Exam 18: The Cell-Division Cycle

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

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.

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

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?

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

Which of the following statements is FALSE?

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.

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.

The concentration of mitotic cyclin (M cyclin)

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. phase metaphase Cdks interphase microtubules condensation intraphase mitosis cytokinesis kinesins myosins meiosis phase phase phase - - - 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.

Which of the following statements is TRUE?

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

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

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? (A) NO p98 (B) PLUS p98 Figure 18-19

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

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

The principal microtubule-organizing center in animal cells is the

Disassembly of the nuclear envelope

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