Given below are groups of terms. In each group the first pair indicates the relationship between the two terms. Rewrite and complete the second pair on a similar basis. Example: Oxygen : Inspiration :: Carbon dioxide : Expiration
Adenine : Thymine :: Cytosine : __________
Topic: Base pairing
Answer
GUANINE.
The relationship is BASE : ITS COMPLEMENTARY PARTNER. By CHARGAFF’S RULE, adenine always pairs with thymine and cytosine always pairs with guanine.
THE RULE BEHIND IT: a PURINE (double-ringed: adenine, guanine) always pairs with a PYRIMIDINE (single-ringed: thymine, cytosine). This keeps the two strands an even distance apart all along the helix — two double rings would bulge and two single rings would pinch.
The numbers differ too: A–T is held by TWO hydrogen bonds, G–C by THREE. DNA rich in G and C is therefore harder to separate.
A memory aid: the two CURVED letters C and G pair together; the two ANGULAR letters A and T pair together.