WHY hume direction milta hai? Kyunki backbone ko phosphodiester bonds jodti hain jo hamesha ek nucleotide ke 5′-phosphate ko agley ke 3′-OH se link karti hain. Ye bond directional hai: phosphate end ≠ hydroxyl end. To ek strand mein hota hai:
ek 5′ end = free phosphate (upar kuch attached nahi)
ek 3′ end = free –OH (neeche kuch attached nahi)
WHY synthesis 5′→3′ mein locked hai: DNA polymerase ek incoming nucleotide pakadta hai jo 5′-triphosphate ke roop mein aata hai. Bond banane ki energy is incoming nucleotide se do phosphates kaatne se aati hai. Chain ka existing 3′-OH incoming 5′-phosphate par attack karta hai. Isliye naya nucleotide hamesha 3′ end par add hota hai → growth sirf 5′→3′ hoti hai. Kabhi ulti nahi.
5' A T G C C 3' ← top strand
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3' T A C G G 5' ← bottom strand
WHY unhe antiparallel hona chahiye? Base pairing ki geometry ki wajah se. Stable hydrogen bonds banane ke liye, A ka T se aur G ka C se sahi atom-to-atom alignment ke saath face karna zaroori hai. Dono sugar-phosphate backbones bases ko us geometry mein tab hi hold kar sakti hain jab strands opposite directions mein point karein — jaise do interlocking zippers, ek doosre ke muqable mein ulta. Parallel strands bases ko alignment se bahar twist kar deti aur H-bonds ban hi nahi sakti.
Socho do trains parallel tracks par opposite directions mein khadi hain. Har train mein ek engine (5′ end) aur ek caboose (3′ end) hai. DNA mein, dono strands ye opposite-facing trains hain jo apne middle seats (bases A–T, G–C) se aapas mein chipki hain. "Builder" robot sirf caboose end par naye train cars add kar sakta hai, engine par kabhi nahi. Kyunki dono trains opposite taraf face karti hain, robot ko ek train smoothly extend karna aasaan lagta hai, lekin doosri ko wo chote-chote pieces mein patch karta hai!
