Biology / Unit 6 Bio 1205 short lesson, about 20 seconds
Complementary Base Pairing
Captions available
The idea
In double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine, so a purine always faces a complementary pyrimidine across the helix.
In practice
A strand reading 5'-ATGC-3' pairs against a complementary strand reading 3'-TACG-5', matching Chargaff's measured A=T and G=C ratios.
Where people slip
Base pairing constrains which base sits opposite another — it does not fix the sequence running along either strand itself.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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