametogenesis.

1. Egg Production: meiosis occurs within the ovary; for example, in plants only one of the four products of meiosis develops into an egg (the other three degenerate or serve some other function).
2. Sperm Production: meiosis occurs within the testes; for example, in plants each original cell (called a spermatocyte) that undergoes meiosis produces four viable sperm.

Procedure

1. Examine under low power (100 X) and draw a cross section of an ovary from a prepared slide. Make a second drawing of a follicle under high power (400 X). Include one or more follicles in your drawing. Each follicle contains an egg, known as an oocyte. Label the following: follicle and outer ovary wall.
2. Examine a prepared slide of testes cross section under high power. Note the numerous canals with sperm. Draw a canal and Label the following: seminiferous tubules, spermatogonia, spermatocyte (cell that undergoes meiosis) and sperm.

(a) Mitosis of a spermatogonial stem cell involves a single cell division that results in two identical, diploid daughter cells (spermatogonia to primary spermatocyte). Meiosis has two rounds of cell division: primary spermatocyte to secondary spermatocyte, and then secondary spermatocyte to spermatid. This produces four haploid daughter cells (spermatids). (b) In this electron micrograph of a cross-section of a seminiferous tubule from a rat, the lumen is the light-shaded area in the center of the image. The location of the primary spermatocytes is near the basement membrane, and the early spermatids are approaching the lumen (tissue source: rat). EM × 900.

Germinal epithelium of the testicle. 1) basal lamina, 2) spermatogonia, 3) spermatocyte 1st order, 4) spermatocyte 2nd order, 5) spermatid, 6) mature spermatid, 7) Sertoli cell, and 8) occlusive junctions

Order of changes in ovary. 1) Menstruation, 2) Maturing follicle, 3) Mature follicle, 4) Ovulation, 5) Corpus luteum,  and 6) Deterioration of corpus luteum