Transcription

Transcription is copying the information of a stretch of DNA into RNA. It follows the same base-pairing rule, except that RNA has uracil (U) instead of thymine, so A pairs with U. Only one strand of the DNA is copied in any one stretch: copying both would make two RNAs that stick to each other and could not become proteins, and they would code for two different proteins.

A transcription unit has three parts:

• a promoter, where RNA polymerase binds to start;
• the structural gene, which is copied;
• a terminator, where copying stops.

The two DNA strands have names. The template strand (running 3'→5') is the one actually read. The other, the coding strand (5'→3'), is not read, but its sequence is the same as the RNA's, with T in place of U. By convention, the promoter is said to lie towards the 5' end (upstream) of the coding strand.

In bacteria, one RNA polymerase makes all kinds of RNA. It needs help: the σ (sigma) factor to start, and the ρ (rho) factor to stop. Bacteria have no nucleus, so their mRNA can be translated while it is still being made.

In eukaryotes, there are three RNA polymerases, each with its own job:

• RNA polymerase I makes most ribosomal RNAs (28S, 18S, 5.8S).
• RNA polymerase II makes the precursor of mRNA, hnRNA (heterogeneous nuclear RNA).
• RNA polymerase III makes tRNA, 5S rRNA and small nuclear RNAs (snRNAs).

Eukaryotic genes are split: the coding parts, the exons, are interrupted by non-coding parts, the introns. The hnRNA is processed before it leaves the nucleus:

• Splicing: the introns are cut out and the exons joined together.
• Capping: an unusual nucleotide (methyl guanosine triphosphate) is added to the 5' end.
• Tailing: a tail of 200-300 adenine nucleotides is added to the 3' end.

The result is mature mRNA, which goes out to the cytoplasm to be translated.