Which is the correct base pairing found in a normal DNA molecule?
- (a)Adenine pairs with Thymine
- (b)Adenine pairs with Guanine
- (c)Adenine pairs with Cytosine
- (d)Thymine pairs with Guanine
Correct — A, Adenine pairs with Thymine. In the DNA double helix the two strands are held together by complementary base pairing: adenine (A) always pairs with thymine (T) through two hydrogen bonds, and guanine (G) always pairs with cytosine (C) through three hydrogen bonds. So the correct pairing among the options is adenine with thymine.
- (b)Adenine pairs with Guanine — Adenine and guanine are both purines (double-ring bases). Two purines are too bulky to fit across the helix, and this pairing does not occur — adenine pairs with the pyrimidine thymine.
- (c)Adenine pairs with Cytosine — Adenine pairs with thymine, not cytosine. Cytosine's partner is guanine (G-C), so an A-C pair is not the normal base pairing.
- (d)Thymine pairs with Guanine — Thymine pairs with adenine and guanine pairs with cytosine. A T-G pairing breaks the complementary rule, so it is wrong.
DNA is a double helix of two strands whose nitrogenous bases pair specifically across the middle: adenine with thymine (A-T) and guanine with cytosine (G-C). This complementary pairing, described by Watson and Crick and consistent with Chargaff's rule, lets each strand act as a template to copy the other during replication.
Use the purine-pyrimidine rule: a large purine (A or G) always pairs with a small pyrimidine (T or C), never purine with purine or pyrimidine with pyrimidine. That fixes A-T and G-C as the only valid pairs, so any option pairing A with G, A with C, or T with G must be wrong, leaving adenine-thymine.
- A-T pair is held by two hydrogen bonds; G-C by three hydrogen bonds.
- Adenine and guanine are purines; thymine and cytosine are pyrimidines.
- Chargaff's rule: in DNA the amount of A equals T and the amount of G equals C.
- In RNA, thymine is replaced by uracil, so adenine pairs with uracil.
Adenine pairs only with thymine — so the correct pairing is (a).
- Pairing two purines (A-G) or a wrong purine-pyrimidine set (A-C, T-G).
- Forgetting that in RNA adenine pairs with uracil, not thymine.
DNA questions test the base-pairing rule directly or the number of hydrogen bonds per pair — memorise A-T (2) and G-C (3).
No directly related past PYQ was found.
- practice — not a real PYQ
In a DNA molecule, guanine forms a complementary pair with
- (a)Adenine
- (b)Thymine
- (c)Cytosine
- (d)Uracil
Answer(c) Cytosine — G pairs with C through three hydrogen bonds.
- practice — not a real PYQ
In RNA, the base that replaces thymine and pairs with adenine is
- (a)Guanine
- (b)Cytosine
- (c)Uracil
- (d)Adenine
Answer(c) Uracil — RNA uses uracil in place of thymine.