Essential Cell Biology

Bruce Alberts, Dennis Bray, Karen Hopkin

Chapter 18

The Cell-Division Cycle - all with Video Answers

Educators

LG

Chapter Questions

Problem 1

Consider the following statement: "All present-day cells have arisen by an uninterrupted series of cell divisions extending back in time to the first cell division." Is this strictly true?

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Problem 2

A population of proliferating cells is stained with a dye that becomes fluorescent when it binds to DNA, so that the amount of fluorescence is directly proportional to the amount of DNA in each cell. To measure the amount of DNA in each cell, the cells are then passed through a flow cytometer, an instrument that measures the amount of fluorescence in individual cells. The number of cells with a given DNA content is plotted on the graph below. Indicate on the graph where you would expect to find cells that are in $\mathrm{G}_{1}, \mathrm{S}, \mathrm{G}_{2},$ and mitosis. Which is the longest phase of the cell cycle in this population of cells?

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Problem 3

Why do you suppose cells have evolved a special Go phase to exit from the cell cycle, rather than just stopping in $G_{1}$ and not moving on to S phase?

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Problem 4

What might be the consequences if a cell replicated damaged DNA before repairing it?

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Problem 5

A small amount of cytoplasm isolated from a mitotic cell is injected into an unfertilized frog oocyte, causing the oocyte to enter M phase (see Figure $18-7$ A) A sample of the injected oocyte's cytoplasm is then taken and injected into a second oocyte, causing this cell also to enter M phase. The process is repeated many times until, essentially, none of the original protein sample remains, and yet, cytoplasm taken from the last in the series of injected oocytes is still able to trigger entry into M phase with undiminished efficiency. Explain this remarkable observation.

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Problem 6

If fine glass needles are used to manipulate a chromosome inside a living cell during early M phase, it is possible to trick the kinetochores on the two sister chromatids into attaching to the same spindle pole. This arrangement is normally unstable, but the attachments can be stabilized if the needle is used to gently pull the chromosome so that the microtubules attached to both kinetochores (via the same spindle pole) are under tension. What does this suggest to you about the mechanism by which kinetochores normally become attached and stay attached to microtubules from opposite spindle poles? Is the finding consistent with the possibility that a kinetochore is programmed to attach to microtubules from a particular spindle pole? Explain your answers.

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Problem 7

Consider the events that lead to the formation of the new nucleus at telophase. How do nuclear and cytosolic proteins become properly re-sorted so that the new nucleus contains nuclear proteins but not cytosolic proteins?

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Problem 8

Draw a detailed view of the formation of the new cell wall that separates the two daughter cells when a plant cell divides (see Figure $18-34$ ). In particular, show where the membrane proteins of the Golgi-derived vesicles end up, indicating what happens to the part of a protein in the Golgi vesicle membrane that is exposed to the interior of the Golgi vesicle. (Refer to Chapter 11 if you need a reminder of membrane structure.)

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Problem 9

The Golgi apparatus is thought to be partitioned into the daughter cells at cell division by a random distribution of fragments that are created at mitosis. Explain why random partitioning of chromosomes would not work.

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Problem 10

Why do you think apoptosis occurs by a different mechanism from the cell death that occurs in cell necrosis? What might be the consequences if apoptosis were not achieved in so neat and orderly a fashion, whereby the cell destroys itself from within and avoids leakage of its contents into the extracellular space?

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Problem 11

Roughly, how long would it take a single fertilized human egg to make a cluster of cells weighing $70 \mathrm{kg}$ by repeated divisions, if each cell weighs 1 nanogram just after cell division and each cell cycle takes 24 hours? Why does it take very much longer than this to make a 70 -kg adult human?

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Problem 12

The shortest eukaryotic cell cycles of all-shorter even than those of many bacteria- occur in many early animal embryos. These so-called cleavage divisions take place without any significant increase in the weight of the embryo. How can this be? Which phase of the cell cycle would you expect to be most reduced?

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Problem 13

One important biological effect of a large dose of ionizing radiation is to halt cell division.
A. How does this occur?
B. What happens if a cell has a mutation that prevents it from halting cell division after being irradiated?
C. What might be the effects of such a mutation if the cell is not irradiated?
D. An adult human who has reached maturity will die within a few days of receiving a radiation dose large enough to stop cell division. What does that tell you (other than that one should avoid large doses of radiation)?

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Problem 14

If cells are grown in a culture medium containing radioactive thymidine, the thymidine will be covalently incorporated into the cell's DNA during S phase. The radioactive DNA can be detected in the nuclei of individual cells by autoradiography (i.e., by placing a photographic emulsion over the cells, radioactive cells will activate the emulsion and be labeled by black dots when looked at under a microscope). Consider a simple experiment in which cells are radioactively labeled by this method for only a short period (about 30 minutes). The radioactive thymidine medium is then replaced with one containing unlabeled thymidine, and the cells are grown for some additional time. At different time points after replacement of the medium, cells are examined in a microscope. The fraction of cells in mitosis (which can be easily recognized because the cells have rounded up and their chromosomes are condensed) that have radioactive DNA in their nuclei is then determined and plotted as a function of time after the labeling with radioactive thymidine (Figure $Q 18-14$ ).
A. Would all cells (including cells at all phases of the cell cycle) be expected to contain radioactive DNA after the labeling procedure?
B. Initially there are no mitotic cells that contain radioactive DNA (see Figure $018-14$ ). Why is this?
C. Explain the rise and fall and then rise again of the curve.
D. Estimate the length of the $G_{2}$ phase from this graph.

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Problem 15

One of the functions of M-Cdk is to cause a precipitous drop in M-cyclin concentration halfway through M phase. Describe the consequences of this sudden decrease and suggest possible mechanisms by which it might occur.

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Problem 16

Figure $18-5$ shows the rise of cyclin concentration and the rise of M-Cdk activity in cells as they progress through the cell cycle. It is remarkable that the cyclin concentration rises slowly and steadily, whereas M-Cdk activity increases suddenly. How do you think this difference arises?

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Problem 17

What is the order in which the following events occur during cell division:
A. anaphase
B. metaphase
C. prometaphase
D. telophase
E. lunar phase
F. mitosis
G. prophase
Where does cytokinesis fit in?

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Problem 18

The lifetime of a microtubule in a mammalian cell, between its formation by polymerization and its spontaneous disappearance by depolymerization, varies with the stage of the cell cycle. For an actively proliferating cell, the average lifetime is 5 minutes in interphase and 15 seconds in mitosis. If the average length of a microtubule in interphase is $20 \mu \mathrm{m},$ how long will it be during mitosis, assuming that the rates of microtubule elongation due to the addition of tubulin subunits in the two phases are the same?

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Problem 19

The balance between plus-end-directed and minus-enddirected motor proteins that bind to interpolar microtubules in the overlap region of the mitotic spindle is thought to help determine the length of the spindle. How might each type of motor protein contribute to the determination of spindle length?

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Problem 20

Sketch the principal stages of mitosis, using Panel $18-1$ (pp. $622-623$ ) as a guide. Color one sister chromatid and follow it through mitosis and cytokinesis. What event commits this chromatid to a particular daughter cell? Once initially committed, can its fate be reversed? What may influence this commitment?

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Problem 21

The polar movement of chromosomes during anaphase A is associated with microtubule shortening. In particular, microtubules depolymerize at the ends at which they are attached to the kinetochores. Sketch a model that explains how a microtubule can shorten and generate force yet remain firmly attached to the chromosome.

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Problem 22

Rarely, both sister chromatids of a replicated chromosome end up in one daughter cell. How might this happen? What could be the consequences of such a mitotic error?

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Problem 23

Which of the following statements are correct? Explain your answers.
A. Centrosomes are replicated before $\mathrm{M}$ phase begins.
B. Two sister chromatids arise by replication of the DNA of the same chromosome and remain paired as they line up on the metaphase plate.
C. Interpolar microtubules attach end-to-end and are therefore continuous from one spindle pole to the other.
D. Microtubule polymerization and depolymerization and microtubule motor proteins are all required for DNA replication.
E. Microtubules nucleate at the centromeres and then connect to the kinetochores, which are structures at the centrosome regions of chromosomes.

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Problem 24

An antibody that binds to myosin prevents the movement of myosin molecules along actin filaments (the interaction between actin and myosin is described in Chapter 17 ). How do you suppose the antibody exerts this effect? What might be the result of injecting this antibody into cells
(A) on the movement of chromosomes at anaphase or
(B) on cytokinesis? Explain your answers.

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Problem 25

Look carefully at the electron micrographs in Figure $18-37$ Describe the differences between the cell that died by necrosis and those that died by apoptosis.

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Problem 26

Which of the following statements are correct? Explain your
answers.
A. Cells do not pass from $G_{1}$ into $M$ phase of the cell cycle unless there are sufficient nutrients to complete an entire cell cycle.
B. Apoptosis is mediated by special intracellular proteases, one of which cleaves nuclear lamins.
C. Developing neurons compete for limited amounts of survival factors.
D. Some vertebrate cell-cycle control proteins function when expressed in yeast cells.
E. The enzymatic activity of a Cdk protein is determined both by the presence of a bound cyclin and by the phosphorylation state of the Cdk.

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Problem 27

Compare the rules of cell behavior in an animal with the rules that govern human behavior in society. What would happen to an animal if its cells behaved as people normally behave in our society? Could the rules that govern cell behavior be applied to human societies?

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Problem 28

In his highly classified research laboratory, Dr. Lawrence M. is charged with the task of developing a strain of dog-sized rats to be deployed behind enemy lines. In your opinion, which of the following strategies should Dr. M. pursue to increase the size of rats?
A. Block all apoptosis.
B. Block p53 function.
C. Overproduce growth factors, mitogens, or survival factors.
D. Obtain a taxi driver's license and switch careers.
Explain the likely consequences of each option.

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Problem 29

$\mathrm{PDGF}$ is encoded by a gene that can cause cancer when expressed inappropriately. Why do cancers not arise at wounds in which PDGF is released from platelets?

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Problem 30

What do you suppose happens in mutant cells that
A. cannot degrade M-cyclin?
B. always express high levels of p21?
C. cannot phosphorylate Rb?

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Problem 31

Liver cells proliferate excessively both in patients with chronic alcoholism and in patients with liver cancer. What are the differences in the mechanisms by which cell proliferation is induced in these diseases?

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