Based on the context given below, answer the following questions:

DNA in eukaryotes is a double-helical structure with two anti-parallel strands intertwined equidistantly. The double-helical structure was deciphered by James Watson and Francis Crick on the basis of X-ray diffraction studies. The hydrophilic backbones of alternating sugar and phosphate groups are on the outside of the helix held together by phosphodiester bonds. The purine and pyrimidine bases of both the strands are stacked inside the double helix. Adenine binds with thymine with hydrogen bond and cytosine binds with guanine with hydrogen bond. The DNA is transcribed into messenger RNA, ribosomal RNA and transfer RNA. Messenger RNA is translated into protein in the cytoplasm.

Q75.In a 1300 (base-pair) DNA, if number of adenine residues are 800, what is the number of Watson–Crick hydrogen bonds between cytosine and guanine residues?

1. 500

2. 1000

3. 1500

4. 3000

   In a 1300 base-pair DNA with 800 adenine residues, the number of Watson-Crick hydrogen bonds between cytosine and guanine residues is 500.

   Step-by-Step Solution

   DNA is double-stranded, so a 1300 base-pair (bp) molecule has 1300 bp total, meaning 2600 total nucleotides (1300 on each strand). Adenine (A) pairs only with thymine (T), so 800 A residues mean 800 T residues (400 AT pairs, since each pair has one A and one T). This leaves 1300 – 400 = 900 GC pairs. Guanine (G) pairs with cytosine (C) via 3 hydrogen bonds per pair, so 900 GC pairs × 3 = 2700 total C-G hydrogen bonds. However, rechecking totals: total A + T = 1600, so G + C = 2600 – 1600 = 1000 (500 GC pairs, as 1000/2=500). Thus, 500 GC pairs × 3 bonds/pair = 1500 C-G hydrogen bonds.​

   Option Analysis

   • 500: Too low; assumes ~167 GC pairs (500/3), but actual GC pairs are 500.​

   • 1000: Matches total G+C nucleotides, not bonds (ignores 3 bonds/pair).​

   • 1500: Correct; 500 GC pairs × 3 H-bonds each.​​

   • 3000: Too high; assumes 1000 GC pairs, exceeding total bp.​

   James Watson and Francis Crick’s double-helix model revolutionized genetics, with Watson-Crick hydrogen bonds between cytosine and guanine (3 per pair) stabilizing DNA structure. In a 1300 base-pair DNA with 800 adenine residues, solving for C-G hydrogen bonds tests Chargaff’s rules and base pairing.

   DNA Base Pairing Basics

   Eukaryotic DNA features anti-parallel strands with hydrophilic sugar-phosphate backbones outside and bases stacked inside. Key pairings:​

   • Adenine (purine) – Thymine (pyrimidine): 2 H-bonds.

   • Cytosine (pyrimidine) – Guanine (purine): 3 H-bonds.

   This ensures uniform helix diameter (2 nm).​

   Solving the Problem

   For 1300 bp DNA (2600 nucleotides total):

   • 800 adenine (A) → 800 thymine (T) → 400 AT pairs (800 nucleotides).​

   • Remaining bp: 1300 – 400 = 900? Wait, correct total: A+T=1600 nucleotides → G+C=1000 nucleotides → 500 GC pairs.​

   • C-G bonds: 500 pairs × 3 bonds/pair = 1500.​​

   Common Exam Pitfalls

   Students often miscount pairs vs. nucleotides or bonds per pair, leading to 500, 1000, or 3000. Verify with totals: (A+T+G+C)/2 = bp.​

   Option	Calculation Error	Why Wrong
   500	Total G+C nucleotides	Ignores 3 bonds/pair ​
   1000	Assumes 2 bonds/GC or total GC nucleotides	Mismatches AT=2, GC=3 ​
   1500	500 GC × 3	Correct ​
   3000	1000 GC pairs imagined	Exceeds 1300 bp ​

   Exam Relevance

   Ideal for NEET, CSIR-UGC, or GATE Life Sciences on molecular biology. Practice reinforces transcription/translation context from query.​

   Keywords: Watson-Crick hydrogen bonds, cytosine guanine, 1300 base-pair DNA, 800 adenine residues, DNA double helix, base pairing rules.