Messenger RNA (mRNA) and transcription

Transcription is the synthesis of an RNA copy of a gene.

1. The enzyme RNA polymerase binds to a special nucleotide sequence on DNA called the promoter, which sits at the start of a gene. The orientation of the promoter indicates which strand is to be transcribed.
2. The two DNA strands separate locally, and one strand (the template strand) is used as a mould.
3. RNA polymerase moves along the template, reading it in the 3′→5′ direction and joining complementary RNA nucleotides one by one, so the RNA grows in the 5′→3′ direction. (A pairs with U, T with A, G with C, C with G.)
4. Only one strand of any section is transcribed, even though either could in principle give an RNA.

How it differs from replication: once replication begins it copies all the DNA in the cell; transcription copies only selected portions (genes) and uses RNA nucleotides, producing a single strand.

Polymerases: in prokaryotes, one RNA polymerase transcribes rRNA, mRNA and tRNA; in eukaryotes, there are three different RNA polymerases.

Structure of mRNA:

• At the start is a ribosome-binding site that positions the ribosome so that the first codon, AUG (start codon), is correctly placed for translation.
• At the end of the coding region is a stop codon – UAA, UAG or UGA.
• At the other end is a poly-A tail, a chain of up to ~200 adenine nucleotides, which protects the mRNA from breakdown by cytoplasmic enzymes. The tail is not translated into amino acids.

• Prokaryote vs eukaryote: In prokaryotes, mRNA is ready for translation immediately, and ribosomes can attach to the front end and begin translation while the other end is still being transcribed – transcription and translation occur at the same time because there is no nuclear envelope.
• In eukaryotes, the primary transcript must first be processed in the nucleus before the mature mRNA moves to the cytoplasm.