Essential Cell Biology 5th edition

5′ 3′
3′
Answers A:23
proliferated uncontrollably, it is unlikely that a viable host
organism could be maintained. For this reason, most
transposons move only rarely. Many transposons, for
example, synthesize only infrequent bursts of very small
amounts of the transposase that is required for their
movement.
ANSWER 9–5 Viruses cannot exist as free-living
organisms: they have no internal metabolism, and cannot
reproduce themselves. They thus have none of the
attributes that one normally associates with life. Indeed,
they can even be crystallized. Only inside cells can they
redirect normal cellular biosynthetic activities to the task of
making more copies of themselves. Thus, the only aspect of
“living” that viruses display is their capacity to direct their
own reproduction once inside a cell.
ANSWER 9–6 Although they can harm individuals,
mobile genetic elements do provide opportunities for
homologous recombination events, thereby causing
genomic rearrangements. They could insert into genes,
possibly obliterating splicing signals and thereby changing
the protein produced by the gene. They could also insert
into the regulatory DNA sequences of a gene, where
insertion between an enhancer and a transcription start
site could block the function of the enhancer and therefore
reduce the level of expression of a gene. In addition, the
mobile genetic element could itself contain an enhancer and
thereby change the time and place in the organism where
the gene is expressed.
ANSWER 9–7 With their ability to facilitate genetic
recombination, mobile genetic elements have almost
certainly played an important part in the evolution of
modern-day organisms. They can facilitate gene duplication
and the creation of new genes via exon shuffling, and they
can change the way in which existing genes are expressed.
Although the transposition of a mobile genetic element
can be harmful for an individual organism—if, for example,
it disrupts the activity of a critical gene—these agents of
genetic change may well be beneficial to the species as a
whole.
ANSWER 9–8 About 7.6% of each gene is converted
to mRNA [(5.4 exons/gene × 266 nucleotide pairs/exon)/
(19,000 nucleotide pairs/gene) = 7.6%]. Protein-coding
genes occupy about 28% of Chromosome 22 [(700 genes ×
19,000 nucleotide pairs/gene)/(48 × 10 6 nucleotide pairs) =
27.7%]. However, over 90% of this DNA is made of introns.
ANSWER 9–9 This statement is probably true. For
example, nearly half our DNA is composed of defunct
mobile genetic elements. And only about 10% of the human
genome appears to be under positive selection. However,
it is possible that future research will uncover functions for
some portion of our DNA that now seems unimportant.
ANSWER 9–10 The HoxD cluster is packed with complex
and extensive regulatory DNA sequences that direct each
of its genes to be expressed at the correct time and place
during development. Insertions of mobile genetic elements
into the HoxD cluster were probably selected against
because they would disrupt proper regulation of these
genes.
ANSWER 9–11
A. The exons in the human β-globin gene correspond to
the positions of sequence similarity (in this case identity)
with the cDNA, which is a direct copy of the mRNA and
thus contains no introns. The introns correspond to the
regions between the exons. The positions of the introns
and exons in the human β-globin gene are indicated
in Figure A9–11A. Also shown (in open bars) are
sequences present in the mature β-globin mRNA (and in
the gene) that are not translated into protein.
B. From the positions of the exons, as defined in Figure
A9–11A, it is clear that the first two exons of the
human β-globin gene have counterparts, with similar
sequence, in the mouse β-globin gene (Figure A9–11B).
However, only the first half of the third exon of the
human β-globin gene is similar to the mouse β-globin
gene. The similar portion of the third exon contains
sequences that encode protein, whereas the portion that
is different represents the 3′ untranslated region of the
gene. Because this portion of the gene does not encode
protein (nor does it contain extensive regulatory DNA
sequences), its sequence is probably not constrained
and the mouse and human sequences have drifted apart.
C. The human and mouse β-globin genes are also similar at
their 5′ ends, as indicated by the cluster of points along
the same diagonal as the first exon (Figure A9–11B).
These sequences correspond to the regulatory DNA
sequences upstream of the start sites for transcription.
Functional sequences, which are under selective
pressure, diverge much more slowly than sequences
without function.
D. The diagon plot shows that the first intron, although it is
not conserved in sequence, it is nearly the same length
in the human and mouse genes; however, the length of
(A) POSITIONS OF HUMAN β-GLOBIN EXONS
(B) HOMOLOGY BETWEEN MOUSE
AND HUMAN GENES
human β-globin cDNA
5′ mouse β-globin gene
Figure A9–11
5′ human β-globin gene
3′
5′
human β-globin gene 3′
ECB5 eA9.11-A9.11