TRANSCRIPTION
During transcription ("copying"), messenger RNA (mRNA) is synthesized as a complementary copy of one of the DNA strands, the transcribed strand. Messenger RNA carries genetic information in the form of sets of three bases called codons, each of which specifies one amino acid. Messenger RNA codons are translated consecutively, thus specifying the linear sequence of amino acids in the polypeptide chain.

Messenger RNA is synthesized by DNA-dependent RNA polymerase enzymes. RNA is formed from nucleotide subunits, each of which contains the sugar ribose, a single base (uracil, adenine, guanine, or cytosine), and three phosphates. RNA polymerase initially binds to a special DNA sequence called the promoter region.

As transcription proceeds, incoming nucleotides pair with complementary bases on the transcribed DNA strand. The same base-pairing rules are followed as in DNA replication, except that uracil is substituted for thymine. RNA synthesis proceeds in a 5' to 3' direction, which means that the template DNA strand is read in a 3' to 5' direction.