Essential Cell Biology 5th edition

Answers A:13
rate (µmole/min)
2
1
0 5 10
substrate (µM)
[S] (µM)
0.08
0.12
0.54
1.23
1.82
2.72
4.94
10.00
min
µmole
1
rate
DATA FOR A AND B
1
[S]
6
4
2
1
µM
12.50
8.30
1.85
0.81
0.55
0.37
0.20
0.10
rate
(µmole/min)
0.15
0.21
0.70
1.1
1.3
1.5
1.7
1.8
1
rate
0 4 8 12
1
[S]
1
µM
Figure A4−26
min
µmole
6.7
4.8
1.4
0.91
0.77
0.67
0.59
0.56
B. True. G-C base pairs are held together by three
hydrogen bonds, whereas A-T base pairs are held
together by only two.
ANSWER 5–2 Histone octamers occupy about 9% of the
volume of the nucleus. The volume of the nucleus is
V = 4/3 × 3.14 × (3 × 10 3 nm) 3
V = 1.13 × 10 11 nm 3
The volume of the histone octamers is
V = 3.14 × (4.5 nm) 2 × (5 nm) × (32 × 10 6 )
V = 1.02 × 10 10 nm 3
The ratio of the volume of histone octamers to the nuclear
volume is 0.09; thus, histone octamers occupy about 9% of
the nuclear volume. Because the DNA also occupies about
9% of the nuclear volume, together they occupy about 18%
of the volume of the nucleus.
ANSWER 5–3 In contrast to most proteins, which
accumulate amino acid changes over evolutionary time,
the functions of histone proteins must involve nearly all of
their amino acids, so that a change in any position would be
deleterious to the cell.
ANSWER 5–4 Men have only one copy of the X
chromosome in their cells; a defective gene carried on it
therefore has no backup copy. Women, on the other hand,
have two copies of the X chromosome in their cells, one
inherited from each parent, so a defective copy of the
gene on one X chromosome can generally be compensated
for by a normal copy on the other chromosome. This
is the case with regard to the gene that causes color
blindness. However, during female development, one X
chromosome in each cell is inactivated by compaction into
heterochromatin, shutting down gene expression from that
chromosome (see Figure 5−28). This inactivation occurs
at random in each cell to one or the other of the two X
chromosomes, and therefore some cells of the woman will
express the mutant copy of the gene, whereas others will
express the normal copy. This process results in a retina
in which, on average, only every other cone cell is colorsensitive,
and women carrying the mutant gene on one X
chromosome therefore see colored objects with reduced
resolution.
A woman who is color-blind must have two defective
copies of this gene, one inherited from each parent.
Her father must therefore carry the mutation on his X
chromosome; because this is his only copy of the gene, he
would be color-blind. Her mother could carry the defective
gene on either or both of her X chromosomes: if she carried
it on both, she would be color-blind; if she carried it on
one, she would have color vision but reduced resolution, as
described above. Several different types of inherited color
blindness are found in the human population; this question
applies to only one type.
ANSWER 5–5
A. The complementary strand reads
5ʹ-TGATTGTGGACAAAAATCC-3ʹ. Paired DNA strands have
opposite polarity, and the convention is to write a singlestranded
DNA sequence in the 5ʹ-to-3ʹ direction.
B. The DNA is made of four nucleotides (100% = 13% A +
x% T + y% G + z% C). Because A pairs with T, the two
nucleotides are represented in equimolar proportions in
DNA. Therefore, the bacterial DNA in question contains
13% thymine. This leaves 74% [= 100% – (13% + 13%)]
for G and C, which also form base pairs and hence are
equimolar. Thus y = z = 74/2 = 37% of each.
C. A single-stranded DNA molecule that is N nucleotides
long can have any one of 4 N possible sequences.
D. To specify a unique sequence that is N nucleotides long,
4 N has to be larger than 3 × 10 6 . Thus,
4 N > 3 × 10 6 , solved for N, gives N > ln(3 × 10 6 )/
ln(4) = 10.7. Thus, on average, a sequence of only
11 nucleotides in length is unique in the genome.
Performing the same calculation for the genome size of
an animal cell yields a minimal stretch of 16 nucleotides.
This shows that a relatively short sequence can mark
a unique position in the genome and is sufficient, for
example, to serve as an identity tag for one specific
gene.
ANSWER 5–6 If the wrong bases were frequently
incorporated during DNA replication, genetic information
could not be inherited accurately. Life, as we know it, could
not exist. Although the bases can form hydrogen-bonded
pairs as indicated, these do not fit into the structure of the
double helix. The angle at which the A base is attached to