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Chapter 6

Nucleic Acids

CHAPTER OUTLINE
Introduction
6.1 Composition of nucleic acids and synthesis of nucleotides
6.2 Replication of nuclear DNA
6.3 DNA repair
6.4 DNA recombination
6.5 Organellar DNA
6.6 DNA transcription
6.7 Characteristics and functions of RNA
6.8 RNA processing

Masahiro Sugiura
Ytaka Takeda

 

 

 

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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