Essential Cell Biology 5th edition

A:54 Answers
eyed fly and a white-eyed fly, every progeny fly will
inherit one functional copy of the White gene from one
parent and one functional copy of the Ruby gene from
the other parent. Note that the normal white allele
produces brick-red eyes and the mutated form of the
gene produces white eyes. Because each of the mutant
alleles is recessive to the corresponding wild-type allele,
the progeny will have the wild-type phenotype—brickred
eyes.
B. Garnet, ruby, vermilion, and carnation complement one
another and the various alleles of the White gene (that
is, when these mutant flies are mated with each other,
they produce flies with a normal eye color); thus each
of these mutants defines a separate gene. In contrast,
white, cherry, coral, apricot, and buff do not complement
each other; thus, they must be alleles of the same gene,
which has been named the White gene. Thus, these nine
different eye-color mutants define five different genes.
C. Different alleles of the same gene, like the five alleles
of the White gene, often have different phenotypes.
Different mutations compromise the function of the
gene product to different extents, depending on the
location of the mutation. Alleles that do not produce any
functional product (null alleles), even if they result from
different DNA sequence changes, do have the same
phenotype.
ANSWER 19–18 SNPs are single-nucleotide differences
between individuals for which two or more variants are each
found at high frequency in the population. In the human
population, SNPs occur roughly once per 1000 nucleotides
of sequence. Many have been identified and mapped in
various organisms, including millions in the human genome.
SNPs, which are detected by sequencing, serve as physical
markers whose genomic locations are known. By tracking a
mutant gene through different matings, and correlating the
presence of the gene with the co-inheritance of particular
SNP variants, one can narrow down the potential location of
a gene to a chromosomal region that may contain only a few
genes. These candidate genes can then be tested for the
presence of a mutation that could account for the original
mutant phenotype (see Figure 19–38).
Chapter 20
ANSWER 20–1 The horizontal orientation of the
microtubules will be associated with a horizontal orientation
of cellulose microfibrils deposited in the cell walls. The
growth of the cells will therefore be in a vertical direction,
expanding the distance between the cellulose microfibrils
without stretching them (see Figure 20−6). In this way, the
stem will rapidly elongate; in a typical natural environment,
this will hasten emergence from darkness into light.
ANSWER 20–2 As three collagen polypeptide chains have
to come together to form the triple helix, a single defective
polypeptide chain will impair assembly, even if normal
chains are present at the same time. Collagen mutations are
therefore dominant; that is, they have a deleterious effect
even in the presence of a normal copy of the gene.
ANSWER 20–3 The remarkable ability to swell and thus
occupy a large volume of space depends on the negative
charges. These attract a cloud of positive ions, chiefly Na + ,
which by osmosis draw in large amounts of water, thus
giving proteoglycans their unique properties. With fewer
negative charges, proteoglycans will attract less water and
occupy less space. By contrast, uncharged polysaccharides
such as cellulose, starch, and glycogen (all composed
entirely of glucose subunits) are easily compacted into fibers
or granules.
ANSWER 20–4 Focal contacts are common in connective
tissue, where fibroblasts exert traction forces on the
extracellular matrix, and in cell culture, where cell crawling
is observed. The forces for pulling on the matrix or for
crawling are generated by the actin cytoskeleton. In mature
epithelia, focal contacts are presumably rare because the
cells are largely fixed in place and have no need to crawl
over the basal lamina or actively pull on it.
ANSWER 20–5 Suppose a cell is damaged so that its
plasma membrane becomes leaky. Ions present in high
concentration in the extracellular fluid, such as Na + and
Ca 2+ , then rush into the cell, and valuable metabolites leak
out. If the cell were to remain connected to its healthy
neighbors by open gap junctions, these cells too would
suffer from the damage. But the influx of Ca 2+ into the sick
cell causes its gap junctions to close immediately, effectively
isolating the cell and preventing damage from spreading in
this way.
ANSWER 20–6 Ionizing (high-energy) radiation tears
through matter, knocking electrons out of their orbits and
breaking chemical bonds. In particular, it creates breaks and
other damage in DNA, and thus causes cells to arrest in the
cell cycle to allow time to repair the damaged DNA before
proceeding to cell division (discussed in Chapter 18). If the
damage is so severe that it cannot be repaired, cells usually
kill themselves by undergoing apoptosis.
ANSWER 20–7 Cells in the gut epithelium are exposed to
a quite hostile environment, containing digestive enzymes
and many other substances that vary drastically from day
to day depending on the food intake of the organism.
These epithelial cells form a first line of defense against
potentially hazardous compounds and mutagens that
we consume or are ubiquitous in our environment. Rapid
turnover of epithelial cells protects the organism from
harmful consequences, as wounded and sick epithelial cells
are discarded (along with undamaged ones during the
normal course of gut epithelium renewal). If an epithelial cell
started to divide inappropriately as the result of a mutation,
for example, it and its unwanted progeny would most often
simply be discarded by natural disposal from the tip of the
villus: even though such mutations must occur often, they
rarely give rise to a cancer.
A neuron, on the other hand, lives in a highly protected
environment, largely insulated from the outside world. Its
function depends on a complex system of connections
with other neurons—a system that is created during
development and is not easy to reconstruct if the neuron
subsequently dies.
ANSWER 20–8 Every cell division generates one additional
cell; so if the cells were never lost or discarded from the
body, the number of cells in the body should equal the
number of divisions plus one. The number of divisions is
1000-fold greater than the number of cells because, in the
course of a lifetime, 1000 cells are discarded by mechanisms
such as apoptosis for every cell that is retained in the body.