Essential Cell Biology 5th edition

196 CHAPTER 5 DNA and Chromosomes
QUESTIONS
QUESTION 5–5
A. The nucleotide sequence of one DNA strand of a DNA
double helix is 5ʹ-GGATTTTTGTCCACAATCA-3ʹ.
What is the sequence of the complementary strand?
B. In the DNA of certain bacterial cells, 13% of the
nucleotides contain adenine. What are the percentages of
the other nucleotides?
C. How many possible nucleotide sequences are there for a
stretch of single-stranded DNA that is N nucleotides long?
D. Suppose you had a method of cutting DNA at specific
sequences of nucleotides. How many nucleotides long
(on average) would such a sequence have to be in order
to make just one cut in a bacterial genome of 3 × 10 6
nucleotide pairs? How would the answer differ for the
genome of an animal cell that contains 3 × 10 9 nucleotide
pairs?
QUESTION 5–6
An A-T base pair is stabilized by only two hydrogen bonds.
Hydrogen-bonding schemes of very similar strengths can
also be drawn between other base combinations that
normally do not occur in DNA molecules, such as the A-C
and the A-G pairs shown in Figure Q5−6. What would
happen if these pairs formed during DNA replication and
the inappropriate bases were incorporated? Discuss why
this does not often happen. (Hint: see Figure 5–4.)
the bases have quite different structures (Figure Q5–7).
Bases V, W, X, and Y have replaced bases A, T, G, and C.
Look at these structures closely. Could these DNA-like
molecules have been derived from a living organism that
uses principles of genetic inheritance similar to those used
by organisms on Earth?
B. Simply judged by their potential for hydrogen-bonding,
could any of these extraterrestrial bases replace terrestrial
A, T, G, or C in terrestrial DNA? Explain your answer.
H
N
H
O
N
C
C
N
X
H
C
N
H
N
C
N
C
C
C
C
C
N
H
V
O
H
N
H
H
H
O
H
N
H
C
C
C
N
Y
H
C
N
H
N
C
N
N
C
C
C
C
N
H
O
N
W
H
H
3′ 5′ 3′ 5′
O
N
C
adenine
H
H
5′
H
C
N
H
N
N
Figure Q5−6
C
C
C
QUESTION 5–7
C
C
N
H
N
C
N
H
cytosine
H
C
N
H
3′
adenine
guanine
A. A macromolecule isolated from an extraterrestrial source
superficially resembles DNA, but closer analysis reveals that
ECB5 EQ5.06/Q5.06
5′
H
N
H
H
N
C
C
N
N
C
N
H
N
C
N
C
C
C
C
C
N
N
H
O
H
N
C
H
3′
H
Figure Q5−7
QUESTION 5–8
The two strands of ECB5 a DNA EQ5.07/Q5.07 double helix can be separated
by heating. If you raised the temperature of a solution
containing the following three DNA molecules, in what
order do you suppose they would “melt”? Explain your
answer.
A. 5ʹ-GCGGGCCAGCCCGAGTGGGTAGCCCAGG-3ʹ
3ʹ-CGCCCGGTCGGGCTCACCCATCGGGTCC-5ʹ
B. 5ʹ-ATTATAAAATATTTAGATACTATATTTACAA-3ʹ
3ʹ-TAATATTTTATAAATCTATGATATAAATGTT-5ʹ
C. 5ʹ-AGAGCTAGATCGAT-3ʹ
3ʹ-TCTCGATCTAGCTA-5ʹ
QUESTION 5–9
The total length of DNA in one copy of the human genome
is about 1 m, and the diameter of the double helix is about
2 nm. Nucleotides in a DNA double helix are stacked (see
Figure 5–4B) at an interval of 0.34 nm. If the DNA were
enlarged so that its diameter equaled that of an electrical
extension cord (5 mm), how long would the extension cord
be from one end to the other (assuming that it is completely
stretched out)? How close would the bases be to each
other? How long would a gene of 1000 nucleotide pairs be?