Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Information flows DNA → RNA → protein. Transcription makes the RNA; translation (Topic 6.4) makes the protein.
- What an RNA does depends on its base sequence and on the shape it folds into by pairing with itself.
- RNA polymerase reads one strand, the template strand, and builds a complementary RNA. No primer is needed.
- In eukaryotes the first transcript (pre-mRNA) is processed in the nucleus before export. Prokaryotic mRNA is generally used without this processing.
Key relationships
| RNA | Job |
|---|---|
| mRNA | carries a gene’s information from DNA in the nucleus to a ribosome in the cytoplasm |
| tRNA | carries a specific amino acid; its anticodon pairs with an mRNA codon |
| rRNA | structural and working part of the ribosome, with proteins |
| Transcription rule | Detail |
|---|---|
| Template read | 3′ → 5′ |
| RNA built | 5′ → 3′ (nucleotides added to the 3′ end) |
| Pairing | template A → U, T → A, G → C, C → G |
| RNA sequence | same as the coding strand, with U for T |
| Processing step | Where on the mRNA | Effect |
|---|---|---|
| Cap (modified G, “GTP cap”) | 5′ end | helps the ribosome recognise the mRNA |
| Poly-A tail | 3′ end | makes the mRNA more stable |
| Splicing | introns removed, exons joined | gives the mature mRNA; alternative splicing gives several versions |
Assumptions behind the numbers
- Lengths of mature mRNA are sums of the exons kept; cap and tail are left out unless the question includes them.
- Data on mRNA stability compare versions that differ in one feature only (for example, tail or no tail).
Mistakes to avoid
- Writing the mRNA as identical to the template strand.
- Forgetting to reverse a template written 5′ → 3′, so the ends are labelled the wrong way.
- Putting T in an RNA sequence.
- Saying introns are removed from the DNA.
- Swapping the jobs of the cap (recognition) and the tail (stability).
- Saying RNA polymerase needs a primer.
Quick self-check
- Template 3′-TACGTT-5′. mRNA? (5′-AUGCAA-3′)
- Which strand does the mRNA resemble? (The coding strand, with U instead of T.)
- A gene has three exons of 200, 150 and 90 nt. Mature mRNA length if all are kept, and if the middle exon is skipped? (440 nt; 290 nt)
- An mRNA lacks its poly-A tail. Prediction? (It is broken down sooner, so less protein is made from it.)
Next: practice questions.