Chapter 12

The cancer-causing forms of the Ras protein are involved in which of the following processes?

relaying a signal from a growth factor receptor

The decline of MPF activity at the end of mitosis is due to _____.

the degradation of cyclin.

A group of cells is assayed for DNA content immediately following mitosis and is found to have an average of 8 picograms of DNA per nucleus. How many picograms would be found at the end of S and the end of G2?

16;16

If there are 20 chromatids in a cell, how many centromeres are there?

20

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. In which stage of the cell cycle did the nucleus contain 6 picograms of DNA?

G2

The first gap in the cell cycle (G1) corresponds to _____.

Normal cell growth and function

Some cells have several nuclei per cell. How could such multinucleated cells be explained?

The cell underwent repeated cytokinesis but no mitosis.

Myosin is a motor protein involved in animal cell cytokinesis. It binds to ATP or ADP, causing the myosin to move with respect to actin. What is the effect of the interaction between myosin and actin?

The cleavage furrow deepens.

Once a cell completes mitosis, molecular division triggers must be turned off. What happens to MPF during mitosis?

The cyclin component of MPF is degraded.

Regulatory proteins that prevent a cell from entering the S phase under conditions of DNA damage are also known as _____.

Tumor suppressors

Which cytoskeletal proteins are important constituents of the contractile structures that form the cleavage furrows involved in animal cell cytokinesis?

actin

The MPF protein complex turns itself off by _____.

activating a process that destroys cyclin components

The microtubule-organizing center found in animal cells is an identifiable structure present during all phases of the cell cycle. Specifically, it is known as the _____.

centrosome

Proteins that are involved in the regulation of the cell cycle and that show fluctuations in concentration during the cell cycle are called _____.

cyclins

Vinblastine is a standard chemotherapeutic drug used to treat cancer. Because it interferes with the assembly of microtubules, its effectiveness must be related to _____.

disruption of mitotic spindle formation

Tumor-suppressor genes _____.

encode proteins that help prevent uncontrolled cell growth

Neurons and some other specialized cells divide infrequently because they _____.

have entered into G0

The product of the p53 gene _____.

inhibits the cell cycle

The M-phase checkpoint ensures that all chromosomes are attached to the mitotic spindle. If this does not happen, cells would most likely be arrested in _____.

metaphase

If a cell has accumulated DNA damage, it is unlikely to _____

pass the G2 checkpoint

At which phase are centrioles beginning to move apart in animal cells?

prophase

Which of the following does NOT occur during mitosis?

replication of DNA

Movement of the chromosomes during anaphase would be most affected by a drug that prevents _____.

shortening of microtubules

In human and many other eukaryotic species' cells, the nuclear membrane has to disappear to permit _____.

the attachment of microtubules to kinetochores

Researchers pulsed rapidly dividing cultured cells with radioactive thymidinefor 30 minutes. The cells were then exposed to a solution containing non-radiolabeled thymidine. Cells were analyzed at 2-hour intervals. At the 2-hour time point, no cells appeared to be dividing. Only after 4 hours did some labeled cells appear to be in M phase. This result can be explained in the following way:

wrong answer; The synthesis (S) phase is lengthy, about 12 hours in most cell types, and the radioactive thymidine was not present long enough for most cells to be labeled. The cells were arrested in a nondividing state because of the treatment and could not enter M phase until several hours after the label was removed.

Exposure of zebrafish nuclei to meiotic cytosol resulted in phosphorylation of NEP55 and L68 proteins by cyclin-dependent kinase 2. NEP55 is a protein of the inner nuclear membrane, and L68 is a protein of the nuclear lamina. What is the most likely role of phosphorylation of these proteins in the process of mitosis?

wrong answer; They are involved in the disassembly and dispersal of the nucleolus.

Cyclin-dependent kinase (Cdk) is _____.

wrong answer; the enzyme that catalyzes the attachment of chromosomes to microtubules present only during the S phase of the cell cycle