270 Chapter 8 • RNA: Transcription and Processing We know that information is not transferred directly from DNA to protein, because, in a eukaryotic cell, DNA is in the nucleus, whereas protein is synthesized in the cytoplasm. Information transfer from DNA to pro-tein requires an intermediate. That intermediate is RNA. Although DNA and RNA are nucleic acids, RNA differs from DNA in that (1) it is usually single stranded rather than a double helix, (2) it has the sugar ribose rather than deoxyribose in its nucleotides, (3) it has the pyrimidine base uracil rather than thymine, and (4) it can serve as a biological catalyst. The similarity of RNA to DNA suggests that the Fow of information from DNA to RNA relies on the complementarity of bases, which is also the key to DNA replication. RNA is copied, or transcribed, from a tem-plate DNA strand into either a functional RNA (such as tRNA or rRNA), which is never translated into poly-peptides, or a messenger RNA, from which proteins are synthesized. In prokaryotes, all classes of RNA are transcribed by
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