Deciphering the Genetic Code

There are two steps in the path from genes to proteins. In the first step, called transcription, the region of the double-stranded DNA corresponding to a specific gene is copied into an RNA molecule, called messenger RNA (mRNA), by an enzyme called RNA polymerase. In the second step, called translation, the mRNA directs the assembly of amino acids in a specific sequence to form a chain of amino acids called a polypeptide. This process is accomplished by ribosomes, special amino acid-bearing RNA molecules called transfer RNAs (tRNAs), and other translation factors. The newly synthesized polypeptides form proteins, which have functional and structural roles in cells. All proteins are synthesized by this process.

The precise order of amino acids assembled during translation is determined by the order of nucleotides in the mRNA. These nucleotides are a direct copy of the linear sequence of the nucleotides in one of the two complementary DNA strands, which have been transcribed using a code in which every three bases of the RNA specify an amino acid. DNA and RNA molecules both have directionality, which is indicated by reference to either the 5' ("five prime") end or the 3' ("three prime") end.

The code is always read in the 5' to 3' direction, using adjacent, non-overlapping three-base units called codons. Since there are four different nucleotides (also called bases) in RNA (abbreviated A, C, G, and U), there are sixty-four (43) different codons, and each codon specifies a particular amino acid. There are only twenty different amino acids, however, so many of the amino acids can be specified by more than one codon, a circumstance that is known as degeneracy. The list of mRNA codons specific for a given transcription messenger RNA formation from a DNA sequence polypeptide chain of amino acids ribosomes protein-RNA complexes at which protein synthesis occurs nucleotides the building blocks of RNA or DNA

The steps of translation. Initiation: RNA binds to the small ribosomal subunit, a tRNA binds to the RNA, and the large subunit attaches to the small subunit. Elongation: Successive tRNAs bind to the A site, form a peptide bond to the growing amino acid chain, and then move to the P site. The spent tRNA is ejected. Termination: A termination or stop codon binds a release factor, which dissociates two ribosomal subunits. The peptide (amino acid chain) is released. Adapted from Curtis and Barnes, 1994.

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