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18.1 Mutations are
changes in the genetic message.
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• A
mutation is any change in the hereditary message.
• Mutations that change one or a few nucleotides are
called point mutations. They may arise as a result of damage from ionizing
or ultraviolet radiation, chemical mutagens, or errors in pairing during
DNA replication.
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1. What
are pyrimidine dimers? How do they form? How are they repaired? What may
happen if they are not repaired?
2. Explain how slipped mispairing can cause deletions and
frame-shift mutations.
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18.2 Cancer results
from mutation of growth-regulating genes.
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• Cancer
is a disease in which the regulatory controls that normally restrain cell
division are disrupted.
• A variety of environmental factors, including ionizing
radiation, chemical mutagens, and viruses, have been implicated in causing
cancer.
• The best way to avoid getting cancer is to avoid exposure
to mutagens, for example, those in cigarette smoke.
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3. What
is transfection? What has it revealed about the genetic basis of cancer?
4. About how many genes can be mutated to cause cancer? Why
do most cancers require mutations in multiple genes?
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18.3 Recombination
alters gene location.
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• Recombination
is the creation of new gene combinations. It includes changes in the position
of genes or fragments of genes as well as the exchange of entire chromosomes
during meiosis.
• Genes may be transferred between bacteria when they
are included within small circles of DNA called plasmids.
• Transposition is the random movement of genes within
transposons to new locations in the genome. It is responsible for many
naturally occurring mutations, as the insertion of a transposon into a
gene often inactivates the gene.
• Crossing over involves a physical exchange of genetic
material between homologous chromosomes during the close pairing that
occurs in meiosis. It may produce chromosomes that have different combinations
of alleles.
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5. What
is genetic recombination? What mechanisms produce it? Which of these mechanisms
occurs in prokaryotes, and which occurs in eukaryotes?
6. What is a plasmid? What is a transposon? How are plasmids
and transposons similar, and how are they different?
7. What are mismatched pairs? How are they corrected? What
effect does this correction have on the genetic message?
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18.4 Genomes are continually
evolving.
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·There are
four different classes of eukaryotic genes that differ primarily in gene
copy number.
·Many eukaryotic genes exist as single-copy genes. Segmental duplications
contain whole blocks of genes that have been copied. Multigene families
are clusters of related but different genes. A gene copied many times
in series is a tandem cluster.
·Noncoding sequences comprise a large portion of the eukaryotic
genome.
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8. What kinds of genes
exist in multigene families? How are these families thought to have evolved?
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