Essential Cell Biology 5th edition

A:18 Answers
use of the word dogma, which is a belief that cannot be
doubted. I did appreciate this in a vague sort of way but
since I thought that all religious beliefs were without serious
foundation, I used the word in the way I myself thought
about it, not as the world does, and simply applied it to a
grand hypothesis that, however plausible, had little direct
experimental support at the time.” (Francis Crick, What Mad
Pursuit: A Personal View of Scientific Discovery. Basic Books,
1988.)
ANSWER 7–2 Actually, the RNA polymerases are not
moving at all in the micrograph, because they have been
fixed and coated with metal to prepare the sample for
viewing in the electron microscope. However, before they
were fixed, they were moving from left to right, as indicated
by the gradual lengthening of the RNA transcripts. The RNA
transcripts are not fully extended because they begin to
fold up and interact with proteins as they are synthesized;
this is why they are shorter than the corresponding DNA
segments.
ANSWER 7–3 At first glance, the catalytic activities of
an RNA polymerase used for transcription could replace
the primase that operates during DNA replication. Upon
further reflection, however, there would be some serious
problems. (1) The RNA polymerase used to make primers
would need to initiate every few hundred bases, which is
much more often than promoters are spaced on the DNA.
Initiation would therefore need to occur in a promoterindependent
fashion or many more promoters would
have to be present in the DNA, both of which would be
problematic for the synthesis of mRNA. In addition, RNA
polymerase normally begins transcription on doublestranded
DNA, whereas the DNA replication primers are
synthesized using single-stranded DNA. (2) Similarly, the
RNA primers used in DNA replication are much shorter than
mRNAs. The RNA polymerase would therefore need to
terminate much more frequently than during transcription.
Termination would need to occur spontaneously (i.e.,
without requiring a terminator sequence in the DNA) or else
many more terminators would need to be present. Again,
both of these scenarios would be problematic for mRNA
production. Although it might be possible to overcome this
problem if special control proteins became attached to RNA
polymerase during replication, the problem has been solved
by the evolution of separate enzymes with specialized
properties. Some small DNA viruses, however, do utilize
the host RNA polymerase to make RNA primers for their
replication.
ANSWER 7–4 This experiment demonstrates that, once
an amino acid has been coupled to a tRNA, the ribosome
will trust the tRNA and “blindly” incorporate that amino
acid into the position according to the match between
the codon and anticodon. We can therefore conclude that
a significant part of the correct reading of the genetic
code—that is, the matching of a codon in an mRNA with
the correct amino acid—is performed by the synthetase
enzymes that correctly match tRNAs and amino acids.
ANSWER 7–5 The mRNA will have a 5ʹ-to-3ʹ polarity,
opposite to that of the DNA strand that serves as
the template. Thus the mRNA sequence will read
5ʹ-GAAAAAAGCCGUUAA-3ʹ. The N-terminal amino acid
coded for by GAA is glutamic acid. UAA specifies a stop
codon, so the C-terminal amino acid is coded for by CGU
and is an arginine. Note that the usual convention in
describing the sequence of a gene is to give the sequence
of the DNA strand that is not used as a template for RNA
synthesis; this sequence is the same as that of the RNA
transcript, with T written in place of U.
ANSWER 7–6 The first statement is probably correct:
RNA is thought to have been the first self-replicating
catalyst and, in modern cells, is no longer self-replicating.
We can debate, however, whether this represents a “loss.”
RNA now serves many roles in the cell: as messengers,
as adaptors for protein synthesis, as primers for DNA
replication, as regulators of gene expression, and as
catalysts for some of the most important reactions,
including RNA splicing and protein synthesis.
ANSWER 7–7
A. False. Ribosomes can make any protein that is specified
by the particular mRNA that they are translating. After
translation, ribosomes are released from the mRNA and
can then start translating a different mRNA. It is true,
however, that a ribosome can only make one type of
protein at a time.
B. False. mRNAs are translated as linear polymers; there
is no requirement that they have any particular folded
structure. In fact, such structures that are formed by
mRNA can inhibit its translation, because the ribosome
has to unfold the mRNA in order to read the message it
contains.
C. False. Ribosomal subunits can exchange partners after
each round of translation. After a ribosome is released
from an mRNA, its two subunits dissociate and enter a
pool of free small and large subunits from which new
ribosomes assemble around a new mRNA.
D. False. Ribosomes are not individually enclosed in a
membrane.
E. False. The position of the promoter determines the
direction in which transcription proceeds and therefore
which of the two DNA strands is used as the template.
Transcription of the other strand would produce an
mRNA with a completely different (and in most cases
meaningless) sequence.
F. False. RNA contains uracil but not thymine.
G. False. The level of a protein depends on its rate of
synthesis and degradation but not on its catalytic
activity.
ANSWER 7–8 Because the deletion in the Lacheinmal
mRNA is internal, it likely arose from incorrect splicing
of the pre-mRNA. The simplest interpretation is that the
Lacheinmal gene contains a 173-nucleotide-long exon
(labeled “E2” in Figure A7−8), and that this exon is lost
(“skipped”) during the processing of the mutant precursor
mRNA (pre-mRNA). This could occur, for example, if the
mutation changed the 3ʹ splice site in the preceding intron
(“I1”) so that it was no longer recognized by the splicing
machinery (a change in the CAG sequence shown in Figure
7–20 could do this). The snRNP would search for the next
available 3ʹ splice site, which is found at the 3ʹ end of the
next intron (“I2”), and the splicing reaction would therefore
remove E2 together with I1 and I2, resulting in a shortened
mRNA. The mRNA is then translated into a defective
protein, resulting in the Lacheinmal deficiency.
Because 173 nucleotides do not amount to an integral