Complementary base pairing at Higher Level: The same statement as SL. At HL a seven-mark answer on DNA's structure and function needs this idea in it: pairing makes each strand a template for replication and for transcription.
Practise this as you read
- Explain what pairing allows: accurate copying and gene expression.
- Use the word template in every answer.
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The big idea: Because each base has only one partner, each DNA strand holds the information for the other. That is how DNA is copied exactly.
The points to remember
- Each base pairs with one partner only: A with T, G with C.
- The pairs are held by hydrogen bonds between the bases.
- To copy DNA, the hydrogen bonds break and the two strands separate.
- Each old strand is a template: free nucleotides pair with its bases, A with T, G with C.
- The result is two DNA molecules with the same base sequence: the information is copied accurately.
Remember it as: One strand tells you the other.
Real example: PCR in a COVID-19 test: A PCR test copies a short piece of the virus's genetic material millions of times in a few hours. Every copy is made by free nucleotides pairing with a template strand, A with T and G with C.
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The big idea: The same pairing rules let the cell read DNA: a gene is copied into RNA, and the RNA is used to make a protein.
The points to remember
- To use a gene, one DNA strand is a template for making RNA (transcription).
- RNA nucleotides pair with the DNA bases: A with U, T with A, G with C, C with G.
- So the RNA carries an exact copy of the gene's information.
- In translation, each tRNA pairs with three bases of the mRNA in the same complementary way.
- Pairing one base with its partner also keeps the two DNA strands an even distance apart, a stable structure.
U, not T, in RNA: When RNA is made, adenine on the DNA pairs with uracil, not thymine.
How it is asked: A three-mark Explain on why complementary base pairing matters, and a Paper 1A item on what makes copying accurate.
IB-style questionExplain[3 marks]
Explain the significance of complementary base pairing in DNA.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.