DNA, RNA, and Their Building Blocks
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are long-chained organic molecules that contain genetic code information (see Chapter 4). From a chemical point of view, in terms of building blocks of life, their composition and structure can be quite simply explained.
The backbone of DNA and RNA is a succession of sugars (deoxyribose for DNA, ribose for RNA) linked by phosphate bridges (PO42-) (Fig. 3.5). The "letters" of the genetic code, which are connected to each sugar molecule, are the purine bases adenine (A) and guanine (G) and the pyrimidine bases cytosine (C), thymine (T) (T in DNA only), and uracil (U) (in RNA only) (Fig. 3.6). The association of the sugar
with one of the bases is called a nucleoside. If a phosphate is linked to a nucleoside, the resulting structure is called a nucleotide. The polynucleotide is named according to the sugar in its structure: DNA if the sugar is a deoxyribose, and RNA if the sugar is a ribose.
The genetic code is encrypted in the specific succession of purine and pyrimidine bases. It contains information about which succession of amino acids is to be built in which proteins (see Chapter 4). This information is transferred to the ribosomes, the "proteins factory," via RNA (Fig. 3.2). Ribosomes are composed of RNA and proteins.
All living systems function according to this very same mechanism. What makes a mouse different from an elephant or a mushroom is the number and succession of the "letters" in the DNA molecules.
DNA and RNA are usually seen as carriers of genetic information. This picture has changed with the discovery of the self-splicing of certain RNAs. These observations laid the foundation for the concept of catalytic RNA, the ribozymes (see Chapter 4). This catalytic capacity of RNA led to the idea of the RNA world that might have preceded life as we know it today: a simplified cellular biochemical machinery in which RNA plays all roles that are essential for life; it carries and transmits information, and it catalyzes the reactions.
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