Chapter 14

In what way is cell division a link between humans and the earliest eukaryotic cells?

Suppose you are labeling a population of cells growing asynchronously with \(\left[^{3} \mathrm{H}\right]\) thymidine. \(\mathrm{G}_{1}\) is 6 hours, \(\mathrm{S}\) is 6 hours, \(\mathrm{G}_{2}\) is 5 hours, and \(\mathrm{M}\) is 1 hour. What percentage of the cells would be labeled after a 15 -minute pulse? What percentage of mitotic cells would be labeled after such a pulse? How long would you have to chase these cells before you saw any labeled mitotic chromosomes? What percentage of the cells would have labeled mitotic chromosomes if you chased the cells for 18 hours?

Give four distinct mechanisms by which a Cdk can be in activated.

A syncytium is a "cell" that contains more than one nucleus; examples are a skeletal muscle fiber and a blastula of a fly embryo. These two types of syncytia arise by very different pathways. What two mechanisms can you envision that could lead to formation of syncytia? What does this tell you about the relationship between mitosis and cytokinesis?

If you were to add \(\left[^{3} \mathrm{H}\right]\) thymidine to a cell as it underwent replication (S phase) prior to beginning meiosis, what percentage of the chromosomes of the gametes produced would be labeled? If one of these gametes (a sperm) were to fertilize an unlabeled egg, what percentage of the chromosomes of the two-cell stage would be labeled?

If the haploid number of chromosomes in humans is 23 and the amount of nuclear DNA in a sperm is \(1 \mathrm{C}\), how many chromosomes does a human cell possess in the following stages: metaphase of mitosis, prophase I of meiosis, anaphase I of meiosis I, prophase II of meiosis, anaphase II of meiosis. How many chromatids does the cell have at each of these stages? How much DNA (in terms of numbers of \(\mathrm{C}\) ) does the cell have at each of these stages?

Plot the amount of DNA in the nucleus of a spermatogonia from the \(\mathrm{G}_{1}\) stage prior to the first meiotic division through the completion of meiosis. Label each of the major stages of the cell cycle and of meiosis on the graph.

How many centrioles does a cell have at metaphase of mitosis?

Suppose you incubate a meiotic cell in \(\left[^{3} \mathrm{H}\right]\) thymidine between the leptotene and zygotene stages, then you fix the cell during pachytene and prepare an autoradiograph. You find that the chiasmata are sites of concentrations of silver grains. What does this tell you about the mechanism of recombination?

Assume for a moment that crossing-over did not occur. Would you agree that you received half of your chromosomes from each parent? Would you agree that you received one-quarter of your chromosomes from each grandparent? Would the answer to these questions change if you allowed for crossing-over to have occurred?

It was noted on page 586 that centrosome duplication and DNA synthesis are both initiated by cyclin \(\mathrm{E}-\mathrm{Cdk} 2,\) which becomes active at the end of \(G_{1} .\) A recent study found that if cyclin \(\mathrm{E}-\mathrm{Cdk} 2\) is activated at an earlier stage, such as the beginning of \(\mathrm{G}_{1}\), that centrosome duplication begins at that point in the cell cycle, but that DNA replication is not initiated until S phase would normally begin. Provide a hypothesis to explain why DNA synthesis does not begin as well. You might look back at Figure 13.20 for further information.