The big idea: Each rung of the DNA ladder is one purine plus one pyrimidine. Both kinds of rung are the same length, so the helix is the same width all the way along.
The points to remember
- Purines have two rings: adenine and guanine. Pyrimidines have one ring: cytosine and thymine.
- Complementary pairing always joins one purine to one pyrimidine: A with T, and C with G.
- An A–T pair and a C–G pair have the same length.
- So the two backbones stay the same distance apart: the helix has the same 3-D structure whatever the sequence.
- Two purines would be too long to fit, two pyrimidines too short: that regular shape helps keep the helix stable.
Remember it as: Big plus small: two rings meet one.
Real example: cardboard bases: In 1953 James Watson cut the four bases out of cardboard and moved them around on his desk. He saw that an A–T pair and a G–C pair had the same shape and size, which is how the pairs fit inside one regular helix.
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How it is asked: A one-mark List on Paper 1B (which bases are purines, which are pyrimidines), or a short Explain of why the helix keeps the same shape.
IB-style questionList[1 mark]
A table shows the percentages of the four bases in DNA from several organisms. List the bases that are purines and the bases that are pyrimidines.
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See the answer broken into clear points, with guidance on how each point earns marks.
The same idea, asked as an Explain:
IB-style questionExplain[2 marks]
Explain why the DNA double helix has the same width whatever its base sequence.
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See the answer broken into clear points, with guidance on how each point earns marks.