Explain how RNA polymerase recognizes where transcription should begin. Describe the promoter, the terminator, and the transcription unit.

Transcription is the process by which a DNA nucleotide sequence is used as a template for the synthesis of an RNA molecule, which can be mRNA, tRNA or rRNA, by means of the enzyme RNA dependent DNA polymerase. Transcription in prokaryotes and eukaryotes differs in several respects, so they are studied separately.

1. The precursors of RNA synthesis are the four ribonucleotides 5 'triphosphates (adenosine 5'-triphosphate, guanosine 5'-triphosphate, cytosine 5'-triphosphate and uridine 5'-triphosphate).

2. In the condensation reaction between the 5 'trisphosphate group of the incoming nucleotide and the 3'-OH group of the last nucleotide in the chain, the incoming nucleotide loses its two terminal phosphate groups. Its α group is used in the phosphodiester bond that binds it to the chain. This reaction occurs at the polymerase catalytic site.