Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CHAPTER 1 END-OF-CHAPTER PROBLEMS
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Summary
Eukaryotic cells, by definition, keep their DNA in a separate membrane-enclosed
compartment, the nucleus. They have, in addition, a cytoskeleton for support and
movement, elaborate intracellular compartments for digestion and secretion, the
capacity (in many species) to engulf other cells, and a metabolism that depends on
the oxidation of organic molecules by mitochondria. These properties suggest that
eukaryotes may have originated as predators on other cells. Mitochondria—and,
in plants, chloroplasts—contain their own genetic material, and they evidently
evolved from bacteria that were taken up into the cytoplasm of ancient cells and
survived as symbionts.
Eukaryotic cells typically have 3–30 times as many genes as prokaryotes, and
often thousands of times more noncoding DNA. The noncoding DNA allows for
great complexity in the regulation of gene expression, as required for the construction
of complex multicellular organisms. Many eukaryotes are, however, unicellular—among
them the yeast Saccharomyces cerevisiae, which serves as a simple
model organism for eukaryotic cell biology, revealing the molecular basis of many
fundamental processes that have been strikingly conserved during a billion years of
evolution. A small number of other organisms have also been chosen for intensive
study: a worm, a fly, a fish, and the mouse serve as “model organisms” for multicellular
animals; and a small milkweed serves as a model for plants.
Powerful new technologies such as genome sequencing are producing striking
advances in our knowledge of human beings, and they are helping to advance our
understanding of human health and disease. But living systems are incredibly complex,
and mammalian genomes contain multiple closely related homologs of most
genes. This genetic redundancy has allowed diversification and specialization of
genes for new purposes, but it also makes biological mechanisms harder to decipher.
For this reason, simpler model organisms have played a key part in revealing
universal genetic mechanisms of animal development, and research using these
systems remains critical for driving scientific and medical advances.
What we don’t know
• What new approaches might
provide a clearer view of the anaerobic
archaeon that is thought to have
formed the nucleus of the first
eukaryotic cell? How did its symbiosis
with an aerobic bacterium lead to the
mitochondrion? Somewhere on Earth,
are there cells not yet identified that
can fill in the details of how eukaryotic
cells originated?
• DNA sequencing has revealed a rich
and previously undiscovered world
of microbial cells, the vast majority
of which fail to grow in a laboratory.
How might these cells be made more
accessible for detailed study?
• What new model cells or organisms
should be developed for scientists to
study? Why might a concerted focus
on these models speed progress
toward understanding a critical
aspect of cell function that is poorly
understood?
• How did the first cell membranes
arise?
Problems
Which statements are true? Explain why or why not.
1–1 Each member of the human hemoglobin gene
family, which consists of seven genes arranged in two clusters
on different chromosomes, is an ortholog to all of the
other members.
1–2 Horizontal gene transfer is more prevalent in single-celled
organisms than in multicellular organisms.
1–3 Most of the DNA sequences in a bacterial genome
code for proteins, whereas most of the DNA sequences in
the human genome do not.
Discuss the following problems.
1–4 Since it was deciphered four decades ago, some
have claimed that the genetic code must be a frozen accident,
while others have argued that it was shaped by natural
selection. A striking feature of the genetic code is its
inherent resistance to the effects of mutation. For example,
a change in the third position of a codon often specifies the
same amino acid or one with similar chemical properties.
The natural code resists mutation more effectively (is less
susceptible to error) than most other possible versions, as
illustrated in Figure Q1–1. Only one in a million computer-generated
“random” codes is more error-resistant than
the natural genetic code. Does the extraordinary mutation
resistance of the genetic code argue in favor of its origin as
a frozen accident or as a result of natural selection? Explain
your reasoning.
number of codes (thousands)
20
15
10
5
0
0
natural
code
5 10 15
susceptibility to mutation
Figure Q1–1 Susceptibility to mutation of the natural code shown
relative to that of millions of computer-generated alternative genetic
codes (Problem 1–4). Susceptibility measures the average change in
amino acid properties caused by random mutations in a genetic code.
A small value indicates that mutations tend to cause minor changes.
(Data courtesy of Steve Freeland.)
Q1.1
20