A protein is composed of smaller molecules called amino acids , and the structure and function of the protein is determined by the sequence of its amino acids. A sequence of three nucleotide bases, called a triplet, is the genetic code word, or codon, that specifies a particular amino acid. A protein consisting of 100 amino acids is thus encoded by a DNA segment consisting of 300 nucleotides. Of the two polynucleotide chains that form a DNA molecule, only one strand contains the information needed for the production of a given amino acid sequence. The other strand aids in replication.
Protein synthesis begins with the separation of a DNA molecule into two strands.
The mRNA leaves the cell nucleus and attaches to the ribosomes , specialized cellular structures that are the sites of protein synthesis. In a process called translation, the amino acids are linked together in a particular sequence, dictated by the mRNA, to form a protein. A gene is a sequence of DNA nucleotides that specify the order of amino acids in a protein via an intermediary mRNA molecule. Substituting one DNA nucleotide with another containing a different base causes all descendant cells or viruses to have the altered nucleotide base sequence. As a result of the substitution, the sequence of amino acids in the resulting protein may also be changed.