Transcription

Transcription copies a gene into messenger RNA. It is a faithful copy, but in another alphabet.

The principle

An enzyme, RNA polymerase, locally opens the double helix and reads one of the two strands — the template strand — assembling the complementary RNA.

        non-template strand  A  T  G  C  C  T
                             |  |  |  |  |  |
   DNA                       |  |  |  |  |  |
        TEMPLATE strand      T  A  C  G  G  A
                             |  |  |  |  |  |
                             v  v  v  v  v  v
        messenger RNA        A  U  G  C  C  U

The complementarity rule

   DNA read      ->    RNA built
   ---------           ---------
     A           ->        U          (not T: RNA contains none)
     T           ->        A
     C           ->        G
     G           ->        C

Only one difference from DNA replication: an A on the read strand calls for a U, not a T.

A convenient consequence: the resulting mRNA is identical to the non-template strand, except that Ts are replaced by Us. That is why the non-template strand is also called the coding strand — it reads directly as the sequence of the message.

How it proceeds

1. INITIATION   RNA polymerase recognises a signal sequence (the promoter)
                upstream of the gene, and opens the double helix

2. ELONGATION   it moves along the template strand, adding
                complementary nucleotides one by one

3. TERMINATION  on reaching a stop signal it detaches
                and the messenger RNA is released

   ==========[promoter]=====GENE=====[stop]==========   DNA
                    >>>>>>>>>>>>>>
                    RNA polymerase advances
                       ~~~~~~~~~~~~~   mRNA being formed

Maturation, in eukaryotes

In organisms with a nucleus, a gene is not one continuous stretch: it alternates coding sequences, the exons, and non-coding ones, the introns.

pre-messenger RNA:  [exon1]--intron--[exon2]--intron--[exon3]
                              |  splicing: introns are cut out
                              v
mature messenger RNA: [exon1][exon2][exon3]

This cutting, splicing, holds a surprise: depending on which exons are kept, the same gene can produce several different proteins. This is alternative splicing, and it largely explains a result that stunned biologists in 2001: a human being has only about 20,000 genes — barely more than a microscopic worm — yet makes well over 100,000 different proteins.

Regulating transcription

Not all cells transcribe the same genes, nor at the same time. Regulatory proteins bind near the promoter to activate or repress transcription, in response to the cell's state, its type, or outside signals such as hormones.

It is this level of control that keeps a liver cell a liver cell, even though it also contains the genes for brain and skin.

Summary

• Transcription copies a DNA strand into messenger RNA, via RNA polymerase.
• Complementarity: A→U, T→A, C→G, G→C.
• The mRNA reproduces the non-template (coding) strand, with U instead of T.
• Three steps: initiation (promoter), elongation, termination.
• In eukaryotes, splicing removes the introns and keeps the exons.
• Alternative splicing lets one gene produce several proteins.