Help with Questions 3 and 6

This document provides help for answering questions 3 and 6 in the Mitosis and Meiosis Drawing Exercise.

Question 3

Question 3 is: 
How many chromosomes and chromatids are in each of the following human cells? primary spermatocyte, secondary spermatocyte, spermatid, sperm, primary oocyte, secondary oocyte, first polar body, ovum, second polar body

During meiosis, the number of chromosomes is reduced by 1/2. This happens because a normal body cell has two of each chromosome but the pair separates during Metaphase I of meiosis. It is helpful to consult your drawings of chromosome movement to see this. The explanations below refer to drawings in The Biology Web. Click the document Mitosis and Meiosis Summary, Drawings and Photographs. 

Scroll down to Summary of the Phases of Meiosis. This hypothetical cell has 8 chromosomes (2N=8). A human cell has 46 (2N=46). Notice that in Meiosis I (Prophase I, Metaphase I, Anaphase I, and Telophase I) all of the cells have 8 chromosomes and 16 chromatids. Human cells in these phases have 46 chromosomes and 92 chromatids.

At the start of meiosis II (Prophase II, Metaphase II) the cells have half as many chromosomes because each pair of chromosomes separated during Metaphase I (the first division) and then when the cell divided at the end of Meiosis I, each of the resulting daughter cells had one chromosome from each pair.

The number of chromosomes and chromatids in a cell during Interkinesis is the same as in Prophase II so it is not shown on the diagram. Human cells in Interkinesis are called secondary oocytes (females) or secondary spermocytes (males). The cells in the diagrams have half as many chromosomes (4) and each chromosome has 2 chromatids. Therefore, a human cell would also have 1/2 as many chromosomes (1/2 X 46) and each of these would have 2 chromatids.

During anaphase II, the chromosomes split. This doubles the number of chromosomes. Notice that in the drawings, each cell now has twice as many chromosomes (8) and each chromosome has one chromatid (8 chromatids per cell).

When each of the two cells in Telophase II pinches into two, each of the resulting daughter cells has 1/2 the original number of chromosomes (4) and each chromosome has one chromatid. Human cells that have completed meiosis also have 1/2 the original number of chromosomes and each chromosome has one chromatid. These daughter cells in human females consist of an egg (ovum) and second polar bodies. In males they are spermatids. During the maturation of spermatids to sperm, the number of chromosomes and chromatids remains unchanged.

Question 6

6a)  A woman has a gene for blond hair color on one chromosome and a gene for brown hair on the homologous chromosome. What percentage of her body cells will contain the gene for blond hair?

To answer question 6a, you need to understand that all of a persons body cells are produced by mitosis from a single fertilized egg and that mitosis produces identical daughter cells. Therefore all of the cells in a persons body are identical with respect to DNA and chromosomes. If a person has a gene fro blond hair, all of her cells will have blond hair.

6b)  What percentage of her primary oocytes will contain the gene for blond hair?

To answer question 6b, you should understand that a primary oocyte is the same as a body cell. It is a normal diploid cell. It has not yet undergone meiosis.

6c)  If cell A (above) was in prophase I, how many genes for blond hair would be in the cell?

To answer question 6c, you need to understand that each chromosome in cell A has two chromatids (check your drawings). You also need to know that each of the two chromatids that compose a chromosome are identical. If there is a gene on one of them, the same identical gene will be found on the other one.

6d)  If cell "C" contains a gene for blond hair, what gene does cell B contain?

Question 6d can be answered by referring to the drawings Mitosis and Meiosis Summary, Drawings and Photographs in The Biology Web. Scroll down to Summary of the Phases of Meiosis. Suppose that in the cell showing Metaphase I, the top pink chromosome on the left contained a gene for blond hair and the blue chromosome next to it contained a gene for brown hair. In Anaphase I, the chromosomes separate. After the cell splits (Interkinesis and Prophase II) one of the daughter cells will have the gene for blond (the pink chromosome) and the other will have the gene for brown.

6e) If cell "D" contains the gene for blond hair, what gene does cell "E" contain?

Question 6e can be answered by looking at the chromosomes during Metaphase II on the drawings. Suppose that the top chromosome in the cell on the left has a gene for blond hair. Each chromatid in a chromosome is identical so that when the chromosome splits, the two resulting daughter cells will each have a gene for blond hair.

6f) If cell "D" contains the gene for blond hair, what gene does cell "F" contain?

Question 6f can be answered by going back to Metaphase I on the drawings. Suppose that the gene for blond hair is on the pink chromosome shown on the upper half of the drawing. The gene for brown hair must therefore be on the blue chromosome on the upper right. These two chromosomes separate during Anaphase I. The pink chromosome with the blond gene is in the cell on the left during Prophase II and the chromosome with the gene for brown hair is in the cell on the right. When these two cells divide during Meiosis II, the Prophase II cell on the left with the gene for blond hair will produce the two daughter cells shown on the left. They will each contain a gene for blond hair. The Prophase II cell shown on the right with the gene for brown hair will produce the two daughter cells shown on the right. Each of these two daughter cells on the right will have a gene from brown hair.