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An important nucleic acid in addition to DNA is r ibo n ucleic a cid (RNA). Some viruses use RNA as the genetic
material, and even those organisms that employ DNA must first convert the genetic information into RNA for the
information to be accessible or functional. Structurally, RNA is quite similar to DNA.
It is a linear polymer made up of a
limited number of repeating monomers, each composed of a sugar, a phosphate, and a base. The sugar is ribose instead
of deoxyribose (hence, RNA) and one of the bases is uracil (U) instead of thymine (T). Unlike DNA, an RNA molecule
usually exists as a single strand, although significant segments within an RNA molecule may be double stranded, with G
pairing primarily with C and A pairing with U. This intrastrand base-pairing generates RNA molecules with complex
structures and activities, including catalysis.
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RNA has three basic roles in the cell. First, it serves as the intermediate in the flow of information from DNA to protein,
the primary functional molecules of the cell. The DNA is copied, or transcribed, into messenger RNA (mRNA), and the
mRNA is translated into protein. Second, RNA molecules serve as adaptors that translate the information in the nucleic
acid sequence of mRNA into information designating the sequence of constituents that make up a protein. Finally, RNA
molecules are important functional components of the molecular machinery, called ribosomes, that carries out the
translation process. As will be discussed in Chapter 2, the unique position of RNA between the storage of genetic
information in DNA and the functional expression of this information as protein as well as its potential to combine
genetic and catalytic capabilities are indications that RNA played an important role in the evolution of life.
1.1.4. Proteins, Encoded by Nucleic Acids, Perform Most Cell Functions
A major role for many sequences of DNA is to encode the sequences of proteins, the workhorses within cells,
participating in essentially all processes. Some proteins are key structural components, whereas others are specific
catalysts (termed enzymes) that promote chemical reactions. Like DNA and RNA, proteins are linear polymers.
However, proteins are more complicated in that they are formed from a selection of 20 building blocks, called amino
acids, rather than 4.
The functional properties of proteins, like those of other biomolecules, are determined by their three-dimensional
structures. Proteins possess an extremely important property: a protein spontaneously folds into a well defined and
elaborate three-dimensional structure that is dictated entirely by the sequence of amino acids along its chain (Figure 1.6).
The self-folding nature of proteins constitutes the transition from the one-dimensional world of sequence information to
the three-dimensional world of biological function. This marvelous ability of proteins to self assemble into complex