Introduction to the Genetic Code

The sequence of a coding strand of DNA, read in the direction from 5′ to 3′, consists of nucleotide triplets (codons) corresponding to the amino acid sequence of a polypeptide read from N-terminus to C-terminus. Sequencing of DNA and proteins makes it possible to compare corresponding nucleotide and amino acid sequences directly. There are 64 codons; each of four possible nucleotides can occupy each of the three positions of the codon, making 4³ =64 possible trinucleotide sequences. In the (nearly) universal genetic code, used in the translation of prokaryotic genes and of nuclear genes of eukaryotes, each of these codons has a specific meaning in translation: 61 codons represent amino acids and 3 codons cause the termination of translation.
The breaking of the genetic code originally showed that genetic information is stored in the form of nucleotide triplets, but it did not reveal which amino acid is specified by each triplet codon. Before the advent of DNA sequencing, codon assignments were deduced on the basis of two types of in vitro studies. A system involving the translation of synthetic polynucleotides was introduced in 1961, when Nirenberg showed that polyuridylic acid (poly[U]) directs the assembly of phenylalanine into polyphenylalanine. This result means that UUU must be a codon for phenylalanine. In a later, second system, a trinucleotide was used to mimic a codon, thus causing the corresponding aminoacyl-tRNA to bind to a ribosome.
By identifying the amino acid component of the aminoacyl-tRNA, the meaning of the codon could be found. The two techniques together assigned meaning to all of the codons that represent amino acids.
The assignment of amino acids to codons is not random but shows relationships in which the third (3′) base has less effect on codon  meaning. In addition, chemically similar amino acids are often represented by related codons. The meaning of a codon that encodes an amino acid is determined by the tRNA that corresponds to it; the meaning of the termination codons is determined directly by protein factors .