Test yourself on Protein Synthesis — Transcription and Translation with exam style questions, marked instantly.
Flashcards that come back just before you would forget them.
A revision plan that remembers what you know and shows what to do next.
See which topics you have nailed and which need another look.
Quiz yourself on Protein Synthesis — Transcription and TranslationGet your flashcardsBuild my revision planTrack my progress🎓 Higher Tier Extension: Semi-Conservative DNA Replication
This section extends beyond protein synthesis. Semi-conservative replication appeared in the 2022 Higher paper (8 marks) and requires a separate mechanism explanation.
Before a cell can undergo mitosis (or before a gene can be transcribed), the DNA must be copied so each daughter cell or each transcription event has access to the full genome. This copying process is called DNA replication, and it is described as semi-conservative because each new DNA molecule retains one original strand and one newly synthesised strand.
How semi-conservative replication works:
- Helicase (an enzyme) unwinds the DNA double helix and breaks the hydrogen bonds between complementary base pairs, separating the two strands.
- Each original strand acts as a template — free DNA nucleotides in the nucleus pair with complementary bases on the template strand by hydrogen bonding:
- A pairs with T; T pairs with A; G pairs with C; C pairs with G
- Note: DNA replication uses thymine — only RNA uses uracil
- DNA polymerase joins the new nucleotides together by covalent bonds, building the new complementary strand.
- The result is two identical DNA double helices, each consisting of one original (conserved) strand and one newly synthesised strand — hence semi-conservative.
Why "semi-conservative"?
Three possible models of replication were proposed historically:
- Conservative: original double helix kept intact; entirely new copy made
- Semi-conservative: each new molecule = one old strand + one new strand ✓ (correct model)
- Dispersive: old and new DNA scattered randomly throughout both new molecules
The Meselson-Statt experiment (1958), using heavy and light nitrogen isotopes, proved the semi-conservative model. OCR A does not require you to recall the experiment details — but you must be able to explain why the model is called semi-conservative.
Comparing DNA replication with transcription:
| Feature | DNA Replication | Transcription |
|---|---|---|
| Location | Nucleus | Nucleus |
| Template | Both DNA strands (each acts as template) | Template strand of DNA only |
| Product | Two identical DNA double helices | Single-stranded mRNA |
| Key enzyme | Helicase (unwinds) + DNA polymerase (joins) | RNA polymerase (unwinds and joins) |
| Bases used | A, T, G, C (DNA nucleotides) | A, U, G, C (RNA nucleotides) |
| When it occurs | S phase of interphase (before cell division) | Whenever a protein is needed |
Make this stick
Signing in gives access to all PrepWise features.