BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 25. RNA INFORMATION AND TRANSCRIPTION

In this chapter, we examine the phenotypic expression of the Genetic information contained in DNA. The primary function of genes is to encode the Proteins synthesized within The Cell. However, DNA itself is not used directly as a template for Protein Synthesis. Instead, RNA (ribonucleic acid) molecules serve this role as templates. Under normal conditions, the flow of genetic information in a cell proceeds in the following direction:

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In this chapter, we will see that certain RNA molecules act as intermediate carriers of information in protein synthesis, while others form an integral part of the protein-synthesizing machinery. We will then examine RNA Synthesis, which proceeds according to a DNA template sequence. This is The process of METABOLISM/31.html">Transcription, followed by Translation, during which RNA templates direct protein synthesis. This topic is discussed in greater detail in Chapter 27.

The involvement of RNA in protein synthesis seemed likely as early as 1940, several years before DNA was proven to be the hereditary material. Torbjörn Caspersson studied the distribution of RNA and DΝΑ in individual Eukaryotic Cells using light Microscopy. He based his work on the fact that both RNA and DΝΑ strongly absorb ultraviolet light at a wavelength of 260 nm, but only DΝΑ stains intensely with the Feulgen reagent. Caspersson discovered that almost all DΝΑ is localized in The Nucleus, whereas RNA is predominantly found in the Cytoplasm. Jean Brachet arrived at the same Conclusion by fractionating cells into nuclear and cytoplasmic components and determining their DΝΑ and RNA content. Furthermore, he found that RNA in the cytoplasmic fraction is part of small particles and is associated with protein. These RNA-protein particles were shown to be the sites of PROTEIN SYNTHESIS AND are now known as Ribosomes.

25.1. Structure of RNA

Like DΝΑ, RNA is a long, unbranched molecule composed of NUCLEOTIDES linked by 3' → 5' phosphodiester bonds. The number of nucleotides in an RNA molecule ranges from as few as 75 to many thousands. The Covalent Structure of RNA differs from that of DΝΑ in two respects. As the name implies, the sugar moiety in RNA is ribose rather than deoxyribose. Ribose contains a 2'-hydroxyl group that is absent in deoxyribose. The other difference is that one of the four bases in RNA is uracil (U) instead of thymine, which is found in DΝΑ. Like thymine, uracil can form Base Pairs with adenine; however, the uracil molecule lacks the methyl group present in thymine.

RNA molecules are single-stranded, with the exception of certain viral RNAs. Consequently, the Nucleotide Composition of RNA does not obey Chargaff's rules of complementarity. Indeed, in most RNA molecules, The amount of adenine differs from that of uracil, and the amount of guanine differs from that of cytosine. Nevertheless, RNA molecules contain double-stranded regions that form hairpin loops (Fig. 25.2). In these regions, A pairs with U, and G pairs with C. In addition, G can form a base pair with U, though it is less stable than the GC pair (Section 27.6). Base pairing in RNA hairpins is often imperfect. Some opposing bases within a hairpin are non-complementary, and one or more bases in one of the strands may bulge out from the double-helical region to facilitate the pairing of other bases. The fraction of double-helical segments varies widely among different RNAs, reaching up to 50% in the most typical cases.

Fig. 25.1. Structure of a portion of an RNA chain

Fig. 25.2. RNA can fold back on itself to form double-Helical structures