Essential Cell Biology 5th edition

A:48 Answers
ANSWER 18–13
A. Radiation leads to DNA damage, which activates a
regulatory mechanism (mediated by p53 and p21; see
Figure 18−15) that arrests the cell cycle until the DNA
has been repaired.
B. The cell will replicate damaged DNA and thereby
introduce mutations in the daughter cells when the cell
divides.
C. The cell will be able to divide normally, but it will be
prone to mutations, because some DNA damage always
occurs as the result of natural irradiation caused, for
example, by cosmic rays. The mechanism mediated
by p53 is mainly required as a safeguard against the
devastating effects of accumulating DNA damage; this
mechanism is not required for the natural progression of
the cell cycle in undamaged cells.
D. Cell division in humans is an ongoing process that does
not cease upon reaching maturity, and it is required
for survival. Blood cells and epithelial cells in the skin
or lining the gut, for example, are being constantly
produced by cell division to meet the body’s needs; each
day, your body produces about 10 11 new red blood cells
alone.
ANSWER 18–14
A. Only the cells that were in the S phase of their cell cycle
(i.e., those cells making DNA) during the 30-minute
labeling period contain any radioactive DNA.
B. Initially, mitotic cells contain no radioactive DNA
because these cells were not engaged in DNA synthesis
during the labeling period. Indeed, it takes about two
hours before the first labeled mitotic cells appear.
C. The initial rise of the curve corresponds to cells that
were just finishing DNA replication when the radioactive
thymidine was added. The curve rises as more labeled
cells enter mitosis; the peak corresponds to those cells
that had just started S phase when the radioactive
thymidine was added. The labeled cells then exit from
mitosis, and are replaced by unlabeled mitotic cells,
which were not yet in S phase during the labeling period.
After 20 hours, the curve starts rising again, because the
labeled cells enter their second round of mitosis.
+
+
+
+
+
+
+
+
overlapping interpolar
microtubules of mitotic spindle
D. The initial two-hour lag before any labeled mitotic
cells appear corresponds to the G 2 phase, which is the
time between the end of S phase and the beginning of
mitosis. The first labeled cells seen in mitosis were those
that were just finishing S phase (DNA synthesis) when
the radioactive thymidine was added.
ANSWER 18–15 Loss of M cyclin leads to inactivation of
M-Cdk. As a result, the M-Cdk target proteins become
dephosphorylated by phosphatases, and the cells exit from
mitosis: they disassemble the mitotic spindle, reassemble
the nuclear envelope, decondense their chromosomes, and
so on. The M cyclin is degraded by ubiquitin-dependent
destruction in proteasomes, and the activation of M-Cdk
leads to the activation of APC/C, which ubiquitylates
the cyclin, but with a substantial delay. As discussed in
Chapter 7, ubiquitylation tags proteins for degradation in
proteasomes.
ANSWER 18–16 M cyclin accumulates gradually as it is
steadily synthesized. As it accumulates, it will tend to form
complexes with the mitotic Cdk molecules that are present.
The Cdk in these complexes is inhibited by phosphorylation
(see Figure 18–10). After a certain threshold level has been
reached, M-Cdk is activated by the phosphatase Cdc25.
Once activated, M-Cdk acts to enhance the activity of the
activating phosphatase; this positive feedback leads to the
complete activation of M-Cdk (see Figure 18−17). Thus,
M cyclin accumulation acts like a slow-burning fuse, which
eventually helps trigger the explosive self-activation of
M-Cdk. The precipitous destruction of M cyclin terminates
M-Cdk activity, and a new round of M cyclin accumulation
begins.
ANSWER 18–17 The order is F, C, B, A, D. Together, these
five steps are referred to as mitosis (E). Cytokinesis is the
last step in M phase, which overlaps with anaphase and
telophase. Mitosis and cytokinesis are both part of M phase.
ANSWER 18–18 If the growth rate of microtubules is the
same in mitotic and in interphase cells, their length is
proportional to their lifetime. Thus, the average length of
microtubules in mitosis is 1 μm (= 20 μm × 15 s/300 s).
ANSWER 18–19 As shown in Figure A18–19, the
+
+
+
+
+
+
+
+
spindle pole
+
+
+
+
+
+
+
+
+
+
Figure A18–19
+
+
+
+
plus-end directed
motor proteins
+
+