The Genetic Code

Little more than 50 years have passed since James Watson and Francis Crick reported the structure of deoxyribose nucleic acid (DNA) in an understated article in the journal Nature. This discovery ushered in a new era of discovery in the biological sciences that continues at an ever-increasing pace.

The growth and differentiation from a single fertilized ovum into a functional organism requires around 2000 proteins, many of which are enzymes that catalyse chemical reactions directly or via intermediates. Other proteins have structural roles, contribute to cell membranes, bind ions or act as hormones. Proteins consist of polypeptides chains, each built from unique sequences of 20 amino acids into chains of between 30 and 3000 amino acids (primary structure). The chains are folded into conformations containing active sites that allow allosteric interaction between enzyme and substrate. Enzymatic reactions require energy, which is supplied by the breakdown of food molecules coupled to the phos-phorylating system. Unlike sugars, which are synthesized by repeating blocks of similar composition and therefore require

Figure 9.1. The double helix.
Figure 9.2. Base-pairing of double-stranded DNA.

a limited number of enzymes, proteins are uniquely irregular, each coded for by a template of DNA, which carries a specific genetic code.

DNA is composed of two strands, each with a deoxyribose sugar 'backbone' to which are attached sequences of two purine (adenine and guanine) and two pyrimidine (cytosine and thymidine) complementary nucleotides. As the 'backbones' are on the outside of the molecule, the complementary purine and pyrimidine bases of opposing strands face each other and are loosely attached by hydrogen bonds: A to T (two H-bonds) and G to C (three H-bonds). The whole molecule is supercoiled into a double helix (Figs 9.1 and 9.2). This arrangement allows a complementary 'negative' strand to be available for replication of the original molecule.

Triplets of bases (codons) form a unique code for each of the 20 amino acids required for the synthesis of proteins (e.g. TGG for tryptophan and AAG for phenylalanine). The code is described as 'degenerate' because from four separate bases, 64 (43) 'triplet' sequences can be constructed, a greater number than that required to synthesize 20 amino acids; that is, each amino acid can have more than one codon. Some code for the same amino acids while others (TAA, TAG and TGA) provide 'stop' signals for the synthetic process. The first two bases are most critical in determining the appropriate amino acid. There is a degree of 'wobble' in that the third base of the codon may differ and yet code for the same amino acid.

Proteins are synthesized by the ribosomes within the cell cytoplasm. DNA cannot pass through the nuclear membrane but ribonucleic acid (RNA), which is of similar composition except that uracil is substituted for thymidine, freely transgresses it. The code from a positive strand of DNA is transcribed into a complementary RNA molecule through the action of DNA-RNA polymerase and is transported to the ribosomes. On the surface of the ribosomes, the RNA strand meets strands of transfer RNA (tRNA), which are chemically linked to amino acids to which they give a genetic identity. By moving over the surface of the ribosome, the RNA recruits amino acids in the required sequence to synthesize a specific protein, the process of 'translation' (Fig. 9.3).

The human chromosome contains some 30 000 functional genes, but 90% of the genome is made up of 'spacers' of 'junk' DNA. Not all the sequences of bases within a gene actively code for amino acids. They are divided into 'exons' which code and 'introns' which do not. As transcription yields an mRNA strand longer than that required to transfer the genetic code, the unnecessary segments are excised and the ends spliced. Recently, it has been recognized that so-called 'junk' DNA may serve distinct and important regulatory functions. Furthermore, RNA species have been shown to have regulatory roles. MicroRNA (miRNA) and small-interfering RNA (siRNA) have been shown to regulate gene expression. Introduction of siRNA complementary to a target mRNA sequence into a cell induces highly specific and potent post-transcriptional silencing of gene expression. Although poorly understood, this phenomenon appears to contribute to genomic stability by suppressing potentially damaging destabilizing elements such as transposons and may act as an antiviral defence mechanism. Gene silencing induced by antisense oligonucleotides and siRNA have shown therapeutic potential in early preclinical studies.

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