Important Terms

genome (p. 16) prokaryote (p. 17) eukaryote (p. 17) eubacteria (p. 18) archaea (p. 18) nucleus (p. 18) histone (p. 18) chromatin (p. 18) homologous pair (p. 20) diploid (p. 20) haploid (p. 20) telomere (p. 21) origin of replication (p. 21) sister chromatid (p. 22)

cell cycle (p. 22) interphase (p. 22) M phase (p. 22) mitosis (p. 22) cytokinesis (p. 22) prophase (p. 23) prometaphase (p. 23) metaphase (p. 23) anaphase (p. 23) telophase (p. 23) meiosis (p. 29) fertilization (p. 29) prophase I (p. 29) synapsis (p. 29)

bivalent (p. 29) tetrad (p. 29) crossing over (p. 29) metaphase I (p. 30) anaphase I (p. 30) telophase I (p. 30) interkinesis (p. 30) prophase II (p. 30) metaphase II (p. 31) anaphase II (p. 31) telophase II (p. 31) recombination (p. 33) microsporocyte (p. 36) microspore (p. 36)

megasporocyte (p. 36) megaspore (p. 36) spermatogenesis (p. 38) spermatogonium (p. 38) primary spermatocyte (p. 38) secondary spermatocyte (p. 38) spermatid (p. 38) oogenesis (p. 38) oogonium (p. 38) primary oocyte (p. 38) secondary oocyte (p. 38) first polar body (p. 38) ovum (p. 38) second polar body (p. 38)

Worked Problems

1. A student examines a thin section of an onion root tip and records the number of cells that are in each stage of the cell cycle. She observes 94 cells in interphase, 14 cells in prophase, 3 cells in prometaphase, 3 cells in metaphase, 5 cells in anaphase, and 1 cell in telophase. If the complete cell cycle in an onion root tip requires 22 hours, what is the average duration of each stage in the cycle? Assume that all cells are in active cell cycle (not G0).

This problem is solved in two steps. First, we calculate the proportions of cells in each stage of the cell cycle, which correspond to the amount of time that an average cell spends in each stage. For example, if cells spend 90% of their time in interphase, then, at any given moment, 90% of the cells will be in interphase. The second step is to convert the proportions into lengths of time, which is done by multiplying the proportions by the total time of the cell cycle (22 hours).

Step 1. Calculate the proportion of cells at each stage. The proportion of cells at each stage is equal to the number of cells found in that stage divided by the total number of cells examined:

Interphase

94/ /120

= 0.783

Prophase

/120

= 0.117

Prometaphase

3/ /120

= 0.025

Metaphase

3/ /120

= 0.025

Anaphase

5/ /120

= 0.042

Telophase

1/120

= 0.008

We can check our calculations by making sure that the proportions sum to 1.0, which they do.

Step 2. Determine the average duration of each stage. To determine the average duration of each stage, multiply the proportion of cells in each stage by the time required for the entire cell cycle:

Interphase 0.783 X 22 hours = 17.23 hours

Prophase 0.117 X 22 hours = 2.57 hours

Prometaphase 0.025 X 22 hours = 0.55 hour Metaphase 0.025 X 22 hours = 0.55 hour

Anaphase 0.042 X 22 hours = 0.92 hour

Telophase 0.008 X 22 hours = 0.18 hour

2. A cell in G1 of interphase has 8 chromosomes. How many chromosomes and how many DNA molecules will be found per cell as this cell progresses through the following stages: G2, metaphase of mitosis, anaphase of mitosis, after cytokinesis in mitosis, metaphase I of meiosis, metaphase II of meiosis, and after cytokinesis of meiosis II?

Remember the rules that we learned about counting chromosomes and DNA molecules: (1) to determine the number of chromosomes, count the functional centromeres; and (2) to determine the number of DNA molecules, count the chromatids. Think carefully about when and how the numbers of chromosomes and DNA molecules change in the course of mitosis and meiosis.

The number of DNA molecules increases only in S phase, when DNA replicates; the number of DNA molecules decreases only when the cell divides. Chromosome number increases only when sister chromatids separate in anaphase of mitosis and anaphase II of meiosis (homologous chromosomes, not chro-matids, separate in anaphase I of meiosis). Chromosome number, like the number of DNA molecules, is reduced only by cell division.

Let us now apply these principles to the problem. A cell in G1 has 8 chromosomes, each consisting of a single chromatid; so 8 DNA molecules are present in G1. DNA replicates in S stage; so, in G2, 16 DNA molecules are present per cell. However, the two copies of each DNA molecule remain attached at the centromere; so there are still only 8 chromosomes present. As the cell passes through prophase and metaphase of the cell cycle, the number of chromosomes and DNA molecules remains the same; so, at metaphase, there are 16 DNA molecules and 8 chromosomes. In anaphase, the chromatids separate and each becomes an independent chromosome; at this point, the number of chromosomes increases from 8 to 16. This increase is temporary, lasting only until the cell divides in telophase or subsequent to it. The number of DNA molecules remains at 16 in anaphase. The number of DNA molecules and chromosomes per cell is reduced by cytokinesis after telophase, because the 16 chromosomes and DNA molecules are now distributed between two cells. Therefore, after cytokinesis, each cell has 8 DNA molecules and 8 chromosomes, the same numbers that were present at the beginning of the cell cycle.

Now, let's trace the numbers of DNA molecules and chromosomes through meiosis. At G1, there are 8 chromosomes and 8 DNA molecules. The number of DNA molecules increases to 16 in S stage, but the number of chromosomes remains at 8 (each chromosome has two chromatids). The cell therefore enters metaphase I with 16 DNA molecules and 8 chromosomes. In anaphase I of meiosis, homologous chromosomes separate, but the number of chromosomes remains at 8. After cytokinesis, the original 8 chromosomes are distributed between two cells; so the number of chromosomes per cell falls to 4 (each with two chromatids). The original 16 DNA molecules also are distributed between two cells; so the number of DNA molecules per cell is 8. There is no DNA synthesis during interkinesis, and each cell still maintains 4 chromosomes and 8 DNA molecules through metaphase II. In anaphase II, the two chromatids of each chromosome separate, temporarily raising the number of chromosomes per cell to 8, whereas the number of DNA molecules per cell remains at 8. After cytokinesis, the chromosomes and DNA molecules are again distributed between two cells, providing 4 chromosomes and 4 DNA molecules per cell. These results are summarized in the following table:

Stage

Number of chromosomes per cell

Number of DNA molecules per cell

Stage

Number of chromosomes per cell

Gi

0 0

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