Deck 18: The Cell-Division Cycle

Question 1

A cell that is terminally differentiated will

A)replicate its DNA.
B)reenter the cell cycle only once a year.
C)dismantle the cell-cycle control system.
D)arrest after S phase.

Answer 1

C

Question 2

Cells in the G₀ state

A)do not divide.
B)cannot reenter the cell cycle.
C)have entered this arrest state from either G₁ or G₂.
D)have duplicated their DNA.

Answer 2

A

Question 3

The G₁ DNA damage checkpoint

A)causes cells to proceed through S phase more quickly.
B)involves the degradation of p53.
C)is activated by errors caused during DNA replication.
D)involves the inhibition of cyclin-Cdk complexes by p21.

Answer 3

D

Question 4

The Retinoblastoma (Rb) protein blocks cells from entering the cell cycle by

A)phosphorylating Cdk.
B)marking cyclins for destruction by proteolysis.
C)inhibiting cyclin transcription.
D)activating apoptosis.

Question 5

Which of the following events does NOT usually occur during interphase?

A)Cells grow in size.
B)The nuclear envelope breaks down.
C)DNA is replicated.
D)The centrosomes are duplicated.

Question 6

A mutant yeast strain stops proliferating when shifted from 25°C to 37°C.When these cells are analyzed at the two different temperatures, using a machine that sorts cells according to the amount of DNA they contain, the graphs in Figure 18-2 are obtained. Figure 18-2
Which of the following would NOT explain the results with the mutant?

A)inability to initiate DNA replication
B)inability to begin M phase
C)inability to activate proteins needed to enter S phase
D)inappropriate production of a signal that causes the cells to remain in G₁

Question 7

MPF activity was discovered when cytoplasm from a Xenopus M-phase cell was injected into Xenopus oocytes, inducing the oocytes to form a mitotic spindle.In a control experiment, Xenopus interphase cytoplasm was injected into oocytes and shown not to induce the formation of a mitotic spindle.Which of the following statements is NOT a legitimate conclusion from the control experiment?

A)The piercing of the oocyte membrane by a needle is insufficient to cause mitotic spindle formation.
B)An increased volume of cytoplasm is insufficient to cause mitotic spindle formation.
C)Injection of extra RNA molecules is insufficient to cause mitotic spindle formation.
D)Components of an interphase nucleus are insufficient to cause mitotic spindle formation.

Question 8

Which of the following statements is FALSE?

A)Cdc25 dephosphorylation of Wee1 activates the kinase, promoting the G2/M transition.
B)Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive.
C)Inhibiting the Cdc25 phosphatase will delay the G2/M transition.
D)The activating phosphatase (Cdc25) removes the phosphates from mitotic Cdk that were added by Wee1, so that M-Cdk will be active.

Question 9

You have isolated a strain of mutant yeast cells that divides normally at 30°C but cannot enter M phase at 37°C.You have isolated its mitotic cyclin and mitotic Cdk and find that both proteins are produced and can form a normal M-Cdk complex at both temperatures.Which of the following temperature-sensitive mutations could NOT be responsible for the behavior of this strain of yeast?

A)inactivation of a protein kinase that acts on the mitotic Cdk kinase
B)inactivation of an enzyme that ubiquitylates M cyclin
C)inactivation of a phosphatase that acts on the mitotic Cdk kinase
D)a decrease in the levels of a transcriptional regulator required for producing sufficient amounts of M cyclin

Question 10

You engineer yeast cells that express the M cyclin during S phase by replacing the gene regulatory sequences of the M cyclin gene with those of the S cyclin gene.Keeping in mind that yeast cells have one common Cdk that binds to all cyclins, which of the following outcomes is LEAST likely during this experiment?

A)There will be both M cyclin-Cdk and S cyclin-Cdk complexes in the cell during S phase.
B)Some substrates that are normally phosphorylated in M phase will now be phosphorylated in S phase.
C)G₁ cyclin-Cdks will be activated earlier in G1.
D)S-Cdk targets will be phosphorylated during S phase.

Question 11

Which of the following statements about the cell cycle is FALSE?

A)Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.
B)An unfavorable environment can cause cells to arrest in G₁.
C)A cell has more DNA during G₂ than it did in G₁.
D)The cleavage divisions that occur in an early embryo have short G₁ and G₂ phases.

Question 12

Which of the following is NOT good direct evidence that the cell-cycle control system is conserved through billions of years of divergent evolution?

A)A yeast cell lacking a Cdk function can use the human Cdk to substitute for its missing Cdk during the cell cycle.
B)The amino acid sequences of cyclins in plants are similar to the amino acid sequences of cyclins in humans.
C)The Cdk proteins in humans share conserved phosphorylation sites with the Cdk proteins in yeast.
D)Yeast cells have only one Cdk, whereas humans have many Cdks.

Question 13

Levels of Cdk activity change during the cell cycle, in part because

A)the Cdks phosphorylate each other.
B)the Cdks activate the cyclins.
C)Cdk degradation precedes entry into the next phase of the cell cycle.
D)cyclin activity change during the cycle.

Question 14

Mitogens are

A)extracellular signals that stimulate cell division.
B)transcription factors important for cyclin production.
C)kinases that cause cells to grow in size.
D)produced by mitotic cells to keep nearby neighboring cells from dividing.

Question 15

Which of the following descriptions is consistent with the behavior of a cell that lacks a protein required for a checkpoint mechanism that operates in G₂?

A)The cell would be unable to enter M phase.
B)The cell would be unable to enter G₂.
C)The cell would enter M phase under conditions when normal cells would not.
D)The cell would pass through M phase more slowly than normal cells.

Question 16

Which of the following statements is FALSE?

A)DNA synthesis begins at origins of replication.
B)The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk.
C)The phosphorylation and degradation of Cdc6 help to ensure that DNA is replicated only once in each cell cycle.
D)DNA synthesis can only begin after prereplicative complexes assemble on the ORCs.

Question 17

What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only?

A)The cells would not be able to replicate their DNA.
B)The mitotic spindle could not assemble.
C)The cells would get larger and larger.
D)The cells produced would get smaller and smaller.

Question 18

Progression through the cell cycle requires a cyclin to bind to a Cdk because

A)the cyclins are the molecules with the enzymatic activity in the complex.
B)the binding of a cyclin to Cdk is required for Cdk enzymatic activity.
C)cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle.
D)without cyclin binding, a cell-cycle checkpoint will be activated.

Question 19

The concentration of mitotic cyclin (M cyclin)

A)rises markedly during M phase.
B)is activated by phosphorylation.
C)falls toward the end of M phase as a result of ubiquitylation and degradation.
D)is highest in G₁ phase.

Question 20

In which phase of the cell cycle do cells check to determine whether the DNA is fully and correctly replicated?

A)at the transition between G₁ and S
B)when cells enter G₀
C)during M
D)at the end of G₂

Question 21

Which of the following statements about kinetochores is TRUE?

A)Kinetochores assemble onto chromosomes during late prophase.
B)Kinetochores contain DNA-binding proteins that recognize sequences at the telomere of the chromosome.
C)Kinetochore proteins bind to the tubulin molecules at the minus end of microtubules.
D)Kinetochores assemble on chromosomes that lack centromeres.

Question 22

How does S-Cdk help guarantee that replication occurs only once during each cell cycle?

A)It blocks the rise of Cdc6 concentrations early in G₁.
B)It phosphorylates and inactivates DNA helicase.
C)It phosphorylates and inactivates Cdc6.
D)It promotes the assembly of a prereplicative complex.

Question 23

Which of the following does not occur during M phase in animal cells?

A)growth of the cell
B)condensation of chromosomes
C)breakdown of nuclear envelope
D)attachment of chromosomes to microtubules

Question 24

At the end of DNA replication, the sister chromatids are held together by the

A)kinetochores.
B)securins.
C)cohesins.
D)histones.

Question 25

Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells?

A)assembly of the contractile ring
B)decondensation of chromosomes
C)reassembly of the nuclear lamina
D)transcription of nuclear genes

Question 26

Which of the following statements is TRUE?

A)Anaphase A must be completed before anaphase B can take place.
B)In cells in which anaphase B predominates, the spindle will elongate much less than in cells in which anaphase A dominates.
C)In anaphase A, both kinetochore and interpolar microtubules shorten.
D)In anaphase B, microtubules associated with the cell cortex shorten.

Question 27

Which of the following statements is FALSE?

A)Cytokinesis in plant cells is mediated by the microtubule cytoskeleton.
B)Small membrane vesicles derived from the Golgi apparatus deliver new cell-wall material for the new wall of the dividing cell.
C)The phragmoplast forms from the remains of interpolar microtubules of the mitotic spindle.
D)Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall.

Question 28

Which of the following statements is FALSE?

A)The cleavage furrow is a puckering of the plasma membrane caused by the constriction of a ring of filaments attached to the plasma membrane.
B)The cleavage furrow will not begin to form in the absence of a mitotic spindle.
C)The cleavage furrow always forms perpendicular to the interpolar microtubules.
D)The cleavage furrow always forms in the middle of the cell.

Question 29

Which letter is associated with the line that is pointing to the interpolar microtubules in Figure 18-29? Figure 18-29

A)A
B)B
C)C
D)D
E)E

Question 30

Which of the following statements is TRUE?

A)The mitotic spindle is largely made of intermediate filaments.
B)The contractile ring is made largely of microtubules and actin filaments.
C)The contractile ring divides the nucleus in two.
D)The mitotic spindle helps segregate the chromosomes to the two daughter cells.

Question 31

Sister chromatid separation occurs because __________ are destroyed by the APC/C.

A)securins
B)cohesins
C)kinetochores
D)condensins

Question 32

Cytokinesis in animal cells

A)requires ATP.
B)leaves a small circular "scar" of actin filaments on the inner surface of the plasma membrane.
C)is often followed by phosphorylation of integrins in the plasma membrane.
D)is assisted by motor proteins that pull on microtubules attached to the cell cortex.

Question 33

A friend declares that chromosomes are held at the metaphase plate by microtubules that push on each chromosome from opposite sides.Which of the following observations does not support your belief that the microtubules are pulling on the chromosomes?

A)the jiggling movement of chromosomes at the metaphase plate
B)the way in which chromosomes behave when the attachment between sister chromatids is severed
C)the way in which chromosomes behave when the attachment to one kinetochore is severed
D)the shape of chromosomes as they move toward the spindle poles at anaphase

Question 34

Condensins

A)are degraded when cells enter M phase.
B)assemble into complexes on the DNA when phosphorylated by M-Cdk.
C)are involved in holding sister chromatids together.
D)bind to DNA before DNA replication begins.

Question 35

Which of the following statements about the anaphase-promoting complex (APC) is FALSE?

A)It promotes the degradation of proteins that regulate M phase.
B)It inhibits M-Cdk activity.
C)It is continuously active throughout the cell cycle.
D)M-Cdk stimulates its activity.

Question 36

You create cells with a version of the origin recognition complex, ORC, that cannot be phosphorylated by S-Cdk and thus cannot be inactivated.Which of the following statements describes the likely consequence of this change in ORC?

A)Cells will enter S phase prematurely.
B)Cells will replicate some regions of the genome more than once in a cell cycle.
C)ORC will be unable to bind to DNA.
D)DNA helicases will not be able to open up the double helix at the replication origin.

Question 37

A cell with nuclear lamins that cannot be phosphorylated in M phase will be unable to

A)reassemble its nuclear envelope at telophase.
B)disassemble its nuclear lamina at prometaphase.
C)begin to assemble a mitotic spindle.
D)condense its chromosomes at prophase.

Question 38

Disassembly of the nuclear envelope

A)causes the inner nuclear membrane to separate from the outer nuclear membrane.
B)results in the conversion of the nuclear envelope into protein-free membrane vesicles.
C)is triggered by the phosphorylation of integrins.
D)must occur for kinetochore microtubules to form in animal cells.

Question 39

Which word or phrase below best describes the phase in mitosis depicted in Figure 18-29? Figure 18-29

A)anaphase
B)prometaphase
C)S-phase checkpoint
D)metaphase

Question 40

The principal microtubule-organizing center in animal cells is the

A)centrosome.
B)centromere.
C)kinetochore.
D)cell cortex.

Question 41

The cytoskeleton of an animal cell changes markedly between G₁ and early M phase (prophase) of the cell cycle.For each of the following sentences, choose one of the options enclosed in square brackets that best describes the changes to the cytoskeleton and its components.
Before mitosis, the number of centrosomes must [increase/decrease].At the beginning of [anaphase/prophase] in animal cells, the centrosomes separate in a process driven partly by interactions between the [plus/minus] ends of microtubules arising from the two centrosomes.Centrosome separation initiates the assembly of the bipolar mitotic spindle and is associated with a sudden [increase/decrease] in the dynamic instability of microtubules.[Interpolar/astral/kinetochore] microtubules are formed in an overlap zone where two microtubules from opposite centrosomes interact.During anaphase [A/B], kinetochore microtubules are shortened, dragging chromosomes toward their spindle pole.

Question 42

Before a cell can enter M phase, two structures from Figure 18-12 must be duplicated.Write the letters corresponding to the lines pointing to these structures, and write the names of the structures next to the letters.
Figure 18-12

Question 43

What is the main molecular difference between cells in a G₀ state and cells that have simply paused in G₁?

Question 44

Apoptosis differs from necrosis in that necrosis

A)requires the reception of an extracellular signal.
B)causes DNA to fragment.
C)causes cells to swell and burst, whereas apoptotic cells shrink and condense.
D)involves a caspase cascade.

Question 45

Why should it be that drugs such as colchicine, which inhibit microtubule polymerization, and drugs such as Taxol^®, which stabilize microtubules, both inhibit mitosis?

Question 46

What would happen to the progeny of a cell that proceeded to mitosis and cell division after entering S phase but had not completed S phase? Keep in mind that highly condensed chromatin, including the centromere region, is replicated late in S phase.Explain your answer.

Question 47

Consider an animal cell that has eight chromosomes (four pairs of homologous chromosomes) in G₁ phase.How many of each of the following structures will the cell have at mitotic prophase?
A.sister chromatids
B.centromeres
C.kinetochores
D.centrosomes
E.centrioles

Question 48

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
APC G₂ phase metaphase
Cdks interphase microtubules
condensation intraphase mitosis
cytokinesis kinesins myosins
meiosis M phase S phase
G₁ phase M-Cdk S-Cdk
G₁-Cdk
The cell cycle consists of an alternation between __________, which appears as a period of dramatic activity under the microscope, and a preparative period called __________, which consists of three phases called __________, __________, and __________.During M phase, the nucleus divides in a process called __________, and the cytoplasm splits in two in a process called __________.The cell-cycle control system relies an increase in the activity of __________ to trigger DNA replication.Inactivation of __________ is required to exit from M phase after chromosome segregation.

Question 49

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
able hexokinase short
asynchronously inhibitory sperm
Cdk long steady
conserved M stimulatory
cyclin maturation promoting factor substrate
divergent oscillate synchronously
egg PI 3-kinase ubiquitin
fibroblast regulin unable
G₁ S uniform
G₂
Many features of __________ cells make them suitable for biochemical studies of the cell-cycle control system.For example, the cells are unusually large and are arrested in a __________-like phase.When the cells are triggered to resume cycling, the cell divisions have especially __________ G₁ and G₂ phases and occur __________.Studies with Xenopus eggs identified a partly purified activity called __________ that drives a resting Xenopus oocyte into M phase.MPF activity was found to __________ during the cell cycle, although the amount of its kinase component, called __________, remained constant.The regulatory component of MPF, called __________, has a __________ effect on MPF activity and plays a part in regulating interactions with its __________s.The components of MPF are evolutionarily __________ from yeast to humans, so that the corresponding human genes are __________ to function in yeast.

Question 51

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase may be used once.
assemble G₁ phosphorylation
condense G₂ polymerization
cytokinesis M S
degrade replicate segregate
A cell contains the most DNA after __________ phase of the cell cycle, the period when a cell __________s its DNA.A cell is smallest in size after __________ phase of the cell cycle, which includes __________, the process by which the cytoplasm is cleaved in two.S and M phases are separated by gap phases, with the first one called the __________ phase and the second one, which follows S phase, called the __________ phase of the cell cycle.During mitosis, the mitotic spindle is built to __________ chromosomes.

Question 52

You have isolated a mutant in which a fraction of the new cells die soon after cell division and a fraction of the living cells have an extra copy of one or more chromosomes.When you grow the cells under conditions in which they transit the cell cycle more slowly, the defect disappears, suggesting that the mitotic spindle and segregation machinery are normal.Propose a basis for the defect.

Question 54

Figure 18-10 shows a living cell from the lung epithelium of a newt at different stages in M phase.Order these light micrographs into the correct sequence and identify the stage in M phase that each represents.
Figure 18-10

Question 55

Which organelle fragments during mitosis?

A)endoplasmic reticulum
B)Golgi apparatus
C)mitochondrion
D)chloroplast

Question 56

Which stage of mitosis in an animal cell does each part of Figure 18-9 represent?
Figure 18-9

Question 57

Examine the schematic representation of centrosome duplication in Figure 18-13.By analogy with DNA replication, would you classify centrosome duplication as conservative or semiconservative? Explain your answer.
Figure 18-13

Question 58

Which of the following statements about apoptosis is TRUE?

A)Cells that constitutively express Bcl2 will be more prone to undergo apoptosis.
B)The prodomain of procaspases contains the catalytic activity necessary for procaspase activation.
C)Bax and Bak promote apoptosis by binding to procaspases in the apoptosome.
D)Apoptosis can be promoted by the release of cytochrome c into the cytosol from mitochondria.

Question 59

Programmed cell death occurs

A)by means of an intracellular suicide program.
B)rarely and selectively only during animal development.
C)only in unhealthy or abnormal cells.
D)only during embryonic development.

Question 60

Name the stage of M phase in which the following events occur.Place the numbers 1-8 next to the letter headings to indicate the normal order of events.
A.alignment of the chromosomes at the spindle equator
B.attachment of spindle microtubules to chromosomes
C.breakdown of nuclear envelope
D.pinching of cell in two
E.separation of two centrosomes and initiation of mitotic spindle assembly
F.re-formation of the nuclear envelope
G.condensation of the chromosomes
H.separation of sister chromatids

Question 61

The lengths of microtubules in various stages of mitosis depend on the balance between the activities of proteins that destabilize microtubules, and microtubule-associated proteins that stabilize them.If you created cells with an increased number of proteins that destabilize molecules, do you predict the length of the mitotic spindle will be longer, shorter, or unchanged, relative to the corresponding stage of mitosis in wild-type cells? What do you predict for a cell with increased numbers of microtubule stabilizing proteins? Explain your reasoning.

Question 62

Imagine that you could microinject cytochrome c into the cytosol of both wild-type cells and cells that were lacking both Bax and Bak, which are apoptosis-promoting members of the Bcl2 family of proteins.Would you expect one, both, or neither of the cell lines to undergo apoptosis? Explain your reasoning.

Question 63

Your friend is intrigued by Cdks and purifies Cdk from the daisy plant.She is able to determine the sequence of 18 amino acids from the daisy Cdk protein.Using these data, she aligns the daisy Cdk sequence with one human Cdk protein and the Cdk proteins from two different kinds of yeast, as shown in Figure 18-25.Sequences identical between the human and fungal proteins have been boxed.
Figure 18-25
Such conserved amino acid sequences are often involved in protein-protein interactions.Indeed, the threonine (T) in the central "YTHE" block is known to be phosphorylated by a kinase that activates Cdks in human and yeast.The surrounding conserved sequences could thus be important for the interaction of this Cdk-activating kinase with Cdk.In the daisy Cdk, however, not all of the amino acids in these conserved blocks match the sequences from yeast and human, as indicated by the arrows marked #1 and #2.
A.When you replace the S.cerevisiae (budding yeast) Cdk with the daisy Cdk, you discover the daisy Cdk does not interact with the Cdk-activating kinase inside the yeast cell.Circle the amino acid in the daisy Cdk that is likely to be the most disruptive to its interaction with Cdk-activating kinase.Explain.
B.Based on the information in Figure 18-25 and what you know about the Cdk1-Cdk-activating kinase interaction, would you predict that the human Cdk will interact with yeast Cdk-activating kinase? Explain.

Question 64

The number of cells in an adult tissue or animal depends on cell proliferation.What else does it depend on?

Question 65

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
anaphase differentiation myostatin
annihilation G₀ nourishment
arrestase G₁ nutrition
Bcl2 G₂ phosphatases
biosynthetic growth factors proliferation
cyclin interphase receptor
cascades ligand S
caspase M survival factors
Cdk mitogens transcription
The survival, __________, and size of each cell in an animal are controlled by extracellular signal molecules secreted by neighboring and distant cells.Many of these signal molecules bind to a cell-surface __________ and trigger various intracellular signaling pathways.One class of signal molecules, called __________, stimulates cell division by releasing the molecular brakes that keep cells in the __________or __________ phase of the cell cycle.Members of a second class of signal molecules are called __________, because they stimulate cell growth and an increase in cell mass.The third class of signal molecules, called __________, inhibits apoptosis by regulating members of the __________ family of proteins.In addition to such stimulatory factors, some signal proteins, such as __________ act negatively on other cells, inhibiting their survival, growth, or proliferation.

Question 66

What is the cause of the massive amount of programmed cell death of nerve cells (neurons) that occurs in the developing vertebrate nervous system, and what purpose does it serve?

Question 67

You have discovered a new protein that regulates microtubule dynamics.First, you isolated proteins from a cellular extract that bound to a tubulin affinity column.You then separated the proteins from each other by loading the mixture of proteins on an ion-exchange column, eluting the column with increasing salt concentration, and collecting small "fractions" of protein as they dripped from the column.To test whether each fraction contained microtubule regulators, you mixed it with fluorescent tubulin and purified centrosomes, and then analyzed the reaction microscopically to measure the size of the astral microtubules formed.You found that fractions 8, 9, and 10 promoted the formation of unusually long astral microtubules.Because electrophoretic separation of the fractions on a gel revealed a plentiful protein with an apparent molecular mass of 98 kD, you named the protein p98.
A.Propose two ways in which p98 might change the dynamic behavior of microtubules to account for the observed change in microtubule length.(Hint: There are four simple possible mechanisms.)
B.Video microscopy of fluorescent tubulin in reactions with purified centrosomes allowed you to follow the behavior of individual microtubules over time.You graphed the changes in microtubule length in the absence (Figure 18-19A) and presence (Figure 18-19B) of p98.Five representative microtubules are shown for each condition.Does p98 alter the rate of microtubule growth or shrinkage? Does p98 alter the frequency of catastrophes (a sudden and rapid decline in microtubule length) or rescues (when a microtubule switches from shrinking to growing)? Explain your answers.
C.After demonstrating the consequences of p98 addition on microtubule dynamics in vitro with the use of purified components, you want to determine whether the protein has the same effects in a complex cellular extract that naturally contains p98.You remove the p98 protein from an extract of Xenopous eggs in mitosis by using antibodies that specifically recognize p98.The p98-depleted extract is then mixed with sperm nuclei, centrosomes, and fluorescent tubulin.How would you expect the microtubules to behave?
Figure 18-19