Nucleic acid structure and function

The chemical constituents of genetic information are deoxyribonucleic acid ( QNA) and ribonucleic acid (RNA). Both molecules consist of linear chains of nitrogenous bases bound to a sugar (ribose) and a phosphate backbone. Because of the way the sugars are joined together, one end of each nucleic acid strand will have a terminal sugar residue in which the carbon atom at position number 5 of the ribose molecule is not linked; the other end has a free carbon atom at position 3. These two ends are termed the 5' (5 prime) and 3' (3 prime) ends respectively. It is usual to describe a DNA or RNA sequence by writing the order of bases in a single strand, in the 5' to 3' direction. DNA contains four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). RNA differs in that it contains uracil instead of thymine.

Two structural differences between DNA and RNA are important for understanding nucleic acid function. First, DNA has a hydroxyl group on part of its sugar constituent whereas RNA has a hydrogen atom. The result is that, in most biological environments, RNA is much more unstable. Second, RNA normally exists as a single molecule, whereas DNA is a double helix in which two strands are held together by weak hydrogen bonds between opposed base pairs ( bp), C joined to G and A to T. The sequence of one strand can therefore be inferred from the other. The two strands are said to be complementary to each other, and this property is exploited whenever DNA is copied (during meiosis, mitosis, or in vitro processes such as amplification of DNA using a polymerase chain reaction).

As befits an unstable molecule, RNA mediates the expression of genetic information; its production, and degradation, are under tight regulation. RNA is translated into a linear order of amino acids in proteins according to a three-letter code (e.g. GAA encodes the amino acid glutamine). DNA acts as a template for the production of RNA in a process termed transcription. But DNA is more than a stable repository of encoded protein sequence information; it also contains information that controls the transcription of RNA.

Disorders of the template function of DNA are the molecular basis of inherited dispositions and illness, and are the subject matter of genetics. By contrast, gene expression (the transcription and translation of RNA) is not entirely genetically predetermined. It is tightly regulated, but in response to changes in the cellular environment which in turn reflect changes in the state of the organism. Unfortunately we do not know much about how gene expression is regulated in eukaryotes, although this subject is rapidly progressing and will certainly have a major impact on the understanding and treatment of human disease. An introduction to this important subject is given below. For further details the reader is recommended to consult the textbook by Strachan and Read. (5)

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