Deck 17: The Cell Cycle

Question 1

Which of the following events occurs in mitotic metaphase?

A) Nuclear envelope breakdown
B) Nuclear envelope reassembly
C) Chromosome attachment to spindle microtubules for the first time
D) Chromosome alignment at the spindle equator
E) Mitotic spindle assembly

Answer 1

D
Explanation: The alignment of chromosomes at the cell equator is the hallmark of metaphase.

Question 2

Consider two mammalian cells, one in G₁ and the other in G₀ (stationary) phase. If they are stimulated to pass the restriction point by the addition of an extracellular proliferation signal, but the signal is then immediately removed, what would you expect to happen?

A) Both cells will replicate their DNA.
B) Only the G₁ cell will replicate its DNA.
C) Only the G₀ cell will replicate its DNA.
D) Only the G₁ cell will start to replicate its DNA, but will stop halfway through the replication and will not reach G₂.
E) Neither of the cells will replicate their DNA.

Answer 2

A
Explanation: Once a cell passes the restriction point, it is committed to DNA replication (and cell division) even if the stimulating signals are removed.

Question 3

How is centrosome duplication similar to DNA replication?

A) They both use a semiconservative mechanism.
B) They are initiated at around the same time in the cell cycle, near the G₁/S transition.
C) They are both controlled in such a way that they replicate once and only once per cell cycle.
D) They are both separated from their sister copies in mitosis.
E) All of the above.

Answer 3

E
Explanation: The centrosomes, like the chromosomes, are duplicated only once during each cell cycle. Centrosome duplication is triggered by the activation of G₁/S-Cdk. Both centrosome duplication and DNA replication employ a semiconservative mechanism. The two new centrosomes remain together on one side of the cell until entry into mitosis.

Question 4

You have been studying the effect of loss-of-function mutations in the Cdk inhibitor protein (CKI) p21. You add the drug fucoxanthin to cell cultures harboring either wild-type or mutant versions of the p21 gene. Fucoxanthin is known to induce cell-cycle arrest in G1. After a day, you add the thymidine analog BrdU to the culture media, collect the cells after an hour, treat them with anti-BrdU antibody and the fluorescent DNA stain DAPI, and finally subject them to fluorescence-activated cell sorting (FACS). The FACS data can be viewed as a two-dimensional dot plot composed of thousands of dots, in which each cell is represented by one dot at coordinates that correspond to the intensities of the DAPI fluorescence signal (X axis) and BrdU fluorescence signal (Y axis) for that cell. Answer the following question(s) according to the simplified dot plot below, generated from your experiment.


-Which one of the FACS plots (a or b) would you expect to correspond to the loss-of-function p21 mutants? Write down a or b as your answer.

Question 5

Which of the time points (A to E) in the following schematic drawing of the mammalian cell cycle represents the restriction point?

Question 6

Consider two kinesin motor proteins at the mitotic spindle midzone: kinesin-5 is a tetrameric motor that walks toward the plus end of both microtubules to which it is attached via its motor domains; kinesin-14, on the other hand, walks toward the minus end of one microtubule while it is attached to another microtubule via its tail domain. How do these motors affect the length of the spindle?

A) They both work to shorten the spindle.
B) Kinesin-5 works to shorten the spindle whereas kinesin-14 works to lengthen it.
C) Kinesin-5 works to lengthen the spindle whereas kinesin-14 works to shorten it.
D) They both work to lengthen the spindle.

Question 7

In the fission yeast Schizosaccharomyces pombe, the proteins Cut1 and Cut2 form a complex that is catalytically inactive. At the onset of anaphase, Cut2 is polyubiquitylated by a large E3 complex containing Cut4, Cut9, Cut23, and other proteins, and is subsequently destroyed. Cut1 then cleaves Rad21, a non-SMC component of a complex that also contains two SMC proteins, thus allowing sister-chromatid separation. Mutations in the cut genes lead to the cut phenotype, in which the cell attempts cytokinesis without chromosome segregation. Indicate which of the following proteins or protein complexes corresponds to or contains the product of the genes cut1 (A), cut2 (B), cut4 (C), and rad21 (D). Your answer would be a four-letter string composed of letters A to D only, e.g. DCAB.
( ) Securin
( ) Cohesin
( ) Anaphase-promoting complex
( ) Separase

Question 8

Indicate whether each of the following descriptions better applies to a Wee1 protein (W) or a Cdc25 protein (C). Your answer would be a four-letter string composed of letters W and C only, e.g. WWCW.
( ) It is a protein kinase.
( ) It activates M-Cdk complexes.
( ) It is activated by M-Cdk complexes.
( ) Its loss in fission yeast results in small cell size.

Question 9

Which one of the following chromosomes has formed stable attachment(s) to the spindle microtubules?

A)
B)
C)
D)
E)

Question 10

Resveratrol is a natural compound found in red grapes (and red wine) and is thought to have beneficial effects in mammals, such as preventing tumor growth and delaying age-related diseases. In vitro, resveratrol and its derivatives have been shown to cause cell-cycle arrest in S phase and induce apoptosis. You have analyzed the DNA content of cultured cells in the presence and absence of these drugs using fluorescence-activated cell sorting. Compared with control cells (dashed line), which of the following curves do you think better represents the DNA content of cells treated with these compounds?

Question 11

The cell cycle can be arrested in mitosis when a single sister-chromatid pair is mono-oriented on the mitotic spindle. If at this point a glass microneedle is used to pull the mono-oriented chromosome toward the pole to which it is not attached, the cell proceeds to anaphase. This observation confirms that …

A) mono-oriented chromosomes are stable.
B) mechanical tension drives the formation of bi-oriented chromosomes.
C) bi-oriented chromosomes activate the spindle assembly checkpoint.
D) lack of mechanical tension at the kinetochore in at least one chromosome prevents entry into anaphase.
E) mechanical tension at the kinetochore in at least one chromosome is required for entry into anaphase.

Question 12

Which of the following is more directly driven by active M-Cdk?

A) Centrosome maturation
B) Centrosome duplication
C) Nuclear envelope reassembly
D) Inactivation of APC/C
E) Cell cleavage

Question 13

Mammalian Cdk inhibitor proteins (CKIs) can be grouped into two families based on their structural and functional differences. The Cip/Kip family proteins (e.g. p21) have a broad binding specificity. These proteins bind preferentially to already formed cyclin-Cdk complexes and thus enhance complex formation. However, they inhibit the kinase activity of most complexes (e.g. S-Cdks), except in the case of G?-Cdk complexes where no inhibition occurs. Consequently, Cip/Kip family proteins have an overall positive effect on Cdk4/6 activity due to their help in bringing the subunits together. In contrast, the inhibitors of the INK4 family (e.g. p16) bind only to the Cdk subunit of G?-Cdks and prevent binding of both the G? cyclins and the Cip/Kip family CKIs. Based solely on these findings, would you expect p16 to activate (A) or inactivate (I) the S-Cdks in the presence of limited amounts of p21?

Question 14

Which of the following simplified diagrams better shows the changes in the concentrations of three major cyclin-Cdk complexes (G?/S-Cdk, S-Cdk, and M-Cdk) in the cell in different stages of the cell cycle?

Question 15

Indicate whether each of the following occurs mainly in G₁ phase (G), S phase (S), or G₂ phase (H) of the cell cycle. Your answer would be a four-letter string composed of letters G, S, and H only, e.g. HGSS.
( ) DNA helicase activation
( ) DNA helicase deposition on DNA at the replication origins
( ) ORC phosphorylation
( ) Licensing of replication origins

Question 16

Fill in the blank: "The … is a large structure formed at the centromeric region of each eukaryotic chromosome. It captures spindle microtubules in mitosis, and therefore serves to attach the chromosomes to the spindle poles."

Question 17

Sort the following schematic diagrams (A to F) to reflect the order of events in a typical eukaryotic M phase. An interphase cell in G₂phase is drawn on the left for comparison. Your answer would be a six-letter string composed of letters A to F only, e.g. BCEADF.

Question 18

You have been studying the effect of loss-of-function mutations in the Cdk inhibitor protein (CKI) p21. You add the drug fucoxanthin to cell cultures harboring either wild-type or mutant versions of the p21 gene. Fucoxanthin is known to induce cell-cycle arrest in G1. After a day, you add the thymidine analog BrdU to the culture media, collect the cells after an hour, treat them with anti-BrdU antibody and the fluorescent DNA stain DAPI, and finally subject them to fluorescence-activated cell sorting (FACS). The FACS data can be viewed as a two-dimensional dot plot composed of thousands of dots, in which each cell is represented by one dot at coordinates that correspond to the intensities of the DAPI fluorescence signal (X axis) and BrdU fluorescence signal (Y axis) for that cell. Answer the following question(s) according to the simplified dot plot below, generated from your experiment.


-Indicate which boxed region (1, 2, or 3) in the FACS plots corresponds better to each of the following phases of the cell cycle. Your answer would be a three-digit number composed of numbers 1 to 3, with each number used once, e.g. 312.
( ) G₁ phase
( ) S phase
( ) G₂ and M phases

Question 19

Indicate true (T) and false (F) statements below regarding mitosis and the changes that it brings about compared to interphase. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTF.
( ) Microtubules become greatly stabilized in mitosis compared to interphase.
( ) The number of ?-tubulin ring complexes in the centrosomes increases greatly during mitosis compared to interphase.
( ) Chromosomes stabilize mitotic microtubules through activation of Ran monomeric G protein.
( ) A bipolar spindle can be formed even in the absence of centrosomes.

Question 20

The following schematic diagram shows the activation of M-Cdk in mitosis. Indicate which proteins below correspond to those indicated as A to D in the diagram. Your answer would be a four-letter string composed of letters A to D only, e.g. BACD.

( ) Wee1
( ) CAK
( ) Cdc25
( ) M-cyclin

Question 21

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.

Question 22

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

Question 23

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.

Question 24

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.

Question 25

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.

Question 26

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.

Question 27

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.

Question 28

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.

Question 29

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

Question 30

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.

Question 31

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.

Question 32

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

Question 33

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.

Question 34

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

Question 35

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.

Question 36

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.

Question 37

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

Question 38

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.

Question 39

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.

Question 40

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.

Question 41

Which of the following cell populations in our body has the highest mitotic index?

A) Neurons
B) Hepatocytes
C) Red blood cells
D) Fibroblasts
E) Skeletal myocytes

Question 42

Indicate true (T) and false (F) statements below regarding meiosis in eukaryotic cells. Your answer would be a four-letter string composed of letters T and F only, e.g. TFFF.
( ) Bivalents form before prophase I.
( ) Crossing-over begins before the synaptonemal complex assembly.
( ) Chiasmata can first be seen under the microscope after the disassembly of the synaptonemal complexes.
( ) All recombination events lead to crossovers.

Question 43

Which one better supports cell proliferation when added to fibroblast cultures: serum or plasma? This activity is due to the presence of mitogens in this fluid. What is responsible for making these mitogens?

A) Serum; red blood cells
B) Serum; platelets
C) Plasma; red blood cells
D) Plasma; white blood cells
E) Plasma; platelets

Question 44

Indicate true (T) and false (F) statements below regarding cellular control of growth and division. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTF.
( ) Replicative cell senescence usually arises due to the accumulation of mutations in genes encoding S- and M-cyclins.
( ) Replicative cell senescence is caused by the induction of apoptosis by p53.
( ) Excessive mitogenic stimulation can result in Mdm2 activation and the induction of apoptosis or cell-cycle arrest.
( ) The Mdm2 inhibitor Arf induces cell-cycle progression.

Question 45

Which of the following proteins is the product of an immediate early gene expressed following mitogenic stimulation of cell-cycle entry?

A) E2F
B) Rb
C) Myc
D) G₁-cyclins
E) All of the above

Question 46

Which of the following events contributes to driving the mammalian cell past the restriction point of the cell cycle?

A) Phosphorylation of Cdh1-APC/C by G₁/S-Cdk
B) Destruction of CKIs that target S-Cdks
C) Phosphorylation of Rb by G₁-Cdk, G₁/S-Cdk, and S-Cdk
D) Activation of E2F gene expression by active E2F protein
E) All of the above

Question 47

Which of the following is an inhibitory extracellular signal for cell proliferation?

A) EGF
B) TGFβ
C) Erythropoietin
D) IGF
E) PDGF

Question 48

In a multicellular organism such as a mammal, loss-of-function mutations in many genes can contribute to the development of cancer. These genes are therefore called tumor suppressors. In their absence, the cell fails to stop progression through the cell cycle under conditions in which normal cells would arrest, paving the way for tumorigenesis. Indicate whether each of the following proteins is (Y) or is not (N) expected to be the product of a tumor suppressor gene based on its function in the cell cycle. Your answer would be a four-letter string composed of letters Y and N only, e.g. YNNN.
( ) Rb
( ) Myc
( ) p53
( ) p21

Question 49

Loss of the gene encoding shugoshin in many multicellular organisms leads to sterility, suggesting defects in meiosis. What would you expect to occur in meiotic cells lacking shugoshin?

A) The homologs fail to separate in anaphase I.
B) The sister chromatids fail to separate in anaphase II.
C) All chromatids separate prematurely in anaphase I.
D) Removal of cohesion between homolog arms fails in prophase I.
E) Removal of cohesion between sister chromatids fails in prophase II.

Question 50

Which division in meiosis is more similar to mitosis? In which division do sister chromatids separate from each other?

A) Meiosis I; meiosis I
B) Meiosis I; meiosis II
C) Meiosis II; meiosis I
D) Meiosis II; meiosis II

Question 51

You have synchronized a culture of HeLa cells so that the cells are all at the same stage in the cell cycle. You then treat the cells with serum containing nutrients and growth factors in the presence or absence of the drug rapamycin. The distribution of cell sizes in the two cell populations is shown below. Based on these results, do you think rapamycin is an activator or inhibitor of the TOR complex? Is the S6 kinase expected to be up-regulated or down-regulated in the presence of rapamycin?

A) Activator; up-regulated
B) Activator; down-regulated
C) Inhibitor; up-regulated
D) Inhibitor; down-regulated
Answers

Question 52

Which of the following events occurs at the onset of S phase in a mitogen-stimulated vertebrate cell?

A) Activation of Cdh1-APC/C
B) Activation of Rb
C) Activation of geminin
D) Activation of Cdc20
E) All of the above

Question 53

Indicate whether each of the following descriptions better applies to mitogens (M), growth factors (G), or survival factors (S). Your answer would be a four-letter string composed of letters M, G, or S only, e.g. GMSG.
( ) They unblock cell-cycle progression
( ) They suppress apoptosis
( ) They trigger a wave of G₁/S-Cdk activity
( ) They inhibit the degradation of cellular macromolecules

Question 54

In the following schematic diagram of a typical eukaryotic cell cycle, choose two major time points (among A to E) at which the cell-cycle control system normally arrests the cycle if DNA damage is detected. Your answer would be a two-letter string composed of letters A to E only in alphabetical order, e.g. CE.

Question 55

Which stage is usually the longest in meiosis?

A) Prophase I
B) Prometaphase I
C) Telophase I
D) Prophase II
E) Prometaphase II

Question 56

In the following schematic diagram of a meiotic bivalent in diplotene, indicate whether each of the following chromatid pairs have undergone DNA exchange (E) or not (N). The different color of the final form of chromatid 3 is for clarity only. Your answer would be a four-letter string composed of letters E and N only, e.g. NENE.

( ) Chromatids 1 and 3
( ) Chromatids 1 and 4
( ) Chromatids 2 and 3
( ) Chromatids 2 and 4

Question 57

A cell has been subjected to ultraviolet irradiation, causing a significant number of mutations in the genome. Which of the following would you NOT expect to occur as a result?

A) Activation of the protein kinase ATR
B) Activation of the protein kinase Chk1
C) Inactivation of the protein phosphatase Cdc25
D) Binding of p53 to Mdm2
E) Stabilization of p53