Cell bio - Chapter 18 Practice Questions

Cells in the G0 state ________________. 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

A

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

A

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

B

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 being after prereplicative complexes assemble on the ORCs.

B

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? (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.

B

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 chromosome? 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

A. the jiggling movement is simply as sing that the chromosomes are subject to forces from both sides, which could be the microtubules pulling, pushing, or both

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 phase C. The cell would enter M phase under conditions when normal cells would not D. The cells would pass through M phase more slowly than normal cells.

C. Normal cells arrest at the G2 checkpoint if DNA replication is incomplete or DNA is damaged. Cells without this mechanism may enter M phase with unreplicated or damaged DNA, whereas normal cells would not.

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 the Cdc6 protein, marking it for destruction D. It promotes the assembly of a prereplicative complex

C. The concentration of Cdc6 rises early in G1.

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 G phase

C. The concentration of mitotic cyclin rises gradually during G2 and is ubiquitylated and degraded during late M phase.

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

D

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

D

Which of the following statements is false? 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

D

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 part of the cell

D

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

D

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

D

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 cell cycle D. Cyclin levels change during the cycle

D is correct. Cdks do not phosphorylate each other (a). The Cdks do not activate the cyclins - cyclins activates Cdk (b). Cdks are not degraded during specific phases of the cell cycles (C). The cyclins however are degraded in a cell-cycle-dependent fashion and they are required for Cdk activity. Overall: Cdk activity is dependent on how much cyclin is in the cell because cyclin activates Cdk

Which of the following is not good direct evidence that the cell-cycle control system is conserved though 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 in proteins in yeast D. Yeast cell have only one Cdk, whereas humans have many Cdks

D. Although it is true that yeast cells have on Cdk and human cells have many, this statement does not provide evidence for the conservation of function across evolutionary time.

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

D. In animal cells, kinetochore microtubules cannot form if the chromosomes in the nucleus are separated from the microtubules in the cytoplasm because of the presence of the nuclear envelope.

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

D. It doesn't make sense to monitor DNA replication before S phase because DNA replication has not yet occurred(a). When cells enter G, they do not replicate their DNA (b). During M phase, chromosomes are condensed for chromosome segregation, so it would be difficult for the cell to examine the replicated DNA for errors at that point.

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

D. No contractile ring is formed during plant cytokinesis

Which would be the most obvious outcome of repeated cell cycles consisting of S-phase and M-phase only? A. Cells would not be able to replicate their DNA B. The mitotic spindle could not assemble C. Cells would get larger and larger D. The cells produced would get smaller and smaller

D. The cells produced would get smaller and smaller, as they would not have sufficient time to double their mass before dividing.

the principal microtuble-organizing center in animal cells is the_____________ A. Centrosome B. Centromere C. Kinetochore D. Cell cortex

a

Which organelle fragments during mitosis? A. Endoplasmic reticulum B. Golgi apparatus C. Mitochondrion D. Chloroplast

b

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 apototic cells shrink and condense D. Involves a capase cascade

c

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 catalytic activity necessary procaspase activation C. Bax and Bak promot apoptosis by binding to procaspases in the apoptosome D. Apoptosis is promoted by the release of cytochrome c int the cytosol from mitochondria.

c

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.

A. All cell movement requires ATP

Which of the following does not occur during M phase in animal cells? A. Growth of cells B. Condensation of chromosomes C. Breakdown of nuclear envelope D. Attachment of chromosomes to microtubules

A. Cell size increases throughout interphase and not during M phase. All of the other phenomena are observed in M phase

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 G C. A cell has more DNA during G2 than it did in G1 D. The cleavage divisions that occurs in an early embryo have short G1 and G2 phases

A. Cells vary in the amount of time that it takes to complete a cell cycle.

Cells can pause in G1 when DNA is damaged, and can pause in S when there are replication errors. Indicate whether the mechanism below applies to a G1 arrest, an S-phase arrest, both types of arrest, or neither. A. p53 activates the transcription of a Cdk inhibitor. B. Cyclins are phosphorylated and destroyed. C. Cdk is unable to phosphorylate its substrates. D. The Cdc25 phosphatase is inhibited.

A. G1 arrest B. Neither C. Both D. S-phase arrest

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. Involves a capase cascade

A. Programmed cell death results from an intracellular suicide program that eliminates unneeded, unwanted, or damaged cells. It occurs frequently and happens even in health cells throughout the lifetime of an individual

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

A. The contractile ring in an animal cell begins to assemble in anaphase

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

B is correct. Cyclins have no enzymatic activity (a) and cyclin binding to Cdk activates the Cdk (c). As far as we know, cyclin-Cdk binding is not directly monitored by checkpoints (d)

Irradiated mammalian cells usually stop dividing and arrest at a G1 checkpoint. Place the following events in the order in which they occur. A. production of p21 B. DNA damage C. inhibition of cyclin-Cdk complexes D. accumulation and activation of p53

B, D, A, C

You have isolated a strain of mutant yeast cells that divide 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

B. A cells with a mutation that prevents ubiquitylation of M cyclin could enter mitosis but could not exit mitosis properly

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

B. If the lamins cannot be phosphorylated during mitosis, the cells will be unable to disassemble their nuclear lamina, preventing the breakdown of the nuclear envelope at prometaphase

Which of the following events does not usually occur during interphase? A. Cells grow to size B. The nuclear envelope break down C. DNA is replicated D. The centrosomes are duplicated

B. The nuclear envelope is usually not broken down during interphase. The nuclear envelope usually disappears during prophase

At the end of DNA replication, the sister chromatids are held together by_______ A. Kinetochores B. Securins C. Cohesins D. Histones

C

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

C

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? (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.

C. The transcription of G1 cyclins should not be affected by the S-Cdk (which normally occurs in S phase) or M-Cdk (which likely now exists within the ceell due to the inappropriate expression of M cyclin from the S cyclin promoter)

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 D

C. the APC becomes activated in mid to late M phase

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.

D. the interphase cyctoplasmic extract used as the control would not have contained nuclear components (because the nuclear membrane was intact) and so one cannot conclude that components of an interphase nucleus are insufficient to cause mitotic spindle formation. All other statements are true

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

a