3.3.2
DNA ko ek twisted ladder ki tarah socho. Do side-rails sugar–phosphate
chains hain; rungs base pairs hain jo hydrogen bonds se jude hote hain. Poori
ladder ek spiral mein twist hoti hai (ek helix ). Watson & Crick ki genius
parts invent karne mein nahi thi — balki yeh realize karne mein thi ki shape
(ek right-handed double helix with specific base pairing) explain karti hai ki
DNA information kaise store aur copy kar sakta hai.
Definition Watson–Crick model (1953)DNA ek double helix hai: do antiparallel polynucleotide strands ek
common axis ke around wound hote hain, bahar sugar–phosphate backbone hota
hai aur andar nitrogenous base pairs hote hain, jo hydrogen bonds se
jude hote hain, complementary base pairing (A–T, G–C) follow karte hue.
Core claims, pieces mein tode gaye hain jo tum ek waqt mein ek recall kar sako:
Do strands ek right-handed helix mein wound.Strands antiparallel hain: ek 5 ′ → 3 ′ 3 ′ → 5 ′
Backbone bahar = alternating deoxyribose sugar + phosphate.Bases andar , stacked, axis ke perpendicular.Base pairing : A pairs with T (2 H-bonds) , G pairs with C (3 H-bonds) .Ek purine hamesha ek pyrimidine ke saath pair karta hai → constant rung width.
Helix mein major aur minor grooves hote hain.
Intuition Backbone BAHAR kyun hota haiSugar–phosphate backbone charged aur hydrophilic hota hai (phosphates cell
pH par negative hote hain). Water-loving groups bahar jaate hain taaki watery
cell ka samna kar sakein. Bases flat, aromatic, aur hydrophobic hote hain,
isliye woh andar chhup jaate hain, coins ki tarah stack hote hain paani se
bachne ke liye. Yahi logic cell membrane ka hai — hydrophobic cheez khud ko
daba leti hai.
Intuition PURINE hamesha PYRIMIDINE ke saath kyun pair karta haiPurines (A, G) double-ring hote hain, bade. Pyrimidines (C, T) single-ring
hote hain, chhote. Agar do purines pair karte, toh rung bahut wide hoti; do
pyrimidines, bahut narrow. Bada + chhota = constant width → helix ka ek
uniform diameter hota hai (~2 nm). Constant width ek smooth, regular helix ke
liye zaroori hai.
Worked example A–T aur G–C specifically kyunHydrogen bonds ko ek donor (jaise N–H) ki zaroorat hoti hai jo ek acceptor
(jaise C=O ya N) ke saath align ho.
A–T : adenine donor/acceptor positions offer karta hai jo thymine se exactly
2 jagah match karte hain → 2 H-bonds .G–C : guanine cytosine se exactly 3 jagah match karta hai → 3 H-bonds .
Yeh step kyun? Atoms sirf in specific pairs ke liye sahi se align hote hain.
A–C ya G–T mein donors, donors ke samne aa jaate (repulsion) — chemically
unfavourable. Isliye pairing chemistry se dictate hoti hai , randomly choose
nahi ki jaati.
Har strand ki ek direction hoti hai jo uske sugar carbons se set hoti hai:
5′ phosphate end aur 3′ hydroxyl end. Bases ko face-to-face hydrogen-bond
karne ke liye, ek strand ko doosre ke relative "ulta" hona padta hai — jaise do
log handshake karte hain toh unhe opposite direction face karni padti hai.
Isliye strands 5 ′ → 3 ′ 3 ′ → 5 ′
Intuition 1953 paper ki closing line"Hamare notice se nahi guzra ki jis specific pairing ko humne postulate kiya hai
woh turant ek possible copying mechanism suggest karti hai." Kyunki har base
apna partner dictate karta hai , ek strand doosre ko rebuild karne ke liye
perfect template hai. Ladder split karo → har half apne missing rungs rebuild
karta hai → do identical helices . Yahi semiconservative replication ka
basis hai (dekho DNA Replication Mechanism ).
Common mistake "Do strands identical hote hain."Kyun sahi lagta hai: dono backbone ki tarah base ke saath dikhte hain, isliye
log assume karte hain ki woh copies hain. Fix: woh complementary hote
hain, identical nahi. Jahan ek mein A hota hai, doosre mein T hota hai. Woh
same information mirror form mein carry karte hain, jo exactly yahi allow
karta hai ki ek doosre ko rebuild kar sake.
Common mistake "G–C mein zyada bonds hain, isliye G–C padhna 'stronger information' hai."Kyun sahi lagta hai: 3 > 2 H-bonds. Fix: extra bond sirf G–C regions
ko thermally more stable banata hai (higher melting temperature), zyada
"important" nahi. Zyada G–C → strands ko alag karna mushkil, bas itna hi.
Common mistake "Backbone hydrogen bonds se juda hota hai."Kyun sahi lagta hai: DNA ki baat mein H-bonds har jagah hain. Fix: backbone
strong covalent phosphodiester bonds se juda hota hai. Hydrogen bonds sirf
do strands ko rungs ke across ek saath hold karte hain — purpose se weak, taaki
unzip ho sakein.
Recall Feynman: 12-saal ke bachche ko explain karoDNA ek twisty ladder hai. Do lambe sides sugar aur phosphate se bane hain jo
strongly glued hain. Beech ke steps letters (A, T, G, C) ke pairs hain.
A hamesha T ke saath haath milata hai, aur G hamesha C ke saath haath milata
hai — woh sirf apne sahi partner ke saath fit hote hain, jaise puzzle pieces.
Haath milana (hydrogen bonds) gentle hota hai, isliye ladder beech se split
ho sakti hai. Har half apne partners yaad rakhti hai, isliye woh ek brand-new
matching half bana sakti hai — aur ab tumhare paas do identical ladders hain!
"Pure Silver, AT the GC"
PUR ines = A aur G (PURe As Gold).A denine–T hymine = 2 bonds → "AT = 2" (A-T mein kam letters apart hain).G uanine–C ytosine = 3 bonds → "GC = 3 strong."Strands AntiParallel hain → "A nti = A rrows A part point karte hain."
Bases andar kyun hain aur backbone bahar kyun?
Ek purine ek pyrimidine ke saath kyun pair karta hai?
Base pairing Chargaff's rule [ A ] = [ T ] predict karta hai?
A–T vs G–C mein kitne H-bonds hain, aur usse kya affect hota hai?
Watson–Crick model mein DNA ki overall shape kya hai? Ek right-handed antiparallel double helix.
DNA ke do outer rails (backbone) kya banate hain? Alternating deoxyribose sugar aur phosphate groups, phosphodiester bonds se jude hue.
DNA ladder ke rungs kya banate hain? Nitrogenous bases ke pairs jo hydrogen bonds se jude hote hain.
Kaun se bases ek saath pair karte hain aur kitne H-bonds ke saath? A–T ke saath 2 hydrogen bonds; G–C ke saath 3 hydrogen bonds.
Ek purine hamesha ek pyrimidine ke saath kyun pair karta hai? Bada (double-ring) + chhota (single-ring) ek constant ~2 nm helix width deta hai.
Do strands ke liye "antiparallel" ka kya matlab hai? Ek 5′→3′ chalti hai aur doosri 3′→5′ (opposite directions).
Chargaff's rules kya hain aur model unhe kaise explain karta hai? [A]=[T] aur [G]=[C]; kyunki har A ek T ke saath aur har G ek C ke saath pair karta hai.
Sugar–phosphate backbone bahar kyun hota hai? Yeh charged/hydrophilic hota hai aur paani ka samna karta hai; hydrophobic bases andar chhup jaate hain.
Backbone ko aur do strands ko ek saath kon se bonds hold karte hain? Backbone mein covalent phosphodiester bonds; base pairs ke across weak hydrogen bonds.
B-DNA mein rise per base pair aur bp per turn kitna hota hai? ~0.34 nm per base pair; ~10 bp aur ~3.4 nm per full turn.
Model "turant" ek copying mechanism kyun suggest karta hai? Har base apna partner dictate karta hai, isliye ek strand doosre ko rebuild karne ka template hai.
Zyada G–C information ko zyada important banata hai? Nahi — yeh sirf thermal stability badhata hai (higher melting temperature).
DNA Replication Mechanism — semiconservative copying seedha base pairing se follow karta hai.Nucleotide Structure — woh monomers jo har strand banate hain.Hydrogen Bonding — woh weak force jo strands ko unzip karne deta hai.Chargaff's Rules — experimental data jise model ne explain kiya.DNA vs RNA — kyun RNA single-stranded hota hai aur uracil use karta hai.Transcription — base pairing genes padhne mein reuse hota hai.
Stacked nitrogenous bases
Complementary base pairing
Chargaff's rules A=T, G=C
