THE CELL DIVISION CYCLEOverview of the Cell CycleHow We Know: Discovery of Cyclins and CdksThe Cell-Cycle Control SystemS PhaseM PhaseMitosisCytokinesisControl of Cell Number and Cell SizeCHAPTER 18THE CELL DIVISION CYCLE 2009 Garland Science PublishingOverview of the Cell Cycle18-1 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; use each word or phrase only once.The four phases of the cell cycle, in order, are G1, __________________, __________________, and __________________. A cell contains the most DNA after __________________ phase of the cell cycle. A cell is smallest in size after __________________ phase of the cell cycle. Growth occurs in __________________, __________________, and __________________ phases of the cell cycle. A cell does not enter mitosis until it has completed __________________ synthesis.DNA M proteinG1 nucleotide SG2 organelle18-2 What 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.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.Figure Q18-3Which of the following would not explain the results with the mutant?(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 G118-4 Which of the following events does not usually occur during interphase?(a) Cells grow in size.(b) The nuclear envelope breaks down.(c) DNA is replicated.(d) The centrosomes are duplicated.18-5 What would happen to the progeny of a cell that proceeded to mitosis and cell division after entering S phase but had not completed S phase? Keep in mind that highly condensed chromatin, including the centromere region, is replicated late in S phase. Explain your answer.18-6 Are the statements below true or false? Explain your answer.A. Statement 1: Generally, in a given organism, the S, G2, and M phases of the cell cycle take a defined and stereotyped amount of time in most cells.B. Statement 2: Therefore, the cell-cycle control system operates primarily by a timing mechanism, in which the entry into one phase starts a timer setfor sufficient time to complete the required tasks. After a given amount of time has elapsed, a molecular “alarm” triggers movement to the next phase.18-7 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 eucaryotic 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.18-8 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.(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.How We Know: Discovery of Cyclins and Cdks18-9 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.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__________________-like phase. When the cells are triggered to resume cycling, the cell divisions have especially __________________ G1 and G2 phases and occur __________________. Studies with Xenopus eggs identified a partly purified activity called __________________ that drives a resting Xenopus oocyte into M phase. MPF activity was found to __________________ during the cell cycle, although the amount of its kinase component, called __________________, remained constant. The regulatory component of MPF, called __________________, has a __________________ effect on MPF activity and plays a part in regulating interactions with its __________________s. The components of MPF are evolutionarily __________________ from yeast to humans, so thatthe corresponding human genes are __________________ to function in yeast.able hexokinase shortasynchronously inhibitory spermCdk long steadyconserved M stimulatorycyclin maturation promoting factor substratedivergent oscillate synchronouslyegg PI 3-kinase ubiquitinfibroblast regulin unableG1S uniformG218-10 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 acontrol 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.18-11 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 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 acidsequences of cylins 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.The Cell-Cycle Control System18-12 Levels of Cdk activity change during the cell cycle, in part, because