Transcription

The central dogma says information flows DNA → RNA → protein. Making RNA from DNA is transcription; making protein from RNA is translation. An mRNA lets the cell make many copies of one message and carry it out of the nucleus. Retroviruses (like HIV) reverse the first step, making DNA from RNA: reverse transcription.

Only one strand of DNA, the template (non-coding) strand, is copied. The RNA produced matches the other, coding (sense) strand, except that it has U instead of T. Bacteria have one RNA polymerase: a core enzyme (α₂ββ') which, with the sigma factor added, becomes the complete holoenzyme. Eukaryotes have three: I makes large rRNAs, II makes hnRNA (the precursor of mRNA), and III makes tRNA and 5S rRNA.

Transcription has three stages:

• Initiation: sigma recognises the promoter, the gene's starting signal. The polymerase opens about 17 base pairs of DNA at the TATAAT sequence (the Pribnow box) and starts making RNA without a primer.
• Elongation: a small "bubble" of open DNA travels along the gene as the RNA grows, about 40 nucleotides a second in bacteria.
• Termination: at the terminator, the RNA is released, with or without help from a protein called rho.

Bacteria often make one mRNA for several genes (polycistronic mRNA), and can begin translating it while it is still being made.

In eukaryotes, the first RNA (hnRNA, pre-mRNA) is processed before it leaves the nucleus: a methylated G cap is added at the 5' end; the introns are cut out and the exons joined (splicing); and a poly-A tail is added at the 3' end. The cap and tail protect the mRNA.