18

Front 1

18-4 Which of the following events does not usually occur during interphase?

Back 1

(b) The nuclear envelope breaks down.

Hint 1

(a) Cells grow in size.
(b) The nuclear envelope breaks down.
(c) DNA is replicated.
(d) The centrosomes are duplicated.

Front 2

18-5 In which phase of the cell cycle do cells check to determine whether the DNA is fully and correctly replicated?

Back 2

(d) at the end of G2

Hint 2

(a) at the transition between G1 and S
(b) when cells enter G0
(c) during M
(d) at the end of G2

Front 3

18-8 Which of the following statements about the cell cycle is false?

Back 3

(a) Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.

Hint 3

(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.

Front 4

18-9 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?

Back 4

(c) The cell would enter M phase under conditions when normal cells would not.

Hint 4

(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

Front 5

18-11 Progression through the cell cycle requires a cyclin to bind to a Cdk because _________.

Back 5

(b) the binding of a cyclin to Cdk is required for Cdk enzymatic activity.

Hint 5

(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.

Front 6

18-12 Levels of Cdk activity change during the cell cycle, in part because ________________.

Back 6

(d) cyclin levels change during the cycle.

Hint 6

(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 levels change during the cycle.

Front 7

18-13 The concentration of mitotic cyclin (M cyclin) ________________.

Back 7

(c) falls toward the end of M phase as a result of ubiquitylation and degradation.

Hint 7

(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.

Front 8

18-14 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?

Back 8

(b) inactivation of an enzyme that ubiquitylates M cyclin

Hint 8

(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

Front 9

18-15 You engineer yeast cells that express the M cyclin during S phase by replacing the promoter sequence of the M cyclin gene with that of S cyclin. 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?

Back 9

(c) G1 cyclins will be expressed during S phase.

Hint 9

(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 cyclins will be expressed during S phase.
(d) S-Cdk targets will be phosphorylated during S phase.

Front 10

18-17 Which of the following statements is false?

Back 10

(d) The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.

Hint 10

(a) Mitotic Cdk must be phosphorylated by an activating kinase (Cak) before it is active.
(b) Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive, even if it is phosphorylated by the activating kinase.
(c) Active M-Cdk phosphorylates the activating phosphatase (Cdc25) in a positive feedback loop.
(d) The activating phosphatase (Cdc25) removes all phosphates from mitotic Cdk so that M-Cdk will be active.

Front 11

18-19 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?

Back 11

(d) Components of an interphase nucleus are insufficient to cause mitotic spindle formation.

Hint 11

(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.

Front 12

18-20 Which of the following is not good direct evidence that the cell-cycle control system is conserved through billions of years of divergent evolution?

Back 12

(d) Yeast cells have only one Cdk, whereas humans have many Cdks.

Hint 12

(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.

Front 13

18-22 Mitogens are _____.

Back 13

(a) extracellular signals that stimulate cell division.

Hint 13

(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.

Front 14

18-23 The Retinoblastoma (Rb) protein blocks cells from entering the cell cycle by ______.

Back 14

(c) inhibiting cyclin transcription

Hint 14

(a) phosphorylating Cdk.
(b) marking cyclins for destruction by proteolysis.
(c) inhibiting cyclin transcription.
(d) activating apoptosis.

Front 15

18-25 The G1 DNA damage checkpoint ________________.

Back 15

(d) involves the inhibition of cyclin–Cdk complexes by p21.

Hint 15

(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.

Front 16

18-26 Cells in the G0 state ________________.

Back 16

(a) do not divide.

Hint 16

(a) do not divide.
(b) cannot re-enter the cell cycle.
(c) have entered this arrest state from either G1 or G2.
(d) have duplicated their DNA.


18-28 Which of the following statements is false?

Front 17

18-29 How does S–Cdk help guarantee that replication occurs only once during each cell cycle?

Back 17

(c) It phosphorylates the Cdc6 protein, marking it for destruction.

Hint 17

(a) It blocks the rise of Cdc6 concentrations early in G1.
(b) It phosphorylates and inactivates DNA helicase.
(c) It phosphorylates the Cdc6 protein, marking it for destruction.
(d) It promotes the assembly of a prereplicative complex.

Front 18

18-30 You create cells with a version of Cdc6 that cannot be phosphorylated and thus cannot be degraded. Which of the following statements describes the likely consequence of this change in Cdc6?

Back 18

(b) Cells will be unable to complete DNA synthesis.

Hint 18

(a) Cells will enter S phase prematurely.
(b) Cells will be unable to complete DNA synthesis.
(c) The origin recognition complex (ORC) will be unable to bind to DNA.
(d) Cdc6 will be produced inappropriately during M phase.

Front 19

18-33 Which of the following does not occur during M phase in animal cells?

Back 19

(a) growth of the cell

Hint 19

(a) growth of the cell
(b) condensation of chromosomes
(c) breakdown of nuclear envelope
(d) attachment of chromosomes to microtubules

Front 20

18-34 Condensins ________________.

Back 20

(b) assemble into complexes on the DNA when phosphorylated by M-Cdk.

Hint 20

(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.

Front 21

18-35 At the end of DNA replication, the sister chromatids are held together by the ___________.

Back 21

(c) cohesins.

Hint 21

(a) kinetochores.
(b) securins.
(c) cohesins.
(d) histones.

Front 22

18-36 Which of the following statements is true?

Back 22

(d) The mitotic spindle helps segregate the chromosomes to the two daughter cells.

Hint 22

(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.

Front 23

18-40 The principal microtubule-organizing center in animal cells is the ____________.

Back 23

(a) centrosome.

Hint 23

(a) centrosome.
(b) centromere.
(c) kinetochore.
(d) cell cortex.

Front 24

18-48 Disassembly of the nuclear envelope ________________.

Back 24

(d) must occur for kinetochore microtubules to form in animal cells.

Hint 24

(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.

18-50 Which of the following statements about kinetochores is true?

Front 25

18-52 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?

Back 25

(a) the jiggling movement of chromosomes at the metaphase plate

Hint 25

(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

Front 26

18-54 Which of the following statements about the anaphase-promoting complex (APC) is false?

Back 26

(c) It is continuously active throughout the cell cycle.

Hint 26

(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.

Front 27

18-55 Which of the following statements is true?

Back 27

(d) In anaphase B, microtubules associated with the cell cortex shorten.

Hint 27

(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.

Front 28

18-56 When introduced into mitotic cells, which of the following is expected to impair anaphase B but not anaphase A?

Back 28

(d) an antibody against the motor proteins that move from the minus end of microtubules toward the plus end

Hint 28

(a) an antibody against myosin
(b) ATPγS, a nonhydrolyzable ATP analog that binds to and inhibits ATPases
(c) an antibody against the motor proteins that move from the plus end of microtubules to the minus end
(d) an antibody against the motor proteins that move from the minus end of microtubules toward the plus end

Front 29

18-57 Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells?

Back 29

(a) assembly of the contractile ring

Hint 29

(a) assembly of the contractile ring
(b) decondensation of chromosomes
(c) reassembly of the nuclear lamina
(d) transcription of nuclear genes

Front 30

18-58 A cell with nuclear lamins that cannot be phosphorylated in M phase will be unable to ________________.

Back 30

(b) disassemble its nuclear lamina at prometaphase.

Hint 30

(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.

Front 31

18-61 Cytokinesis in animal cells ________________.

Back 31

(a) requires ATP.

Hint 31

(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.

18-62 Which of the following statements is false?

Front 32

18-63 Which of the following statements is false?

Back 32

(d) Motor proteins walking along the cytoskeleton are important for the contractile ring that guides formation of the new cell wall.

Hint 32

(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.

Front 33

18-64 Which organelle fragments during mitosis?

Back 33

(b) Golgi apparatus

Hint 33

(a) endoplasmic reticulum
(b) Golgi apparatus
(c) mitochondrion
(d) chloroplast

Front 34

18-65 Programmed cell death occurs ________________.

Back 34

(a) by means of an intracellular suicide program.

Hint 34

(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.

Front 35

18-66 Apoptosis differs from necrosis in that necrosis ________________.

Back 35

c) causes cells to swell and burst, whereas apoptotic cells shrink and condense.

Hint 35

(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.

Front 36

18-67 Which of the following statements about apoptosis is true?

Back 36

(d) Apoptosis is promoted by the release of cytochrome c into the cytosol from mitochondria.

Hint 36

(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 is promoted by the release of cytochrome c into the cytosol from mitochondria.

Front 37

18-72 Of the following mutations, which are likely to cause cell-cycle arrest? If you predict a cell-cycle arrest, indicate whether the cell will arrest in early G1, late G1, or G2. Explain your answers.

Back 37

A. a mutation in a gene encoding a cell-surface mitogen receptor that makes the receptor active even in the absence of the mitogen
B. a mutation that destroyed the kinase activity of S-Cdk

Hint 37

A. a mutation in a gene encoding a cell-surface mitogen receptor that makes the receptor active even in the absence of the mitogen
B. a mutation that destroyed the kinase activity of S-Cdk
C. a mutation that allowed G1-Cdk to be active independently of its phosphorylation status
D. a mutation that removed the phosphorylation sites on the Rb protein
E. a mutation that inhibited the activity of Rb

Front 38

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 Q18-3 are obtained. Which of the following would not explain the results with the mutant?

Back 38

b) inability to begin M phase

Hint 38

(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

Front 39

18-41 Which word or phrase below best describes the phase in mitosis depicted in Figure Q18-41?

Back 39

(d) metaphase

Hint 39

(a) anaphase
(b) prometaphase
(c) S-phase checkpoint
(d) metaphase

Front 40

18-42 Which letter is associated with the line that is pointing to the interpolar microtubules in Figure Q18-41?

Back 40

(e) E

Hint 40

(a) A
(b) B
(c) C
(d) D
(e) E

Front 41

18-50 Which of the following statements about kinetochores is true?

Back 41

(a) Kinetochores assemble onto chromosomes during late prophase.

Hint 41

(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.

Front 42

18-62 Which of the following statements is false?

Back 42

(d) The cleavage furrow always forms in the middle of the cell.

Hint 42

(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.