A = T is about two strands, not one
- The idea
- Predict
- See it
- Why
- Check
The idea
“Any DNA strand has equal A and T, and equal G and C, since they pair.”
You've heard "A pairs with T" so often that it sounds true of any strand. The A and its T sit on different strands.
The short version
The mistake: Any DNA strand has equal A and T, and equal G and C, since they pair.
What's actually true: A = T and G = C hold across two paired strands, where every A faces a T. One strand can have any mix.
- An A on one strand pairs with a T on the other, and G pairs with C. Hydrogen bonds hold each pair.
- So a double helix has equal A and T, and equal G and C. That's Chargaff's rule.
- Along one strand, bases can come in any order. Single strands like mRNA usually have unequal amounts of the bases that would pair.
On the exam: A = T and G = C hold across two paired strands. One strand can have any mix.