The multistage process by which information contained in the cell's genetic material is expressed as the amino acid sequence of a protein. All proteins are constructed from about twenty amino acids, the number, type, and sequence of which is unique to a particular protein. The arrangement of amino acids is predetermined by the sequence of bases in the genetic material, DNA (or RNA in some viruses). DNA is mostly confined to the nucleus in eukaryotic cells and contains four types of bases. Sequences of three bases (triplets) correspond to particular amino acids. Hence sequences of triplets (the genetic code) correspond to sequences of amino acids in proteins. The genetic code is first copied (transcription) by the formation of a complementary molecule called * messenger RNA (mRNA). After various modifications in the nucleus, mRNA moves to the * ribosomes in the cytoplasm where protein synthesis occurs. Amino acids in the cytoplasm bind to specific transfer RNA (tRNA) molecules at one end of which are three bases (the anticodon). Which amino acid is incorporated into the protein is determined by whether the three bases on tRNA are complementary to, i.e. can bind with, the triplets on mRNA (the codon). The matching of codons with anticodons and the addition of an amino acid to an existing polypeptide chain is called * translation. See also ribosomal RNA. |
