Genetic Code Definition
Genetic Code refers to the correspondence system between a codon and an amino acid. This code is universal, i.e., it is used by (practically) all existing species, and it is redundant, which means that the same amino acid is encoded by different codons.
From DNA to Protein
Based on the Biology Central Dogma, whenever a cell needs the function associated with a certain protein, DNA has to be transcribed into RNA and RNA translated into Protein.
DNA is made up of 4 different nucleotides (Adenine, Thymine, Guanine and Cytosine) which are randomly repeated and are linked to form the characteristic DNA double helix molecule. This molecule has the main function of storage of all the genetic information necessary for the conception of an organism. The genetic information is arranged in portions of nucleotide sequences called ‘genes’. Since DNA is not a direct match for protein synthesis, it is necessary an intermediate molecule between these two molecules – RNA.
In transcription, DNA (specifically the necessary genes) is copied and transformed into RNA. RNA is distinguished from DNA by being a more unstable molecule (which makes it not such a good store of genetic information), having sequences of much smaller size and being constituted by nucleotides Adenine, Uracil, Guanine and Cytosine.
In translation, the synthesis of proteins takes place having the RNA molecule as a template. Proteins are molecules made up by amino acids and are endowed with the most varied functions, from catalyzing reactions to participating in cell signalling and progress of cell cycle.
Protein synthesis from template RNA is only possible due to the existence of genetic code.
Genetic Code characteristics’
Genetic Code is the ‘language’ that cell uses in the transfer of genetic information (DNA, RNA) and the expression of that information in proteins capable of performing functions. To be ‘interpreted’, RNA is read in clusters of three nucleotides – the codons – by ribosomes, which are molecules responsible for making possible the interaction between nucleotides and amino acids and for creating bonds between amino acids to form proteins
If each nucleotide were to be read separately, there would be only 4 ‘combinations’ and if two nucleotides were read together, there would be only 16 different combinations (which would not yet reach the 20 existing amino acids). Therefore, it is necessary that nucleotides are read in sets of three, making possible to have 64 combinations.
Each codon specifies an amino acid, and these matches are well defined in the genetic code. In the codon reading there is no overlap and the first nucleotide is the most important for amino acids matching. It is followed, in terms of strength of association, by the second nucleotide of the codon. The third nucleotide is what confers redundancy to the genetic code, since in some cases the change in the last nucleotide of the codon does not alter the encoded amino acid (figure 1). This feature of the genetic code is very important in preventing the occurrence of nonsense mutations, which would lead to the production of many proteins without function.
Figure 1 – Genetic Code table’s. From the center to the periphery one reads the first nucleotide, then the second and finally the third; followed by the encoded amino acid name.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Berg J.M., Tymoczko J.L., Stryer L. (2002). Biochemistry (5th edition). W. H. Freeman, New York.
Brown T.A. (2002). Genomes (2nd edition). Wiley-Liss, Oxford.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
Griffiths A.J.F., Miller J.H., Suzuki D.T., Lewontin R.C., Gelbart W.M. (2000). An Introduction to Genetic Analysis (7th edition). W. H. Freeman, New York.
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
