We don't memorise "A pairs with T" — we derive it from two physical constraints.
Conclusion (derived, not memorised): Combine geometry (purine+pyrimidine) AND chemistry (matching H-bond pattern) → only A=T and G≡C survive both filters.
DNA is like a zipper. Each tooth has a special shape. A's tooth only locks into T's tooth, and G's tooth only locks into C's tooth — they're like matching socks. A big tooth always grabs a small tooth so the zipper stays the same width. Because the shapes only match one way, when you unzip the zipper you can rebuild the missing side perfectly — that's how your body copies its instruction book without making mistakes!
Dekho, DNA ek twisted ladder (seedhi) jaisa hota hai, aur har rung (danda) do halves se banta hai jo aapas mein fit hote hain. Lekin koi bhi base kisi se nahi judta — sirf complementary shapes match karti hain. Adenine (A) sirf Thymine (T) ke saath, aur Guanine (G) sirf Cytosine (C) ke saath. Isko hi complementary base pairing kehte hain.
Ye rule yun hi nahi banaya gaya — iske peeche do reasons hain. Pehla geometry: purines (A, G) bade double-ring hote hain, pyrimidines (C, T, U) chhote single-ring. Helix ki width fixed hai, isliye hamesha ek bada + ek chhota join hota hai, warna ladder ya to bulge karegi ya pinch. Dusra chemistry: hydrogen bonds ke donor aur acceptor edges sirf A-T (2 bonds) aur G-C (3 bonds) mein hi properly line-up karte hain. Dono filters lagao to sirf yahi do pairs bachte hain.
Iska faayda? Jab DNA copy hoti hai, dono strands unzip ho jaati hain, aur har purani strand template ban jaati hai. Complementary rule se naye bases automatically apni jagah dock kar lete hain — isliye copy bilkul accurate banti hai. Yahi rule transcription (DNA se RNA, jahan A ke saath U aata hai) aur translation mein bhi kaam karta hai. Yaad rakho: G-C mein 3 bonds hote hain, isliye GC-rich DNA ko todne ke liye zyada heat chahiye. Aur Chargaff ka rule [A]=[T], [G]=[C] sirf double-stranded DNA pe lagta hai, RNA pe nahi.
Test yourself — Biomolecules — Proteins & Nucleic Acids
