Definition
Transcription = synthesis of an RNA molecule using one strand of DNA as template.
| Term | Meaning |
|---|
| Template (antisense) strand | The strand actually read, 3′ → 5′ |
| Coding (sense) strand | The other strand; identical in sequence to the mRNA, with T for U |
| Promoter | The DNA sequence where RNA polymerase binds |
| Transcription unit | Promoter + coding region + terminator |
- RNA is synthesised 5′ → 3′, like DNA
- NO primer is required — a key difference from replication
- Only one strand of a given gene is transcribed; but different genes may use different strands
- Uses ribonucleoside triphosphates; uracil replaces thymine
RNA Polymerases
| Enzyme | Product | Inhibited by |
|---|
| RNA polymerase I | RRNA (nucleolus) | — |
| RNA polymerase II | MRNA and most snRNA | α-Amanitin (death cap mushroom) |
| RNA polymerase III | TRNA and 5S rRNA | High-dose α-amanitin |
| Bacterial RNA polymerase | All bacterial RNA | Rifampicin |
- Bacterial RNA polymerase has a σ (sigma) factor that recognises the promoter and then dissociates
- Bacterial promoter has the −10 (Pribnow, TATAAT) and −35 consensus sequences; eukaryotic promoters have the TATA box at −25 and a CAAT box
Stages
Initiation — polymerase + transcription factors bind the promoter; DNA melts → elongation — RNA synthesised 5′→3′; the DNA rewinds behind → termination — rho-dependent or rho-independent (hairpin) in bacteria; poly-A signal in eukaryotes
Post-transcriptional Processing of Mrna
| Modification | Detail | Purpose |
|---|
| 5′ Capping | 7-methylguanosine joined by an unusual 5′–5′ triphosphate bond | Protects from exonuclease; needed for ribosome binding |
| 3′ Polyadenylation | Poly-A tail of 100–250 adenines, after the AAUAAA signal | Stability and export from the nucleus |
| Splicing | Introns removed, exons joined by the spliceosome (snRNP); GU at the donor, AG at the acceptor site | Produces the mature coding sequence |
| RNA editing | Base changed after transcription | Apo B-48 vs B-100 arise from the same gene this way |
- Alternative splicing lets one gene produce several proteins — a major reason 20,000 human genes yield far more proteins
- Prokaryotes do none of this: they have no introns, no cap and no poly-A tail, and transcription and translation are coupled
Comparison — Replication VS Transcription
| Feature | Replication | Transcription |
|---|
| Product | DNA | RNA |
| Template | Both strands | One strand only |
| Primer | Required | Not required |
| Enzyme | DNA polymerase | RNA polymerase |
| Proofreading | Yes | Little or none |
| Extent | Entire genome | Selected genes only |
| Ligase needed | Yes | No |
Regulation of Gene Expression
- Operon model (Jacob and Monod) in bacteria — the lac operon is inducible, the trp operon repressible
- In eukaryotes — enhancers and silencers, transcription factors, hormone response elements
- Epigenetic control — DNA methylation at CpG islands silences genes; histone acetylation activates them
- Steroid hormones act by binding intracellular receptors that become transcription factors — hence their slow onset and prolonged action
Types of RNA
| RNA | Share | Function |
|---|
| RRNA | 80% | Structural and catalytic core of the ribosome; peptidyl transferase is a ribozyme |
| TRNA | 15% | Adaptor; clover-leaf secondary structure with anticodon and CCA end |
| MRNA | 5% | Carries the coding message; the most heterogeneous and shortest-lived |
| SnRNA | Small | Splicing (spliceosome) |
| MiRNA and siRNA | Small | Gene silencing — degrade or block mRNA |
Applied Aspects
- Rifampicin inhibits bacterial RNA polymerase selectively — the basis of its use in tuberculosis and leprosy; it colours urine and secretions orange
- α-Amanitin from Amanita phalloides blocks RNA polymerase II → fatal hepatic necrosis
- β-Thalassaemia is often caused by a splice-site mutation, so the β-globin mRNA is processed abnormally
- Systemic lupus erythematosus — anti-Sm antibodies are directed against snRNP splicing proteins
- Antisense oligonucleotides and siRNA silence specific mRNAs; several are now licensed drugs