This is why Chargaff's First Rule only holds for double-stranded DNA. Single-stranded DNA or RNA need not obey it exactly, because there's no opposite strand forcing the match. (The second parity rule, by contrast, is the surprising observation that single strands still come out roughly balanced.)
Chargaff's First Rule states which two equalities?
[A]=[T] and [G]=[C].
Why does [A]=[T] hold in dsDNA?
Complementary base pairing — every A on one strand is opposite a T on the other.
Does Chargaff's First Rule apply exactly to single-stranded DNA or RNA?
No; there's no complementary strand to enforce the exact match.
What does [A]+[G] equal in dsDNA?
50% (purines = pyrimidines).
Which bases are purines?
Adenine and Guanine (double-ring).
Which bases are pyrimidines?
Cytosine, Thymine (and Uracil in RNA) — single-ring.
If A = 30%, what are G and C?
G = C = 20% each (using [G]=[C]=50%−[A]).
If [A]=x, give all four base percentages.
[A]=[T]=x and [G]=[C]=50%−x.
What is the classical statement of Chargaff's Second Rule?
Base composition (%GC) differs between species but is constant within a species.
What is the modern "second parity rule"?
Within a single DNA strand, %A ≈ %T and %G ≈ %C (approximate, not forced by pairing).
Is each base necessarily 25%?
No — only A=T and G=C; %GC varies by organism.
How did Chargaff's data help Watson & Crick?
It hinted at specific A–T and G–C pairing, supporting the double-helix model.
Recall Feynman: explain to a 12-year-old
DNA is like a zipper. Each tooth on one side fits exactly one matching tooth on the other side: the "A" tooth only locks with the "T" tooth, and the "G" tooth only locks with the "C" tooth. So if you count all the A-teeth, you'll get the same number as the T-teeth — they always come in pairs! That's all Chargaff's first rule says: in a zipped-up DNA, A and T match in count, and G and C match in count. If the zipper is open (single strand), this exact matching isn't forced.
Dekho, Chargaff ne kya kiya — usne alag-alag organisms ke DNA ko todkar bases (A, T, G, C) ki percentage count ki. Usko ek pattern mila: jitna A hai utna hi T hai, aur jitna G hai utna hi C. Yahi hai Chargaff's First Rule: [A]=[T] aur [G]=[C]. Iska asli reason baad mein pata chala — DNA ke do strands hote hain aur har A ke saamne T, har G ke saamne C aata hai (complementary base pairing). Isiliye count exactly barabar ho jaata hai.
Ek important baat: ye First Rule sirf double-stranded DNA par exactly lagta hai. Agar single strand ho ya RNA ho, to ye exact equality zaroori nahi. Isliye agar exam mein A=21%, T=29% diya ho, to samajh jaana ki ye normal dsDNA nahi hai. (Modern "second parity rule" alag cheez hai — woh kehta hai ek single strand mein bhi %A ≈ %T aur %G ≈ %C hota hai, lekin sirf approximately, pairing ki wajah se nahi.)
Numericals solve karne ka trick bahut simple hai: chaaron base milkar 100% hote hain, aur [A]+[G]=50% hamesha. To agar A diya hai (maano x), to T=A=x, aur seedha formula [G]=[C]=50%−x — bina koi extra info ke. Jaise A=30 ho to T=30, aur G=C=50−30=20 each. Bas itna hi!
Doosra rule (classical wala) kehta hai ki har species ka %GC alag hota hai — ye ek tarah ka DNA fingerprint hai. Isliye ye galat hai sochna ki har base 25% hota hai. 25% tabhi hoga jab %GC = 50% ho, jo aksar nahi hota. Watson aur Crick ne isi Chargaff data ko use karke double helix model banaya — isiliye ye topic bahut important hai!
