Essential Cell Biology 5th edition

Answers A:17
H
N
N
adenine
N
N
H
NH 2
NH 2
N
O
circular DNA
linear DNA
H
N
hypoxanthine
N
N
H
HO
5′ 3′
REMOVAL OF RNA
PRIMER
template strand
new strand
RNA primer
OH 3′
5′
O
O
H
H 2 N
N
N
guanine
N
N
H
H
N
O
xanthine
N
H
N
N
H
5′
HO
3′
DNA SYNTHESIS
5′
OH 3′
H
H
cytosine
N
N
O
H
H
uracil
O
N
N
H
O
(A)
Figure A6−14
(B)
5′
LOST
NUCLEOTIDES
OH 3′
H 3 C
H
thymine
H
H
uracil
Figure A6−13
O
N
O
N
N
N
H
O
H
O
NO CHANGE
NO CHANGE
(as it is in RNA), repair enzymes could not distinguish
whether a uracil is the appropriate base or whether it arose
through spontaneous deamination of cytosine. This dilemma
is not encountered, however, because thymine, rather than
uracil, is used in DNA. Therefore, if a uracil base is found in
ECB5 EA6.13/A6.13
DNA, it can be automatically recognized as a damaged base
and then excised and replaced by cytosine.
ANSWER 6–14
A. DNA polymerase requires a 3ʹ-OH to synthesize DNA;
without telomeres and telomerase, the ends of linear
chromosomes would shrink during each round of DNA
replication. For bacterial chromosomes, which have no
ends, the problem does not arise; there will always be
a 3ʹ-OH group available to prime the DNA polymerase
that replaces the RNA primer with DNA (Figure A6−14).
Telomeres and telomerase prevent the shrinking of
chromosomes because they extend the 3ʹ end of the
template DNA strand (see Figure 6−23). This extension
of the lagging-strand template provides the “space” to
begin the final Okazaki fragments.
B. As shown in Figure A6−14A, telomeres and telomerase
are still needed even if the last fragment of the lagging
strand were initiated by primase at the very 3ʹ end of
chromosomal DNA, inasmuch as the RNA primer must
be removed.
ECB5 A6.14
ANSWER 6–15
A. If the single origin of replication were located exactly in
the center of the chromosome, it would take more than
8 days to replicate the DNA
[= 75 × 10 6 nucleotides/(100 nucleotides/sec)]. The rate
of replication would therefore severely limit the rate of
cell division. If the origin were located at one end, the
time required to replicate the chromosome would be
approximately double this.
B. A chromosome end that is not “capped” with a telomere
would lose nucleotides during each round of DNA
replication and would gradually shrink. Eventually,
essential genes would be lost, and the chromosome’s
ends might be recognized by the DNA damage-response
mechanisms, which would stop cell division or induce cell
death.
C. Without centromeres, which attach mitotic chromosomes
to the mitotic spindle, the two new chromosomes that
result from chromosome duplication would not be
partitioned accurately between the two daughter cells.
Therefore, many daughter cells would die, because they
would not receive a full set of chromosomes.
Chapter 7
ANSWER 7–1 Perhaps the best answer was given by
Francis Crick himself, who coined the term in the mid-1950s:
“I called this idea the central dogma for two reasons, I
suspect. I had already used the obvious word hypothesis
in the sequence hypothesis, which proposes that genetic
information is encoded in the sequence of the DNA
bases, and in addition I wanted to suggest that this new
assumption was more central and more powerful…. As it
turned out, the use of the word dogma caused more trouble
than it was worth. Many years later Jacques Monod pointed
out to me that I did not appear to understand the correct