Transcription: Production of RNA from a DNA Template

The information in genes (DNA) is transcribed [local] into RNA messages which are then translated into protein.

Each amino acid residue in a protein is coded by three nucleotides in DNA. Each triplet of nucleotides is called a codon. The codons for a particular protein used to be called a cistron, but are now more commonly called the structural gene. mRNA is produced 5'-->3', so the template strand of DNA must be read in the 3'-->5' direction. The first base which is transcribed is designated the +1 position or start site. Bases in the 5' direction on the template strand which will be subsequently transcribed are said to be downstream and designated by higher numbers (e.g. +2, +3, +35, +490, etc.).

The sequences immediately upstream from the start site for transcription are the promoter. They form the binding site for the RNA Polymerase which catalyzes the transcription. The promoter contains consensus sequences of bases which are important for polymerase binding; -10 region is TATA box [local]; -35 region (TTGACA). The TATA box binds proteins [local] which associate with RNA polymerase and orient it at the transcription initiation site. The whole unit of promoter and structural gene(s) is called the operon.

RNA polymerase is multimeric. One subunit (sigma) is a transcription factor that helps bind specifically to promoters. Sigma subunit comes off the complex during RNA elongation.

RNA polymerase core protein bound to promoter region

Sequence (no pun intended) of events in E coli.

1. RNA polymerase [local] enters transcription bubble and binds to promoter.
2. RNA polymerase moves to transcription start site.
3. RNA Polymerase is self-priming and begins polymerization by first binding a purine (ATP or GTP). It utilizes the 3'-OH group of the purine as the primer for the first polymerization cycle.
4. It uses the DNA as a template to incorporate the next nucleotide
5. A phosphodiester mbond forms between the first 2 residues of the mRNA.
6. RNA elongation proceeds, sigma dissociates, other proteins bind
7. RNA polymerase encounters a termination sequence on the template and dissociates (additional proteins may be required). Transcription stops.

Review the Mechansim of Transcription section of this site [local].

The process in eukaryotes is more complex than in prokaryotes. One of the major differences is that eukaryotic genes contain introns, intervening sequences of DNA that are spliced out of the mRNA before it is translated.