The sequence of nucleotides along the DNA of an organism within which is incorporated the information necessary for protein synthesis. By controlling the type and amount of protein manufactured the genetic code controls the growth, development, and characteristics of an organism. The observation that DNA could specify the incorporation of twenty different types of amino acids into protein but itself consisted of only four bases implied that more than one base must code for a single amino acid. The shortest sequence of any four bases that could specify all the types of amino acids found in protein is three, since 43 = 64 possible combinations. (Pairs of any four bases could only code for 42 = 16 amino acids, which is not enough.) Thus arose the base triplet or triplet code hypothesis. It was further proposed that the * triplets were sequential, not overlapping. This would account for the almost infinite variation in the sequence of amino acids in a protein. If there was a tendency for some amino acids to follow others in a polypeptide, then this might indicate an overlapping code, i.e. one where the last one or two bases of one triplet acted as the first one or two in the next triplet. However, this is not the case. The theory that the genetic code consists of base triplets received considerable support from frame-shift experiments. These demonstrated that mutations resulting from the addition or loss of three base pairs close together produced protein resembling that of normal individuals. However, mutations resulting from the addition or loss of one, two, or four pairs produced significantly different protein. When three bases were added or lost only a short segment of protein was affected, whereas if other numbers were added or lost, the code for the whole protein was thrown out of sequence. |
