Essential Cell Biology 5th edition

Answers A:21
Figure A8–2
minor groove
minor groove
N
thymine
O
H
N
adenine
cytosine
H
O
H
N
N
N
guanine
H
N
H
N
N
H
N
H
N
N
H
N
H
groove (see Figure 8–4). The minor groove, however,
contains fewer features that distinguish one base from
another than does the major groove.
ANSWER 8–3 Bending proteins can help to bring distant
ECB5 EA8.02/A8.02
DNA regions together that normally would contact each
other only inefficiently (Figure A8–3). Such proteins are
found in both prokaryotes and eukaryotes and are involved
in many examples of transcriptional regulation.
N
H
O
CH 3
H
H
N
H
N
H
O
hydrophobic
group
H-bond
acceptor
H-bond
donor
H-bond
acceptor
H-bond
donor
H-bond
acceptor
H-bond
acceptor
ANSWER 8–4
A. UV light throws the switch from the prophage to the
lytic state: when cI protein is destroyed, Cro is made and
turns off the further production of cI. The virus produces
coat proteins, and new virus particles are made.
B. When the UV light is switched off, the virus remains
in the lytic state. Thus, cI and Cro form a transcription
switch that “memorizes” its previous setting.
C. This switch makes sense in the viral life cycle: UV light
tends to damage the bacterial DNA (see Figure 6−25),
thereby rendering the bacterium an unreliable host for
the virus. A prophage will therefore switch to the lytic
state and leave the “sinking ship” in search of new host
cells to infect.
ANSWER 8–5
A. True. Prokaryotic mRNAs are often transcripts of entire
operons. Ribosomes can initiate translation at the
internal AUG start sites of these “polycistronic” mRNAs
(see Figures 7−40 and 8–6).
B. True. The major groove of double-stranded DNA is
sufficiently wide to allow a protein surface, such as
one face of an α helix, access to the base pairs. The
sequence of H-bond donors and acceptors in the major
groove can then be “read” by the protein to determine
the sequence of the DNA (see Figure A8–2).
C. True. It is advantageous to exert control at the earliest
possible point in a pathway. This conserves metabolic
energy because unnecessary products are not made.
ANSWER 8–6 From our knowledge of enhancers, one
would expect their function to be relatively independent of
their distance from the promoter—possibly weakening as
this distance increases. The surprising feature of the data
(which have been adapted from an actual experiment) is
the periodicity: the enhancer is maximally active at certain
distances from the promoter (50, 60, or 70 nucleotides),
but almost inactive at intermediate distances (55 or 65
nucleotides). The periodicity of 10 suggests that the mystery
can be explained by considering the structure of doublehelical
DNA, which has 10 base pairs per turn. Thus, placing
an enhancer on the side of the DNA opposite to that of the
promoter (Figure A8–6) would make it more difficult for the
activator that binds to it to interact with the proteins bound
at the promoter. At longer distances, there is more DNA to
absorb the twist, and the effect is diminished.
enhancer with bound
transcription regulator
50 bp
RNA polymerase
enhancer with bound
transcription regulator
RNA polymerase
55 bp
Figure A8–3 bending protein
Figure A8–6
60 bp