DNA is the hereditary material responsible for all the characteristics of an organism, and it controls all the activities of a cell. It is able to do this as it carries the information that controls the synthesis of proteins. An important class of proteins is enzymes. These control chemical reactions within the cell, including the synthesis and breakdown of other classes of molecule. Therefore, by controlling which proteins are made at a particular time, in a particular type of cell, DNA is able to control all the characteristics of a cell.
Proteins are made up of amino acids. There are about 20 different types of amino acids commonly found in proteins. The precise number, and sequence, of amino acids makes up the primary structure of a polypeptide chain. A functional protein may consist of a single, or several polypeptide chains.
DNA must therefore carry coded information that determines not only the number and types of amino acids that appear in a polypeptide, but also their exact sequence in the chain.
The code for primary structure of proteins cannot be carried in the sugar-phosphate backbone of DNA since this structure is identical in all DNA molecules. The only part of DNA that varies between different DNA molecules is the base sequence. Therefore, the sequence of bases in DNA must determine the sequence of amino acids in a polypeptide chain. The length of DNA that codes for a single polypeptide chain (or protein) is called a gene and a gene can be thousands of nucleotides long.
- The code cannot be as simple as 1 base coding for 1 amino acid, as this would allow for the coding of only 4 amino acids.
- If the bases are read together in pairs, this would allow for only 16 different combinations i.e.. 16 different amino acids.
- Each amino acid is in fact coded for by a sequence of 3 consecutive nucleotide bases in the DNA chain. This is called the triplet base hypothesis and each triplet of bases is called a codon. The maximum number of combinations this allows is 64, but since there are only 20 common amino acids, some are coded for by more than one triplet codon. The genetic codon is thus described as degenerate.
The genetic code is usually given in tables and books as in the form of RNA that would be complementary to the DNA in the gene. This is because it is messenger RNA that is directly involved in protein synthesis and not the genes themselves.
Some of the triplet codons do not code for any amino acid. These are called non-sense codons and they act as signals to terminate the synthesis of the polypeptide chain i.e.. they act as "full stops" and are called termination codons. Some other triplets code for modified amino acids that act as signals to start the synthesis of the protein chain and these are called initiation codons.
Each triplet of bases represents a sequence in mRNA. Each sequence codes for the amino acid shown. The amino acids have been given their three letter abbreviations. For A level this table does not need to be learned. Questions are regularly set on this topic but the code will be provided in the question if needed to work out the answer.