Problem 160 - A Complete Resource Book in Chemistry for JEE Main

Question

The reason for double helical structure of DNA is operation of (a) van der waals forces (b) hydrogen bonding (c) dipole-dipole moment (d) electrostatic attractions

Step-by-Step Solution

Answer

(b) hydrogen bonding

Step 1: Identify the Forces Involved

First, understand that the DNA structure must be stabilized by interactions between the two strands. These interactions are primarily provided by forces between complementary base pairs.

Step 2: Hydrogen Bonding in Base Pairs

Recognize that hydrogen bonds play a crucial role in stabilizing the DNA double helix. Specifically, adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds.

Step 3: Consider Other Forces

While examining other forces, note that van der Waals forces and electrostatic attractions also contribute to the DNA structure's stability, but hydrogen bonds are primarily responsible for the double helix structure due to specific pairing.

Step 4: Review the Options

From the options given, identify which force is primarily responsible for creating and maintaining the double helical structure of DNA. It's evident that hydrogen bonding between complementary base pairs is the primary factor.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a fundamental force in DNA structure. The backbone of the DNA molecule is not where these bonds occur. Instead, they happen between the nitrogenous bases of adjacent strands. These bases come in pairs, ensuring a stable and consistent structure.

Two crucial base pairs involve hydrogen bonds:

• Adenine (A) pairs with Thymine (T) using two hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) using three hydrogen bonds.

The different number of bonds explains why G-C pairs are stronger than A-T pairs.

Due to this bonding, the structure remains stable, yet flexible enough to allow for processes like replication and transcription. The specificity and strength of hydrogen bonding are central to DNA's role as a genetic material.

Base Pairing

Base pairing is crucial to DNA's function and structure. The specific matching of bases, known as base pairing, is governed by precise chemical interactions.

There are four types of nitrogenous bases in DNA:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

These bases adhere to a specific pairing rule:

• A pairs with T
• G pairs with C

The reason these pairs align so specifically is due to hydrogen bonding, which recognizes complementary shapes and chemical affinities.

Base pairing's reliability ensures genetic information is accurately replicated and transmitted during cell division. It is this pairing that allows DNA to maintain genetic integrity over generations.

Double Helix

The double helix is the iconic shape of DNA, resembling a twisted ladder. This structure is a result of the molecular forces at play, particularly hydrogen bonding and base pairing.

A few key characteristics define the double helix:

• The strands twist around each other in a right-handed coil.
• The sugar-phosphate backbone forms the outside of the helix.
• The nitrogenous bases are inside, paired across the two strands.

The twisting of the strands into the double helix form is stabilized by the hydrogen bonds between paired bases, giving DNA its robust, yet flexible structure.

This configuration not only aids in the stability of the DNA molecule but also plays a vital role during replication and transcription. The helical shape effectively stores immense amounts of genetic information in a compact form, allowing cells to efficiently utilize and duplicate the genetic code.