Genetic Code - Transfer of Information Via The Genetic Code

Transfer of Information Via The Genetic Code

The genome of an organism is inscribed in DNA, or, in the case of some viruses, RNA. The portion of the genome that codes for a protein or an RNA is called a gene. Those genes that code for proteins are composed of tri-nucleotide units called codons, each coding for a single amino acid. Each nucleotide sub-unit consists of a phosphate, a deoxyribose sugar, and one of the four nitrogenous nucleobases. The purine bases adenine (A) and guanine (G) are larger and consist of two aromatic rings. The pyrimidine bases cytosine (C) and thymine (T) are smaller and consist of only one aromatic ring. In the double-helix configuration, two strands of DNA are joined to each other by hydrogen bonds in an arrangement known as base pairing. These bonds almost always form between an adenine base on one strand and a thymine base on the other strand, or between a cytosine base on one strand and a guanine base on the other. This means that the number of A and T bases will be the same in a given double helix, as will the number of G and C bases. In RNA, thymine (T) is replaced by uracil (U), and the deoxyribose is substituted by ribose.

Each protein-coding gene is transcribed into a molecule of the related RNA polymer. In prokaryotes, this RNA functions as messenger RNA or mRNA; in eukaryotes, the transcript needs to be processed to produce a mature mRNA. The mRNA is, in turn, translated on a ribosome into a chain of amino acids otherwise known as a polypeptide. The process of translation requires transfer RNAs which are covalently attached to a specific amino acid, guanosine triphosphate as an energy source, and a number of translation factors. tRNAs have anticodons complementary to the codons in an mRNA and can be covalently "charged" with specific amino acids at their 3' terminal CCA ends by enzymes known as aminoacyl tRNA synthetases, which have high specificity for both their cognate amino acid and tRNA. The high specificity of these enzymes is a major reason why the fidelity of protein translation is maintained.

There are 4³ = 64 different codon combinations possible with a triplet codon of three nucleotides; all 64 codons are assigned to either an amino acid or a stop signal. If, for example, an RNA sequence UUUAAACCC is considered and the reading frame starts with the first U (by convention, 5' to 3'), there are three codons, namely, UUU, AAA, and CCC, each of which specifies one amino acid. Therefore, this 9 base RNA sequence will be translated into an amino acid sequence that is three amino acids long. A given amino acid may be encoded by between one and six different codon sequences. A comparison may be made using bioinformatics tools wherein the codon is similar to a word, which is the standard data "chunk" and a nucleotide is similar to a bit, in that it is the smallest unit. This allows for powerful comparisons across species as well as within organisms.

The standard genetic code is shown in the following tables. Table 1 shows which amino acid each of the 64 codons specifies. Table 2 shows which codons specify each of the 20 standard amino acids involved in translation. These are called forward and reverse codon tables, respectively. For example, the codon "AAU" represents the amino acid asparagine, and "UGU" and "UGC" represent cysteine (standard three-letter designations, Asn and Cys, respectively).