Biology 111B
Study Questions 8


THE CELL CYCLE AND MITOSIS

1. What is mitotic cell division used for in single-celled organisms? In multicellular organisms?

 In single-celled organisms, mitotic cell division creates more genetically identical individuals (a clone of individuals). In multicellular organisms, mitotic cell division is used for growth, development, and tissue renewal as well as sometimes for asexual reproduction.

2. How is DNA packaged in eukaryotic organisms?

 In eukaryotic chromosomes, DNA is packaged around a main protein scaffold and wound around histone proteins.

3. What is a chromosome? A chromatid? Chromatin?

 A chromosome is a single strand of DNA organized around the packaging proteins. It has a single centromere. A chromatid is one of a pair of replicated DNA molecules still connected by a common centromere. Chromatin is the DNA + protein that makes up the chromosome. It was so named early in microscopy because it stained darkly ("chromatin" = "colored stuff").

4. If there are 20 centromeres in a cell, how many chromosomes are there?

 20 centromeres = 20 chromosomes

5. What is a homologous pair of chromosomes?

Homologous chromosomes are two chromosomes that have the same length, the same centromere position, the same banding patterns...because they have the same genes, although not necessarily the same alleles of those genes.

6. What happens during each phase (G1, S, G2, mitosis, cytokinesis) of the cell cycle? Why is it referred to as a cycle?

During G1, the cell performs all the normal metabolic functions of life...many cells spend their lives in G0, the equivalent of G1 for a non-dividing cell. During the S phase, the DNA is replicated forming sister chromatids for each chromosome. During G2, more cytoplasm is synthesized in preparation for cell division. In mitosis, the chromatids are separated (forming chromosomes) into two nuclei. During cytokinesis, the cytoplasm is separated into two separate cells.

7. Measurements of the amount of DNA per nucleus were taken on a large number of cells from a growing fungus. The measured DNA levels ranged from 3 to 6 picograms per nucleus. One nucleus had 5 picograms of DNA. What stage of the cell cycle was this nucleus in?

 During the cell cycle, DNA levels range from a minimum during G1 to a maximum after S phase but before cytokinesis. Since the cell in question has more than the minimum but less than the maximum, it must be in S phase.

8. What is the "goal" of mitosis? Of cytokinesis?

 The "goal" of mitosis is to make two identical copies of a parental cell nucleus. The goal of cytokinesis is to apportion the cytoplasm to these two new daughter cells. Often this apportionment is equal, but occasionally (as in egg cell generation) one daughter cell gets a lot more cytoplasm than the other.

9. What are microtubules and what roles do they play in mitosis?

 Microtubules make up the spindle fibers. The spindle fibers are responsible for pulling the sister chromatids apart and guiding the new chromosomes to opposite ends of the cell to form the daughter nuclei.

10. How does cytokinesis differ in plant and animal cells?

 In animal cells, cytokinesis occurs by pinching in of the cell between the two daughter nuclei. This cleavage is done by microfilaments that contract in a ring around the middle of the cell. In plant cells, the cell wall prevents cleavage, so instead a dividing plant cell forms a new cell wall between the daughter nuclei.

11. How do the daughter cells at the end of mitosis and cytokinesis compare with their parent cell when it was in the:

G1 phase of the cell cycle?

 The two daughter cells are identical to the parent cell when the parent cell was in G1 phase.

G2 phase of the cell cycle?

 The two daughter cells have half the DNA (unreplicated chromosomes) as the parent cell did when it was in G2 (replicated chromosomes).

12. If there are 12 chromosomes in a zygote in the G1 stage of the cell cycle:

How many homologous pairs are present in the cell?

 6 homologous pairs

How many chromosomes were present in the egg?

 6 chromosomes

How many chromosomes were present in the sperm?

  6 chromosomes

13. A cell containing 92 chromatids at the start of mitosis would, at its completion, produce cells containing how many chromosomes?

 46 chromosomes

14. Colchicine is a drug that binds to the protein subunit of microtubules, thereby preventing microtubule formation. What would happen if sea urchin zygotes were placed into colchicine?

 The chromosomes would replicate, but would be unable to complete anaphase. If they survived this, the cells would have doubled their amount of DNA.

15. The DNA content of a group of cells was measured immediately following mitosis and found to be an average of 8 picograms of DNA per nucleus. How much DNA would the cells have:

At the end of S phase?

 16 picograms

At the end of G2 phase?

 16 picograms