Q.By which of the following can the arrangement of nucleotides in DNA be seen?
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Imagine you have a zipper. Each tooth on one side fits perfectly only with a specific tooth on the other side — a left tooth always clicks into a right tooth, and they lock together. If you tried to force a left tooth into another left tooth, the zipper would jam. That simple "lock-and-key" fit is the everyday intuition behind complementary base pairing.
In biology, the "zipper" is the DNA molecule, which is made of two long strands twisted together. Each strand is a chain of smaller units called nucleotides. Every nucleotide contains one of four chemical "letters" — Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The two strands are held together by weak bonds between these letters, but they don't pair randomly. The rule is strict and unchanging:
- A (Adenine) always pairs with T (Thymine)
- G (Guanine) always pairs with C (Cytosine)
This is the complementary base pairing rule. It means that if you know the sequence of letters on one strand, you can instantly write the sequence on the other strand. For example, if one strand reads A–T–G–C, the opposite strand must read T–A–C–G.
The NCERT Class 12 Biology textbook (Chapter 6, "Molecular Basis of Inheritance") states this rule exactly as: "Adenine pairs with Thymine (A = T) and Guanine pairs with Cytosine (G ≡ C)." The double lines (=) and triple lines (≡) indicate the number of hydrogen bonds — A–T has two bonds, G–C has three bonds. This difference in bond strength matters for how DNA unwinds, but the pairing rule itself is what you need to remember.
Why does this matter? Three big reasons:
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Replication: When a cell divides, the two strands of DNA separate. Each strand acts as a template. Using the complementary rule, the cell builds a new partner strand for each old strand. The result is two identical DNA molecules — one for each new cell. Without this rule, copying would be random and life couldn't pass on genetic information accurately.
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Transcription: To make proteins, a cell first makes a temporary copy of a gene in the form of mRNA. The mRNA is built using the complementary rule, but with one change: wherever there is an A on the DNA, the mRNA puts a U (Uracil) instead of T. So the pairing becomes A–U, T–A, G–C, C–G. This ensures the genetic message is faithfully transcribed. …
DNA's nucleotide arrangement is deduced from X-ray crystallography, so the answer is (B).
The precise arrangement of atoms and the double-helical structure of DNA cannot be seen even with an electron microscope. Instead, it is worked out by X-ray crystallography (X-ray diffraction). The famous X-ray diffraction photographs of DNA obtained by Rosalind Franklin and Maurice Wilkins provided the key data (helical shape, spacing) that Watson and Crick used to build their double-helix model.
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- CBSE 2025Set F1 markMCQQ.Which of the following is a correct pair?(a) G ≡ T(b) C = G(c) G ≡ C(d) A ≡ T
›Reveal solutionSolution
Correct base pairing: G ≡ C (three H-bonds) and A = T (two H-bonds).
In double-stranded DNA the bases pair by complementary hydrogen bonding (Chargaff/Watson-Crick rules). Guanine pairs specifically with Cytosine forming THREE hydrogen bonds (shown as G ≡ C), and Adenine pairs with Thymine forming TWO hydrogen bonds (A = T). Checking the …
- CBSE 2025Set ANNUAL1 markMCQQ.In DNA Double helix Adenine base joins with which base -(a) Thymine(b) Guanine(c) Cytosine(d) Uracil
›Reveal solutionSolution
DNA follows Chargaff/Watson-Crick complementary base pairing: A-T (2 H-bonds) and G-C (3 H-bonds).
In the DNA double helix, the purine Adenine always pairs with the pyrimidine Thymine through two hydrogen bonds, while Guanine pairs with Cytosine through three hydrogen bonds. This strict complementarity keeps the diameter of the double helix uniform and is t …
- CBSE 2025Set ANNUAL1 markMCQQ.The distance between the two strands of a DNA molecule ............ from one end to another.(a) increases(b) remains the same(c) decreases(d) none of these
›Reveal solutionSolution
DNA is a uniform double helix — the inter-strand distance never changes along its length.
According to the Watson–Crick model, DNA is a right-handed double helix made of two antiparallel polynucleotide chains coiled around a common axis. The two strands are held together by hydrogen bonds between complementary bases (A=T, G≡C), and because every base pair is a purine paired with a pyrimidine (which keeps the width of each 'rung' of the ladder essentially identical, about 20 Å), the …
- CBSE 2025Set BOTANY1 markMCQQ.Fill in the blank selecting the appropriate one: In a DNA double helix, two polydeoxyribonucleotide molecules are held together by ____ bonds.(a) phosphodiester(b) covalent(c) ionic(d) hydrogen
›Reveal solutionSolution
The two strands of DNA are joined by hydrogen bonds between complementary base pairs, not by covalent, ionic or phosphodiester linkages.
Within a single strand, adjacent nucleotides are joined by strong covalent phosphodiester bonds, forming the sugar-phosphate backbone. The two separate strands, however, are held together across the helix by weak hydrogen bonds between complementary nitrogen bases: adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine thr …
- CBSE 2024Set ANNUAL1 markMCQQ.Which nitrogenous base pairs with guanine nitrogenous base in DNA(a) Adenine(b) Cytosine(c) Thymine(d) Uracil
›Reveal solutionSolution
In DNA, the purine guanine always base-pairs with the pyrimidine cytosine (three H-bonds); adenine pairs with thymine (two H-bonds).
DNA's two strands are held together by hydrogen bonding between specific, complementary nitrogenous base pairs (Watson-Crick base-pairing rules): a purine always pairs with a pyrimidine of a complementary shape. Adenine (A) pairs with Thymine (T) throug …
- CBSE 2024Set ANNUAL1 markMCQQ.By how many hydrogen bonds guanine is bonded with cytosine ?(a) 1(b) 2(c) 3(d) 4
›Reveal solutionSolution
G≡C pairing has 3 hydrogen bonds; A=T pairing has 2.
In the double-stranded DNA helix, the purine guanine (G) always pairs with the pyrimidine cytosine (C) through three hydrogen bonds, making the G-C pair relatively more stable than the A-T pair (which is held together by only two hydrogen bonds). This complementary base pairing (Char …
- CBSE 2024Set ANNUAL1 markQ.If the sequence of one strand of DNA is 5'ATGCATGCATGC-3', the complementary strand in 5'-3' direction will be _____.
›Reveal solutionSolution
Using the base-pairing rule (A↔T, G↔C) and the antiparallel nature of DNA, the complementary strand read 5'→3' is GCATGCATGCAT.
Given strand: 5'-A T G C A T G C A T G C-3'
Step 1 — Pair each base (A with T, G with C):
T A C G T A C G T A C G (this is the complementary sequence, but it is aligned in the 3'→5' direction relative to the new strand, because DNA strands are antiparallel).
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- CBSE 2022Set ANNUAL1 markMCQQ.By which of the following can the arrangement of nucleotides in DNA be seen?(a) electron microscope(b) X-ray crystallography(c) light microscope(d) ultracentrifuge
›Reveal solutionSolution
DNA's nucleotide arrangement is deduced from X-ray crystallography, so the answer is (B).
The precise arrangement of atoms and the double-helical structure of DNA cannot be seen even with an electron microscope. Instead, it is worked out by X-ray crystallography (X-ray diffraction). The famous X-ray diffraction photographs of DNA obtained by Rosalind Franklin and Maurice Wilkins provided the key data (helical shape, spacing) that Watson and Crick used to build their double-helix model.
…
- CBSE 2022Set ANNUAL1 markMCQQ.When a dissolved fragment of DNA is kept at 82 degrees to 90 degrees C, then(a) it has no effect on it(b) it becomes uncoiled (denatured)(c) it splits into a million fragments(d) it changes into RNA
›Reveal solutionSolution
Heating DNA to ~82–90°C denatures (melts) it into single strands, so the answer is (B).
The two strands of the DNA double helix are held together by hydrogen bonds between complementary base pairs (A=T two bonds, G≡C three bonds). When DNA is heated to its melting temperature (around 82–90°C, depending on GC content), these hydrogen bonds break and the double helix unwinds and separates into two single strands. This is called denaturation (DNA melting). On slow cooling …
- CBSE 2021Set ANNUAL1 markMCQQ.What is the distance between two consecutive base pairs?(a) 0.32 nm(b) 0.36 nm(c) 0.34 nm(d) 0.38 nm
›Reveal solutionSolution
Consecutive base pairs in B-DNA are 0.34 nm apart; one full turn (10 bp) spans 3.4 nm.
According to the Watson and Crick double helix model of DNA (1953), the two polynucleotide strands are antiparallel and coiled in a right-handed helix. The distance between two adjacent (consecutive) base pairs along the helix axis is 0.34 nm, and one complete turn of the helix contains 10 base pairs, giving a helical pitch of 3. …
- CBSE 2019Set ANNUAL1 markMCQQ.Which of the following is a wrong pair?(a) G ≡ C(b) T = A(c) A = U(d) T = U
›Reveal solutionSolution
T = U is the incorrect base pair.
Complementary base pairing follows Chargaff's rule: G pairs with C by three hydrogen bonds (G ≡ C), A pairs with T in DNA by two bonds (A = T, so T = A is correct), and A pairs with U in RNA (A = U is correct). Thymine and …
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