Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CHAPTER 17 END-OF-CHAPTER PROBLEMS
1019
Problems
Which statements are true? Explain why or why not.
17–1 Since there are about 10 13 cells in an adult human,
and about 10 10 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 G 1 , S, G 2 ,
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 G 1 , S, or M phase. Explain your reasoning.
number of cells
0
1 2
relative fluorescence per cell
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 Problems is turned p17.05/17.04
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?
17–12 A living cell from the lung epithelium of a newt
is shown at different stages in M phase in Figure Q17–2.
Order these light micrographs into the correct sequence
and identify the stage in M phase that each represents.
(A) (B) (C)
(D) (E) (F)
Figure Q17–2 Light micrographs of a single cell at different stages of
M phase (Problem 17–12). (Courtesy of Conly L. Rieder.)
17–13 Down syndrome (trisomy 21) and Edwards syndrome
(trisomy 18) are Problems the most common p17.14/17.08
autosomal trisomies
seen in human infants. Does this fact mean that these
chromosomes are the most difficult to segregate properly
during meiosis?
17–14 The human genome consists of 23 pairs of chromosomes
(22 pairs of autosomes and one pair of sex chromosomes).
During meiosis, the maternal and paternal sets
of homologs pair, and then are separated into gametes, so
that each contains 23 chromosomes. If you assume that
the chromosomes in the paired homologs are randomly
assorted to daughter cells, how many potential combinations
of paternal and maternal homologs can be generated
during meiosis? (For the purposes of this calculation,
assume that no recombination occurs.)