ESSENTIAL CELL BIOLOGY CH #18
TEST BANK EXAM QUESTIONS AND
ANSWERS
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 - ANSWER-b
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. - ANSWER-c
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. - ANSWER-d
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. - ANSWER-d
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. - ANSWER-d
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. - ANSWER-
a
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. - ANSWER-c
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-d
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. - ANSWER-a
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. - ANSWER-b
TEST BANK EXAM QUESTIONS AND
ANSWERS
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 - ANSWER-b
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. - ANSWER-c
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. - ANSWER-d
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. - ANSWER-d
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. - ANSWER-d
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. - ANSWER-
a
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. - ANSWER-c
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-d
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. - ANSWER-a
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. - ANSWER-b
