Essential Cell Biology 5th edition

Answers A:19
(A) NORMAL
splicing
173 bp
splicing
5′ 3′
cap E1 E2 E3 AAA
pre-mRNA
(B) MUTANT
Figure A7−8
E1 I1 E2 I2 E3
cap
E3 AAA
mRNA
Lacheinmal protein
gene
mutation that inactivates 3′ splice site
E1 I1 E2 I2 E3
mutant gene
splicing
5′ 3′
cap E1 E2 E3 AAA
mutant pre-mRNA
cap
E1
E1
E2
mutant protein
E3 AAA
mutant RNA
number of codons, the lack of this exon in the mRNA will
shift the reading frame at the splice junction. Therefore, the
Lacheinmal protein would be made correctly only through
exon E1. As the ribosome begins translating sequences
in exon E3, it will be in the wrong reading frame and will
therefore will produce a protein sequence that is unrelated
to the Lacheinmal sequence normally encoded by exon E3.
ECB5 EA7.08/
Most likely, the ribosome will soon encounter a stop codon,
which would be expected to occur on average about once in
every 21 codons (there are 3 stop codons in the 64 codons
of the genetic code).
ANSWER 7–9 Sequence 1 and sequence 4 both code
for the peptide Arg-Gly-Asp. Because the genetic code is
redundant, different nucleotide sequences can encode the
same amino acid sequence.
ANSWER 7–10
A. Incorrect. The bonds are not covalent, and their
formation does not require an input of energy.
B. Correct. The aminoacyl-tRNA enters the ribosome at the
A site and forms hydrogen bonds with the codon in the
mRNA.
C. Correct. As the ribosome moves along the mRNA,
the tRNAs that have donated their amino acid to
the growing polypeptide chain are ejected from the
ribosome and the mRNA. The ejection takes place two
cycles after the tRNA first enters the ribosome (see
Figure 7–37).
ANSWER 7–11 Replication. Dictionary definition: the
creation of an exact copy; molecular biology definition: the
act of copying a DNA sequence. Transcription. Dictionary
definition: the act of writing out a copy, especially from one
physical form to another; molecular biology definition: the
act of copying the information stored in DNA into RNA.
Translation. Dictionary definition: the act of putting words
into a different language; molecular biology definition:
the act of polymerizing amino acids into a defined linear
sequence using the information provided by the linear
sequence of nucleotides in mRNA. (Note that “translation”
is also used in a quite different sense, both in ordinary
language and in scientific contexts, to mean a movement
from one place to another.)
ANSWER 7–12 With four different nucleotides to choose
from, a code of two nucleotides could specify 16 different
amino acids (= 4 2 ), and a triplet code in which the position
of the nucleotides is not important could specify 20 different
amino acids (= 4 possibilities of 3 of the same bases +
12 possibilities of 2 bases the same and one different +
4 possibilities of 3 different bases). In both cases, these
maximal amino acid numbers would need to be reduced by
at least 1 because of the need to specify translation stop
codons. It is relatively easy to envision how a doublet code
could be translated by a mechanism similar to that used in
our world by providing tRNAs with only two relevant bases
in the anticodon loop. It is more difficult to envision how
the nucleotide composition of a stretch of three nucleotides
could be translated without regard to their order, because
base-pairing can then no longer be used: AUG, for example,
will not base-pair with the same anticodon as UGA.
ANSWER 7–13 It is likely that in early cells the matching
between codons and amino acids was less accurate than
it is in present-day cells. The feature of the genetic code
described in the question may have allowed early cells to
tolerate this inaccuracy by allowing a blurred relationship
between sets of roughly similar codons and roughly similar
amino acids. One can easily imagine how the matching
between codons and amino acids could have become more
accurate, step by step, as the translation machinery evolved
into that found in modern cells.
ANSWER 7–14 The codon for Trp is 5ʹ-UGG-3ʹ. Thus
a normal tRNA Trp contains the sequence 5ʹ-CCA-3ʹ as
its anticodon (see Figure 7–33). If this tRNA contains a
mutation so that its anticodon is changed to UCA, it will
recognize a UGA codon and lead to the incorporation of
a tryptophan instead of causing translation to stop. Many
other protein-encoding sequences, however, contain UGA
codons as their natural stop sites, and these stops would
also be affected by the mutant tRNA. Depending on the
competition between the altered tRNA and the normal
translation release factors (Figure 7–41), some of these
proteins would be made with additional amino acids at their
C-terminal end. The additional lengths would depend on the
number of codons before the ribosomes encounter a non-
UGA stop codon in the mRNA in the reading frame in which
the protein is translated.
ANSWER 7–15 One effective way of driving a reaction to
completion is to remove one of the products, so that the
reverse reaction cannot occur. ATP contains two high-energy
bonds that link the three phosphate groups. In the reaction
shown, PP i is released, consisting of two phosphate groups
linked by one of these high-energy bonds. Thus PP i can be
hydrolyzed with a considerable gain of free energy, and
thereby can be efficiently removed. This happens rapidly in