1、MCB 细胞增殖 练习题 1 一问答题 1. For each of the following describe whether such cells exist in human and give examples. A. Cells that do not divide. B. Cells that grow, but do not divide. C. Cells that divide, but do not grow. D. Cells that grow and divide. A. Most of the cells in an adult human are in this
2、class, having withdrawn from the cell cycle into G0. Liver cells, for example, remain quiescent for long periods, although they can grow and divide when the need arises. B. Nerve cells grow as they extend axons over long distances, but do not divide. Fat cells can accumulate large quantities of trig
3、lyceride, which causes them to increase in size. C. This is most rare category of cell, but the production of red blood cells is a good example. During production of the red blood cells, precursor reticulocytes undergo five divisions with little increase in overall volume, ultimately generating very
4、 small red blood cells from a much larger precursor cell. D. Most cells in human body grow and divide actively at some point during development, until we become adults. Even in adults some cells continue to grow and divide; most notably, intestinal cells and hematopoietic cells, which must constantl
5、y renew the lining of the gut and the cells in the blood, respectively. Most other cells grow and divide often enough to balance cell death. 2. How many kinetochores are there in a human cell at mitosis? There are 46 human chromosomes, each with two kinetochoresone for each sister chromatidthus, the
6、re are 92 kinetochores in a human cell at mitosis. 3. What are the two distinct cytoskeletal machines that are assembled to carry out the mechanical processes of mitosis and cytokinesis in animal cells? The separation of chromosomes and their distribution to daughter cells is accomplished by the bip
7、olar mitotic spindle, which is composed of microtubules and a variety of microtubule-dependent motors. The division of an animal cell into daughter cells by cytokinesis is accomplished by the contractile ring, which is composed of actin and myosin filaments and is located just under the plasma membr
8、ane. As the ring constricts, it pulls the membrane inward, ultimately dividing the cell in two. MCB 细胞增殖 练习题 2 4. Rb is one example of a category of antiproliferative genes in humans. Typically, when both copies of such genes are lost, cancers develop. Do you suppose that cancer could be eradicated
9、if tumor-suppressor genes such as Rb could be expressed at abnormally high concentrations in all human cells? Explain your answer. Antiproliferative genes such as Rb encode proteins that stop the cell cycle. During normal cell division, these proteins must be turned off. If the proteins were overexp
10、ressed in all cells, it is likely that the machinery that keeps these proteins turned off would be overwhelmed, and cell division would stop. Thus this cure for cancer might be successful, but the patient would be dead. 5. If a cell just entering mitosis is treated with nocodazole, which destabilize
11、s microtubules, the nuclear envelope breaks down and chromosomes condense, but no spindle forms and mitosis arrests. In contrast, if such a cell is treated with cytochalasin D, which destabilizes actin filaments, mitosis proceeds normally but a binucleate cell is generated and proceeds into G1. Expl
12、ain the basis for the different outcomes of these treatments with cytoskeleton inhibitors. What do these results tell you about cell-cycle checkpoints in M phase? Nocodazole treatment disassembles microtubules, preventing formation of a spindle. Because nuclei break down and chromosomes condense, th
13、ese events must be independent of aster formation by centrosomes, for example, and of any other microtubule-dependent process. Mitosis finally arrest because the unattached chromosomes trigger a signal that engages the spindle-attachment checkpoint, which halts the cycle. Treatment with cytochalasin D does not affect mitosis because actin filaments are not involved in the process. Moreover, the disassembly of actin filaments does not trigger a cell-cycle arrest; the cell completes mitosis and forms a binucleate cell. Thus, there does not seem to be a cytokinesis checkpoint.
