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Genetic Code

  The genetic code is the phrase used by life scientists to describe the form in which information is carried by DNA. The information carried by a gene instructs the cell to produce specific protein. The basic structure of a protein is a string-like molecule which is constructed from units called amino acids; the sequence of amino acids in this string gives each protein its special physical and chemical properties. This basic structure of protein molecules has similarities to that of the DNA, which is composed of a chain of subunits called nucleotides. The gentic information which the DNA carries is encoded in this nucleotide sequence, and the process of protein synthesis is thus called translation, from the genetic code of the nucleotides in the DNA to the sequence of amino acids in the protein.

Only four types of nucleotide—called A, T, G and C—make up DNA, while there are more than 20 types of amino acids which can make up a protein. The four ‘letters’ of the DNA alphabet are grouped into three-letter ‘words’ to signify the amino acids: for example, CGT in the DNA code stands for the amino acid alanine. Thus the genetic code is a triplet code in which three nucleotides in sequence (a codon) code for one amino acid type. Some nucleotide triplets do not appear to code for anything, while others act as the punctuation in the genetic ‘script’. In order for the information contained in the form of DNA to be translated into a protein, the DNA sequence must itself be transcribed into an RNA molecule. RNA is closely related to DNA and is constructed from four nucleotides. This ‘messenger’ RNA molecule is small enough to diffuse out of the nucleus into the cytoplasm where special structures called ribosomes read the code and synthesize the protein.

The idea that chemical units might encode genetic information was first suggested in the 19th century, but it was not until the 1950s that it was suggested that a nucleotide sequence might act as a template for amino acid sequence. By the 1970s the code had been cracked so that genetic information could be read in terms of the protein for which it encodes; this knowledge is central to molecular biology and the many disciplines to which it is related. The flow of information is unidirectional—it appears to be a law of biology that an amino acid sequence cannot give rise to a nucleotide sequence. RB

See also genetic engineering.Further reading Bruce Alberts, Molecular Biology of the Cell.



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