Ch 18

Mitogens are _____.

(extracellular signals that stimulate cell division.

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 on sharp increases in the activities of regulatory proteins called __________________, or __________________, to trigger S phase and M phase. Inactivation of __________________ is required to exit from M phase after chromosome segregation.

**The cell cycle consists of an alternation between M phase, which appears as a period of dramatic activity under the microscope, and a preparative period called interphase, which consists of three phases called G1 phase, S phase, and G2 phase. During M phase, the nucleus divides in a process called mitosis, and the cytoplasm splits in two in a process called cytokinesis. The cell-cycle control system relies on sharp increases in the activities of regulatory proteins called cyclin-dependent kinases, or Cdks, to trigger S phase and M phase. Inactivation of M-Cdk is required to exit from M phase after chromosome segregation.

Irradiated mammalian cells usually stop dividing and arrest at a G1 checkpoint. Place the following events in the order in which they occur.

1. DNA damage 2. accumulation and activation of p53 3. production of p21 4. inhibition of cyclin-Cdk complexes

Which of the following statements about apoptosis is true?

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

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?

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

18-17 Which of the following statements is false?

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

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

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

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

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

Many features of egg cells make them suitable for biochemical studies of the cell-cyclecontrol system. For example, the cells are unusually large and are arrested in a G2-like phase. When the cells are triggered to resume cycling, the cell divisions have especially short G1 and G2 phases and occur synchronously. Studies with Xenopus eggs identified a partly purified activity called maturation promoting factor that drives a resting Xenopus oocyte into M phase. MPF activity was found to be oscillate during the cell cycle, although the amount of its kinase component, called Cdk, remained constant. The regulatory component of MPF, called cyclin, has a stimulatory effect on MPF activity and plays a part in regulating interactions with its substrates. The components of MPF are evolutionarily conserved from yeast to humans, so that the corresponding human genes are able to function in yeast.

answer

Condensins ________________.

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

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

assembly of the contractile ring

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

at the end of G2

Programmed cell death occurs ________________.

by means of an intracellular suicide program.

Apoptosis differs from necrosis in that necrosis ________________.

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

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

centrosome.

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

cohesins.

The regulatory component of MPF, called ________, has a stimulatory effect on MPF activity and plays a part in regulating interactions with its substrates.

cyclin

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

cyclin levels change during the cycle.

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

disassemble its nuclear lamina at prometaphase.

Cells in the G0 state ________________.

do not divide.

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 G2-like phase.

egg

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

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

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

growth of the cell

18-3 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?

inability to begin M phase

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?

inactivation of an enzyme that ubiquitylates M cyclin

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

inhibiting cyclin transcription.

The G1 DNA damage checkpoint ________________.

involves the inhibition of cyclin-Cdk complexes by p21.

Disassembly of the nuclear envelope ________________.

must occur for kinetochore microtubules to form in animal cells.

Cytokinesis in animal cells ________________.

requires ATP.

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

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

After the nuclear envelope breaks down, microtubules gain access to the chromosomes and, every so often, a randomly probing microtubule captures a chromosome and ultimately connects to the kinetochore to become a kinetochore microtubule of the spindle.

true

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

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

Which of the following statements is false?

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

MPF activity was found to be oscillate during the cell cycle, although the amount of its kinase component, called _______, remained constant.

Cdk

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?

Cells will be unable to complete DNA synthesis.

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?

G1 cyclins will be expressed during S phase.

Which organelle fragments during mitosis?

Golgi apparatus

Which of the following statements is true?

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

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

It is continuously active throughout the cell cycle.

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

It phosphorylates the Cdc6 protein, marking it for destruction.

Which of the following statements about kinetochores is true?

Kinetochores assemble onto chromosomes during late prophase.

Studies with Xenopus eggs identified a partly purified activity called _____ that drives a resting Xenopus oocyte into M phase.

MPF

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?

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

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

The cells produced would get smaller and smaller.

Which of the following statements is false?

The cleavage furrow always forms in the middle of the cell.

Which of the following statements is false?

The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk.

Which of the following statements is true?

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

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

The nuclear envelope breaks down.

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 __________________ by regulating members of the __________________ family of proteins. In addition to such stimulatory

The survival, proliferation, 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 receptor and trigger various intracellular signaling pathways. One class of signal molecules, called mitogens, stimulates cell division by releasing the molecular brakes that keep cells in the G0 or G1 phase of the cell cycle. Members of a second class of signal molecules are called growth factors, because they stimulate cell growth and an increase in cell mass. The third class of signal molecules, called survival factors, inhibits apoptosis by regulating members of the Bcl2 family of proteins. In addition to such stimulatory factors, some signal proteins such as myostatin act negatively on other cells, inhibiting their survival, growth, or proliferation.