Essential Cell Biology 5th edition

Questions
331
Gene
Amino
Acids
Rates of Change
Nonsynonymous
Synonymous
Histone H3 135 0.0 4.5
Hemoglobin α 141 0.6 4.4
Interferon γ 136 3.1 5.5
VERTEBRATES
Rabbit Whale
Chicken Cat
Cobra
Salamander Human
Cow
Frog
Goldfish
Rates were determined by comparing rat and human
sequences and are expressed as nucleotide changes per site
per 10 9 years. The average rate of nonsynonymous changes
for several dozen rat and human genes is about 0.8.
PLANTS
Barley
to those that do not alter the encoded amino acid (ATC
→ ATT, which gives isoleucine → isoleucine, for example).
(As is apparent in the genetic code, Figure 7−27, there are
many cases where several codons correspond to the same
amino acid.)
A. Why are there such large differences between the
synonymous and nonsynonymous rates of nucleotide
substitution?
B. Considering that the rates of synonymous changes are
about the same for all three genes, how is it possible for
the histone H3 gene to resist so effectively those nucleotide
changes that alter its amino acid sequence?
C. In principle, a protein might be highly conserved
because its gene exists in a “privileged” site in the genome
that is subject to very low mutation rates. What feature of
the data in the table argues against this possibility for the
histone H3 protein?
QUESTION 9–17
Hemoglobin-like proteins were discovered in legumes,
where they function in root nodules to lower the oxygen
concentration, allowing the resident bacteria to fix nitrogen.
These plant “hemoglobins” impart a characteristic pink
color to the root nodules. The discovery of hemoglobin in
plants was initially surprising because scientists regarded
hemoglobin as a distinctive feature of animal blood. It
was hypothesized that the plant hemoglobin gene was
acquired by horizontal transfer from an animal. Many more
hemoglobin-like genes have now been discovered and
sequenced from a variety of organisms, and a phylogenetic
tree of hemoglobins is shown in Figure Q9–17.
A. Does the evidence in the tree support or refute the
hypothesis that the plant hemoglobins arose by horizontal
gene transfer from animals?
B. Supposing that the plant hemoglobin genes were
originally derived by horizontal transfer (from a parasitic
nematode, for example), what would you expect the
phylogenetic tree to look like?
QUESTION 9–18
The accuracy of DNA replication in the human germ-cell
line is such that on average only about 0.6 out of the 6
billion nucleotides is altered at each cell division. Because
most of our DNA is not subject to any precise constraint
on its sequence, most of these changes are selectively
neutral. Any two modern humans chosen at random will
Earthworm
Insect
INVERTEBRATES
Figure Q9–17
Clam
differ by about 1 nucleotide pair per 1000. Suppose we are
all descended from a single pair of ancestors (an “Adam
and Eve”) who were genetically identical and homozygous
(each chromosome was identical to its homolog). Assuming
that all germ-line mutations that arise are preserved
in descendants, how many cell generations must have
elapsed since the days of our original ancestor parents for
1 difference per 1000 nucleotides to have accumulated in
modern humans? Assuming that each human generation
corresponds on average to 200 cell-division cycles in
the germ-cell lineage and allowing 30 years per human
generation, how many years ago would this ancestral couple
have lived?
QUESTION 9–19
Nematode
Paramecium
ECB5 eQ9.18/Q9.18
Chlamydomonas
PROTOZOA
Lotus
Alfalfa
Bean
Reverse transcriptases do not proofread as they synthesize
DNA using an RNA template. What do you think the
consequences of this are for the treatment of AIDS?