From DNA to RNA: transcription

Why an intermediate molecule

A gene is a recipe; a protein is the dish. Between the two, the cell uses a working copy, the messenger RNA — and that intermediate step is no detail.

Why a copy?

In cells with a nucleus, two facts force an intermediate:

DNA is locked in the NUCLEUS          and never leaves it
proteins are built in the CYTOPLASM, by the ribosomes

DNA is the archive: too precious to circulate. The cell therefore makes disposable working copies, messenger RNAs, which leave the nucleus, serve for minutes to hours, and are then destroyed.

   NUCLEUS                       CYTOPLASM
   _______________              ______________________
  |               |            |                      |
  |   DNA  ---->  |  mRNA ---> |  ribosome -> PROTEIN |
  | transcription |            |     translation      |
  |_______________|            |______________________|

This sequence DNA → RNA → protein has a name: the central dogma of molecular biology, stated by Francis Crick in 1958.

DNA and RNA: four differences

                    DNA                        RNA
------------------  -------------------------  -----------------------
sugar               deoxyribose                ribose
bases               A, T, C, G                 A, U, C, G  (U replaces T)
structure           double helix               single strand
lifetime            permanent                  minutes to hours
location            nucleus                    nucleus then cytoplasm

Replacing thymine (T) by uracil (U) is the handiest marker: as soon as a U appears in a sequence, it is RNA.

The advantage of an intermediate

Going through a copy is not gratuitous complication. It enables three things:

- AMPLIFY  : a gene transcribed a thousand times gives a thousand mRNAs,
             hence many proteins from a single DNA copy
- REGULATE : the cell chooses WHICH genes to transcribe, and when
- PROTECT  : the original archive is never exposed to handling

The second explains a puzzling fact: a neuron and a liver cell have exactly the same DNA, but do not transcribe the same genes. Cell differentiation is not a difference in information, it is a difference in expression.

The full route

   gene (DNA)
      |  TRANSCRIPTION        in the nucleus
      v
   messenger RNA
      |  (exit from the nucleus)
      v
   messenger RNA in the cytoplasm
      |  TRANSLATION          on the ribosomes
      v
   chain of amino acids
      |  folding
      v
   functional PROTEIN

Summary