How many hydrogen bonds are formed between Adenine and Thymine?

Appeared in: AIIMS CRE, SNO-2024

Explanation

According to the Watson-Crick model of DNA, base pairing is highly specific.
Adenine (A), a purine, always pairs with Thymine (T), a pyrimidine.
This complementary A-T pair is held together by exactly two hydrogen bonds.
This bonding arrangement is fundamental to the stability and geometry of the DNA double helix.

Option A: A single hydrogen bond would not be sufficient to form a stable base pair within the DNA helix.
Option C: Three hydrogen bonds are formed between Guanine (G) and Cytosine (C), not Adenine and Thymine.
Option D: No base pair in the standard DNA double helix is connected by four hydrogen bonds.

Visual 1: Diagram - A chemical diagram showing the molecular structures of Adenine and Thymine, with dashed lines clearly indicating the two hydrogen bonds forming between them.
Visual 2: Infographic - A comparative infographic showing the A-T pair with two hydrogen bonds alongside the G-C pair with three hydrogen bonds, highlighting the difference in bond strength.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain DNA structure and base pairing as background academic context rather than a clinical decision trigger.
The number of hydrogen bonds affects the stability of the DNA molecule. The G-C bond is stronger than the A-T bond, so DNA regions rich in G-C content are more stable.
This principle is critical in molecular biology techniques like Polymerase Chain Reaction (PCR). The temperature required to separate the DNA strands (denaturation) depends on the G-C content of the DNA sequence.
What if the question asked about the bond between Guanine and Cytosine? The answer would be 'Three', as the G-C pair is stronger and requires a higher temperature to denature, a key consideration in designing PCR primers.

How to Approach the Question

Identify the question type: This is a factual recall question from biochemistry/molecular biology.
Recall the fundamental principles of DNA structure, specifically the Watson-Crick base pairing rules.
Remember that Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
Associate the number of hydrogen bonds with each pair: A-T has two bonds, and G-C has three bonds.
Select the option that correctly states the number of bonds for the A-T pair, which is 'Two'.

Concept Tested & Keywords

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Practise the full AIIMS CRE, SNO-2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.