The figure shows the double strand helix structure of a segment of DNA.
Which of the following statements about the DNA segment shown in the figure is correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The segment contains 14 bases that are either guanine or cytosine.
The two strands of DNA are held together by hydrogen bonds between specific pairs of bases: adenine pairs with thymine and cytosine pairs with guanine. Each base pair therefore has one base from each strand, and each base belongs to one nucleotide (base + sugar + phosphate).
Counting the figure — the segment shows 16 base pairs: 7 G–C pairs (G≡C or C≡G) and 9 A–T pairs (A=T or T=A). Guanine = cytosine = 7; adenine = thymine = 9. Total bases = 16 × 2 = 32.
Option (4) The segment contains 14 bases that are either guanine or cytosine. — Correct. Each of the 7 G–C pairs has one guanine and one cytosine, so 7 × 2 = 14 bases are guanine or cytosine.
Why the other options are incorrect
Option (1) The segment contains 16 nucleotides. — Incorrect. 16 is the number of base pairs, not nucleotides. Each pair has two bases, one in each strand, so the segment has 16 × 2 = 32 nucleotides.
Option (2) The segment contains equal numbers of adenine and guanine bases. — Incorrect. Adenine pairs with thymine, not with guanine, so the number of adenine bases equals the number of A–T pairs (9); guanine equals the number of G–C pairs (7), and 9 ≠ 7.
Option (3) The segment contains fewer thymine bases than cytosine bases. — Incorrect. Thymine bases = 9 (one in each A–T pair) and cytosine bases = 7 (one in each G–C pair), so the segment has more thymine than cytosine, not fewer.
NCERT (2026-27 edition):
10.5.2 Structure of Nucleic Acids – “A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside. … When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety, we get a nucleotide ….”
“…James Watson and Francis Crick gave a double strand helix structure for DNA (Fig. 10.7). Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.”
Fig. 10.7: Double strand helix structure for DNA