lenge is to identify the relevant genes and signal molecules for each cell type.
• How is cell growth coordinated with cell division to ensure that cell size remains constant?
In multicellular animals, cell size, cell division, and cell survival are carefully controlled to ensure that the organism and its organs achieve and maintain an appropriate size. Mitogens stimulate the rate of cell division by removing intracellular molecular brakes that restrain cell-cycle progression in G1. Growth factors promote cell growth (an increase in cell mass) by stimulating the synthesis and inhibiting the degradation of macromolecules. To maintain a constant cell size, proliferating cells employ multiple mechanisms to ensure that cell growth is coordinated with cell division.
number of cells
Which statements are true? explain why or why not.
17–1 Since there are about 1013 cells in an adult human, and about 1010 cells die and are replaced each day, we become new people every three years.
17–2 In order for proliferating cells to maintain a relatively constant size, the length of the cell cycle must match the time it takes for the cell to double in size.
17–3 While other proteins come and go during the cell cycle, the proteins of the origin recognition complex remain bound to the DNA throughout.
17–4 Chromosomes are positioned on the metaphase plate by equal and opposite forces that pull them toward the two poles of the spindle.
17–5 Meiosis segregates the paternal homologs into sperm and the maternal homologs into eggs.
17–6 If we could turn on telomerase activity in all our cells, we could prevent aging.
Discuss the following problems.
17–7 Many cell-cycle genes from human cells function perfectly well when expressed in yeast cells. Why do you suppose that is considered remarkable? After all, many human genes encoding enzymes for metabolic reactions also function in yeast, and no one thinks that is remarkable.
17–8 Hoechst 33342 is a membrane-permeant dye that fluoresces when it binds to DNA. When a population of cells is incubated briefly with Hoechst dye and then sorted in a flow cytometer, which measures the fluorescence of each cell, the cells display various levels of fluorescence as shown in Figure Q17–1.
A. Which cells in Figure Q17–1 are in the G1, S, G2, and M phases of the cell cycle? Explain the basis for your answer.
b. Sketch the sorting distributions you would expect for cells that were treated with inhibitors that block the cell cycle in the G1, S, or M phase. Explain your reasoning.
Figure Q17–1 analysis of Hoechst 33342 fluorescence in a population of cells sorted in a flow cytometer (problem 17–8).
17–9 The yeast cohesin subunit Scc1, which is essential for sister-chromatid cohesion, can be artificially regulated for expression at any point in the cell cycle. If expression is turned on at the beginning of S phase, all the cells divide satisfactorily and survive. By contrast, if Scc1 expression is turned on only after S phase is completed, the cells fail to divide and they die, even though Scc1 accumulates in the nucleus and interacts efficiently with chromosomes. Why do you suppose that cohesin must be present during S phase for cells to divide normally?
17–10 High doses of caffeine interfere with the DNA damage response in mammalian cells. Why then do you suppose the Surgeon General has not yet issued an appropriate warning to heavy coffee and cola drinkers? A typical cup of coffee (150 mL) contains 100 mg of caffeine (196 g/ mole). How many cups of coffee would you have to drink to reach the dose (10 mM) required to interfere with the DNA damage response? (A typical adult contains about 40 liters of water.) 17–11 How many kinetochores are there in a human cell at mitosis?