Question 1

How is Cdc20-APC/C similar to Cdh1-APC/C?
A) They are both active throughout interphase.
B) They are both inhibited by M-Cdk.
C) They both inhibit M-Cdk activity.
D) They are both activated suddenly at the onset of mitosis.
E) They are both inactivated soon after anaphase.

Accepted Answer

Although regulated differently, both APC/C complexes target M-cyclins for degradation.

Question 2

Which of the following motor proteins are more directly involved in anaphase B?
A) Kinesin-5 on interpolar microtubules and dynein on kinetochore microtubules
B) Kinesin-13 on kinetochore microtubules and dynein on astral microtubules
C) Kinesins 4 and 10 on interpolar and astral microtubules and dynein on kinetochore microtubules
D) Kinesin-5 on interpolar microtubules and kinesins 4 and 10 on interpolar and astral microtubules
E) Kinesin-5 on interpolar microtubules and dynein on astral microtubules

Accepted Answer

Two major forces act in anaphase B. Kinesin-5 pushes the two poles away from each other, and dynein pulls the poles toward the cell periphery.

Question 3

To study chromosome movement during anaphase in mammals, you have injected fluorescently labeled tubulin subunits into cells such that microtubules can be seen under a fluorescence microscope. You then use a relatively strong laser beam to bleach the fluorescent dyes in a limited area of a metaphase cell, as indicated in the schematic diagram below, and follow the progression of mitosis under the microscope by time-lapse imaging. You then use the images to measure the change in distance between various components, and plot the results in the graph below. Answer the following question(s) based on these results.
 
-Based on the results from the experiment, which is dominant in this cell: anaphase A or anaphase B? Write down A or B as your answer.

Accepted Answer

In this example, the two poles are pushed away from each other only slightly during anaphase.

Question 4

Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models.
 
-In experiments on the flattened eggs of the sea urchin Tripneustes gratilla, cytokinesis was analyzed after placing a small physical barrier (e.g. a glass needle or an oil droplet) between the spindle and various areas of the cell cortex. Blocks that were located over the equatorial plane prevented furrow formation in the proximal membrane, whereas blocks located in other areas did not affect furrowing. Which model (1 to 3) is NOT supported by these results? Write down 1, 2, or 3 as your answer.

Accepted Answer

The astral relaxation model (3) cannot explain these results.

Question 5

Formin nucleates the growth of parallel actin bundles in the cell. Additionally, myosin motor activity is positively regulated by phosphorylation. The monomeric G protein RhoA is important in cytokinesis, because it directly or indirectly …
A) activates formins and inactivates myosin light-chain phosphatase.
B) inactivates formins and activates myosin light-chain kinases.
C) activates formins as well as myosin light-chain phosphatases.
D) inactivates formins as well as myosin light-chain kinases.
E) None of the above.

Accepted Answer

This G protein activates formin as well as multiple protein kinases, including the Rho-activated kinase Rock. Rock phosphorylates regulatory myosin light chains and also inhibits myosin light-chain phosphatase by phosphorylation.

Question 6

Imagine a prometaphase chromosome pair that is attached to one spindle pole. Which of the following would happen if both arms of the chromosome are severed with a strong laser beam?
A) All chromosome fragments (the centromere-containing fragment and the arm fragments) would be pushed away from the pole.
B) Only the two arm fragments would be pulled toward the pole. The kinetochore-containing fragment would remain stationary.
C) The kinetochore-containing fragment would be pulled toward the pole. The two arm fragments would move away from the pole.
D) The kinetochore-containing fragment would move away from the pole, but the two arm fragments would be pulled toward the pole.
E) All chromosome fragments would be pulled toward the pole.

Accepted Answer

The result of such an experiment has provided evidence for polar ejection forces acting on chromosome arms.

Question 7

Indicate true (T) and false (F) statements below regarding cytokinesis in eukaryotic cells. Your answer would be a four-letter string composed of letters T and F only, e.g. TFFF.
( )	The preprophase band, composed of microtubules and actin filaments, marks the site of cytokinesis in plant cells.
( )	In some cell types, the site of contractile-ring formation is determined before mitosis.
( )	The early cell plate in dividing plant cells appears after phragmoplast formation.
( )	The membrane required for the newly formed cell plate in plant cells is provided by endocytosis from the equatorial plasma membrane.

Accepted Answer

In plant cells, the preprophase band assembles just before mitosis and marks the site of cytokinesis. The assembly of the cell plate between the two daughter cells is then guided by phragmoplast. The membrane required for the formation of cell plate comes from the Golgi apparatus.

Question 8

Treatment of dividing cells with a low dose of the antifungal drug benomyl, which destabilizes microtubules, slows down correct spindle assembly. But at such doses, the spindle is eventually formed and the cells survive. However, mutations in some genes confer benomyl sensitivity: the mutant cells die because they fail to arrest the cell cycle in the presence of unattached kinetochores and progress through anaphase, with disastrous consequences. Which of the following would you expect to be a benomyl-sensitive mutant?
A) A loss-of-function mutant in the gene encoding Mad2.
B) A mutation causing the overexpression of Cdc20.
C) A loss-of-function mutation in the gene encoding a kinase that inhibits Cdc20-APC/C.
D) A loss-of-function mutation in the gene encoding a tubulin subunit.
E) All of the above.

Accepted Answer

All of these mutations make the cells more sensitive to the benzimidazole drug.

Question 9

The microtubule-binding protein Patronin binds to the minus ends of microtubules at the spindle pole and protects them from the effect of catastrophe factors. If Patronin activity is inhibited by injecting an anti-Patronin antibody into Drosophila melanogaster embryos prior to cellularization, anaphase B is suppressed and the spindles are significantly shorter. Which of the forces (1 or 2) in the following schematic diagram do you think is mostly responsible for anaphase B in this organism at this stage? Does the effect of Patronin inhibition resemble that of kinesin-5 inhibition (I) or overactivation (O)? 
A) 1; I
B) 1; O
C) 2; I
D) 2; O

Accepted Answer

The force generated by kinesin-5 to lengthen the spindle in anaphase B is counteracted by catastrophe factors at the poles, which are inhibited by Patronin. Inhibition of Patronin would therefore result in anaphase B suppression.

Question 10

If cells that have started mitosis are treated with nocodazole, a drug that depolymerizes microtubules, what would you expect to happen? Where would you expect Mad2 protein to be localized?
A) The cells would arrest in prometaphase; Mad2 would be localized to the spindle poles.
B) The cells would arrest in telophase; Mad2 would be localized to the spindle poles.
C) The cells would immediately enter anaphase, finish mitosis, and enter G₁; Mad2 would be localized to the spindle poles.
D) The cells would arrest in prometaphase; Mad2 would be localized to almost all kinetochores.
E) The cells would arrest in telophase; Mad2 would be localized to almost all kinetochores.

Accepted Answer

A cell treated with nocodazole fails to assemble a functional spindle, and therefore arrests in prometaphase with the chromosomes left unattached. Mad2 is recruited to unattached kinetochores and activates the spindle assembly checkpoint, preventing the onset of anaphase.

Question 11

Mutant Saccharomyces cerevisiae cells that lack the gene encoding securin can divide more or less normally by mitosis, without significant chromosome segregation defects. Cells harboring a nondegradable version of securin, on the other hand, arrest in metaphase as expected, since they cannot activate separase to enter anaphase. Similarly, cells lacking Cdc20 arrest in metaphase, since they cannot activate APC/C. Finally, cells lacking both securin and Cdc20 arrest in anaphase: they manage to separate sister chromatids, but do not progress much further. These results suggest that in wild-type cells, …
A) degradation of securin is necessary to trigger sister-chromatid separation.
B) degradation of securin is sufficient to trigger sister-chromatid separation.
C) Cdc20-APC/C is NOT necessary for sister-chromatid separation.
D) Cdc20-APC/C is NOT necessary for later events in anaphase.
E) All of the above.

Accepted Answer

Both Cdc20 activation and securin destruction are necessary for sister-chromatid separation because cells in which either of these fail to occur are observed to arrest in metaphase. Securin degradation does not appear to be sufficient for sister-chromatid separation, as the cells lacking securin are more or less healthy, meaning other mechanisms also control the separation. Cdc20 appears to be necessary for anaphase beyond sister-chromatid separation, since the double knockouts still arrest in anaphase despite lacking securin.

Question 12

Indicate whether each of the following phosphorylation events typically activates (A) or inactivates (I) the protein that is being phosphorylated. Your answer would be a four-letter string composed of letters A and I only, e.g. IIIA.
( )	Phosphorylation of Cdc25 by M-Cdk
( )	Phosphorylation of condensin subunits by M-Cdk
( )	Phosphorylation of kinesin-5 by Aurora-A
( )	Phosphorylation of Ndc80 subunits by Aurora-B

Accepted Answer

M-Cdk activates the Cdc25 phosphatase in a positive feedback loop. It also phosphorylates condensin subunits, stimulating DNA binding and supercoiling activity. Along with Aurora-A, it also phosphorylates kinesins such as kinesin-5, stimulating microtubule binding and motor activity. Without tension in the kinetochore, Aurora-B phosphorylates and inactivates components of Ndc80, lowering its affinity for microtubule plus ends.

Question 13

Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models.
 
-In classic experiments carried out in the 1960s and schematized below, an egg of the sand dollar Echinarachnius parma was made into a torus shape using a glass bead on the end of a needle, to study the influence of microtubules on cleavage-furrow positioning. The cells were allowed to undergo two rounds of mitosis. After the second mitosis, an extra furrow (indicated by a red arrowhead) was observed between the two asters that did not have a spindle between them. Which of the models (1 to 3) is more consistent with this observation? Write down 1, 2, or 3 as your answer.

Accepted Answer

The astral stimulation model (1) predicts this outcome. The central spindle stimulation model (2) predicts the formation of two furrows when there are two spindles. However, it is also likely that a midzone-like structure formed between the two spindles in these experiments, and so model 2 could also be correct here. The astral relaxation model (3) predicts the formation of two furrows in the first division.

Question 14

Indicate whether each of the following, when active, tends to activate (A) or inactivate (I) M-Cdks. Your answer would be a four-letter string composed of letters A and I only, e.g. IIAI.
( )	M-Cdk
( )	Cdc20-APC/C
( )	Cdh1-APC/C
( )	Sic1

Accepted Answer

Both Cdc20-APC/C and Cdh1-APC/C target M-cyclins for degradation. M-Cdk, in turn, activates the former and inactivates the latter. Sic1 in budding yeast is an example of a CKI that also inhibits M-Cdks. M-Cdk activates the expression of M-cyclin genes in a positive feedback loop, although its activity is also regulated by negative feedback, for example through Cdc20-APC/C mentioned above.

Question 15

In the following schematic drawing of a vertebrate prometaphase chromosome as seen under a microscope, indicate whether the chromosome is more likely to be closer to pole A or pole B. Write down A or B as your answer.

Accepted Answer

The polar wind, mediated by plus-end directed kinesin motors, gets stronger as the chromosome approaches a pole, and pushes the chromosome arms toward the opposite pole.

Question 16

Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models.
 
-In the early embryos of Caenorhabditis elegans, defects in the formation of astral microtubules increase myosin activity throughout the cell cortex. If the spindle is forced to one side of the cell, the cortical myosin activity is observed mostly at the opposite side of the cell. Which model (1, 2, or 3) better predicts these observations? Write down 1, 2, or 3 as your answer.

Accepted Answer

These observations are in better agreement with the astral relaxation model (3).

Question 17

In most mammalian cells, low M-cyclin protein levels are maintained during G₁ phase. What is mainly responsible for maintaining these low levels?
A) Cdc20-APC/C
B) Cdh1-APC/C
C) Skp2-SCF
D) β-trCP-SCF
E) p27

Accepted Answer

Cdh1-APC/C activity increases in late mitosis and keeps the M-Cdk activity low via M-cyclin degradation during G₁.

Question 18

To study chromosome movement during anaphase in mammals, you have injected fluorescently labeled tubulin subunits into cells such that microtubules can be seen under a fluorescence microscope. You then use a relatively strong laser beam to bleach the fluorescent dyes in a limited area of a metaphase cell, as indicated in the schematic diagram below, and follow the progression of mitosis under the microscope by time-lapse imaging. You then use the images to measure the change in distance between various components, and plot the results in the graph below. Answer the following question(s) based on these results.
 
-Based on the results from your experiment, which force is dominant in chromosome movement in this cell: polar ejection force (E), microtubule flux (F), or kinetochore microtubule plus-end depolymerization (P)? Write down E, F, or P as your answer.

Accepted Answer

In these cells, the chromosomes seem to approach the nearby pole as fast as the bleached zone approaches the pole, meaning that the microtubule subunit flux is enough to explain the poleward chromosome movement.

Question 19

Indicate true (T) and false (F) statements below regarding cytokinesis in animal cells. Your answer would be a four-letter string composed of letters T and F only, e.g. TFFF.
( )	The force for cytokinesis is generated by kinesin motors on microtubule bundles that form the contractile ring.
( )	As the contractile ring constricts, its thickness increases to keep a constant volume.
( )	The midbody forms from bundles of actin and myosin II.
( )	Local activation of Ran GTPase triggers the assembly and contraction of the contractile ring.

Accepted Answer

The contractile ring is based on actin and myosin II, constricts in a dynamic fashion, and gradually disassembles. RhoA GTPase helps assemble this structure.

Question 20

In the following schematic diagram of an early Drosophila embryo, which step (A to C) corresponds to cellularization? Write down A, B, or C as your answer.

Accepted Answer

Cellularization is the coordinated cytokinesis that occurs after 13 mitoses in early fruit-fly development.

Quiz 17: The Cell Cycle

How is Cdc20-APC/C similar to Cdh1-APC/C?

Which of the following motor proteins are more directly involved in anaphase B?

Formin nucleates the growth of parallel actin bundles in the cell. Additionally, myosin motor activity is positively regulated by phosphorylation. The monomeric G protein RhoA is important in cytokinesis, because it directly or indirectly …

Imagine a prometaphase chromosome pair that is attached to one spindle pole. Which of the following would happen if both arms of the chromosome are severed with a strong laser beam?

If cells that have started mitosis are treated with nocodazole, a drug that depolymerizes microtubules, what would you expect to happen? Where would you expect Mad2 protein to be localized?

Indicate whether each of the following, when active, tends to activate (A) or inactivate (I) M-Cdks. Your answer would be a four-letter string composed of letters A and I only, e.g. IIAI. ( ) M-Cdk ( ) Cdc20-APC/C ( ) Cdh1-APC/C ( ) Sic1

In the following schematic drawing of a vertebrate prometaphase chromosome as seen under a microscope, indicate whether the chromosome is more likely to be closer to pole A or pole B. Write down A or B as your answer.

In most mammalian cells, low M-cyclin protein levels are maintained during G₁ phase. What is mainly responsible for maintaining these low levels?

In the following schematic diagram of an early Drosophila embryo, which step (A to C) corresponds to cellularization? Write down A, B, or C as your answer.