The RNA synthesis or ADN transcription occurs in the nucleus and requires: - DNA strand to act as a template. - Enzymes, specially the RNA-polymerase. - Ribonucleotides of Adenine, Guanine, Cytosine and Uracil. This consist of three maturation of the 3'-->5' direction while the sense of RNA synthesis is 5'-->3'. This is fulfilled because the enzyme selects the ribonucleotide whose base is complementary to the DNA chain that acts as a template, and binds it by means of a nucleotide bond, releasing a phosphate group.