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 Key Components of DNA Structure

The double helical structure of DNA consists of two strands that coil around each other, forming a spiral shape. The primary interactions contributing to this structure are between the nitrogenous bases of the nucleotides.

Step 2: Understand Forces Involved in DNA

In a DNA molecule, the strands are held together by specific interactions between the nitrogenous bases. These bases pair through specific intermolecular forces.

Step 3: Analyze Each Option

- (a) Van der Waals Forces: These are weak, non-specific interactions occurring between all atoms. - (b) Hydrogen Bonding: These are stronger interactions and specifically occur between complementary nitrogenous bases in DNA to hold the strands together. - (c) Dipole-Dipole Moment: These are interactions between molecules with permanent dipole moments, but less relevant to DNA's overall double helix structure. - (d) Electrostatic Attractions: Generally involve ionic interactions, not typically the main force for DNA structure.

Step 4: Determine the Primary Force

The strongest and most specific interactions responsible for the double helix are hydrogen bonds between adenine-thymine and guanine-cytosine pairs.

Step 5: Choose the Correct Answer

Considering the role of various forces, the primary reason for the double helical structure of DNA is (b) hydrogen bonding.

Key Concepts

Hydrogen Bonding

One of the essential features of DNA's double helix structure is hydrogen bonding. These bonds are strong attractions between hydrogen atoms and electronegative atoms like nitrogen and oxygen. In DNA, hydrogen bonds form between nitrogenous bases, which are part of the nucleotides making up the DNA strands. Each base on one strand pairs specifically with another base on the opposite strand, and this pairing is stabilized by hydrogen bonds.

These specific pairings are:

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

Hydrogen bonds are crucial because they hold the two strands of the DNA helix together while still allowing them to separate during processes like DNA replication and transcription. This dual nature of stability and accessibility underlies the functionality of DNA as a genetic material.

Double Helix

The double helix is the iconic shape of DNA, resembling a twisted ladder. This structure emerges from the coiling of two complementary strands of nucleotides into a spiral. Each rivulet in this helix is composed of alternating sugar and phosphate molecules, forming the backbone of DNA, while the rungs consist of paired nitrogenous bases.

The double-helical structure is not only aesthetic but foundational to DNA's role in genetics. The twisting ensures efficient packaging of DNA within the cell nucleus by allowing long DNA molecules to fit into the compact space. Moreover, this configuration facilitates the key functions of replication and information storage. When cells prepare to divide, the strands unwind slightly, enabling enzymes to replicate the genetic code accurately. This elegant structure accounts for both stability and flexibility.

Nitrogenous Bases

Nitrogenous bases are the heart of genetic encoding in DNA. These are organic molecules that contain nitrogen and form part of the nucleotides, which are the building blocks of DNA. The sequence of these bases on the DNA strand encodes genetic information, determining hereditary traits.

The four nitrogenous bases in DNA are:

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

These bases pair specifically: Adenine with Thymine and Cytosine with Guanine. This specificity in pairing is crucial to maintaining genetic fidelity. Each base pair forms part of the double helix’s "rungs," contributing to genetic code stability and the precise duplication of DNA during cell division. Understanding the role and pairing of nitrogenous bases is key to grasping how genetic information is stored and transmitted in living organisms.