DNA and RNA are chemically very similar. The primary structures of both are linear polymers composed of monomers called nucleotides. Cellular RNAs range in length from less than one hundred to many thousands of nu-cleotides. Cellular DNA molecules can be as long as several hundred million nucleotides. These large DNA units in association with proteins can be stained with dyes and visualized in the light microscope as chromosomes, so named because of their stainability.
A Nucleic Acid Strand Is a Linear Polymer with End-to-End Directionality
DNA and RNA each consist of only four different nucleotides. Recall from Chapter 2 that all nucleotides consist of an organic base linked to a five-carbon sugar that has a phosphate group attached to carbon 5. In RNA, the sugar is ribose; in DNA, deoxyribose (see Figure 2-14). The nucleotides used in synthesis of DNA and RNA contain five different bases. The bases adenine (A) and guanine (G) are purines, which con-
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