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The nucleic acids—deoxyribonucleic acid (DNA)
and ribonucleic acid (RNA)—are polymers that can
store and transmit genetic information. The blueprints
for the biochemical machines that manufacture
living organisms are encoded in the DNA molecules
that make up the genome of the cell. During transcription,
sequences of DNA serve as templates for the synthesis
of RNA. Certain RNAs, called messenger RNAs (mRNAs),
are subsequently decoded by ribosomes during translation
(see Chapter 9). The information that is stored
in the translated sequence of mRNA specifies the
amino acid sequence of one or more proteins, which
ultimately determine the phenotypic characteristics
of the organism. When cells divide, DNA replication
generates a duplicate set of genetic instructions
for the new cell. DNA replication and repair are
important processes because the survival of the
individual organism depends on the stability of
its genome. However, long-term survival of a population
can be promoted by the genetic variation that
results from changes in the DNA blueprints of
its individual members.
Living cells store
genetic information in the form of double-stranded
DNA. In contrast, viral genomes consist of either
double-stranded or single-stranded nucleic acids
and contain either DNA or RNA. The genomes of
viruses are generally small and encode only a
few of the proteins that are required for viral
propagation. To replicate their nucleic acids
and multiply, viruses must therefore exploit the
biochemical machinery of a host cell. For example,
when an RNA virus infects a cell, the cellular
machinery can translate the viral RNA directly
into protein, or can use the viral genome as a
template and synthesize complimentary RNAs for
subsequent translation. Some RNA viruses encode
reverse transcriptase, an enzyme that uses RNA
as a template for DNA synthesis. Once this enzyme
has catalyzed the reverse transcription of a DNA
copy of the viral RNA genome, the transcriptional
and translational machinery of the host cell produces
the other components necessary for virus multiplication.
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