Question

The reason for double helical structure of DNA is operation of [2003] (a) van der waals forces (b) hydrogen bonding (c) dipole-dipole moment (d) electrostatic attractions

Short answer

(b) hydrogen bonding

Step-by-step solution

Understanding the Question

The question asks for the reason behind the double helical structure of DNA, with multiple choice options given. We need to determine which type of interaction or force is primarily responsible for this structure.

Evaluating the Options

Go through each option and understand their roles in the context of DNA structure: - (a) Van der Waals forces: These weak forces contribute to DNA stability but are not the main reason for the double helix. - (b) Hydrogen bonding: Hydrogen bonds occur between specific nitrogenous bases, contributing significantly to the double helix. - (c) Dipole-dipole moment: While these interactions occur in molecules, they are not specific to the double helix formation. - (d) Electrostatic attractions: These occur between charged particles but are not primarily responsible for the double helix form.

Identifying the Correct Option

Hydrogen bonding between the complementary base pairs (adenine with thymine and guanine with cytosine) is crucial for the formation of the DNA double helix. These bonds keep the two strands of the helix together, forming the ladder-like structure that twists into a helix.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a type of weak, yet highly significant, interaction that occurs between molecules. Despite its relative weakness compared to covalent bonds, it is fundamental when it comes to the structure of DNA. In DNA, hydrogen bonds form specifically between the nitrogenous bases of the two strands. This interaction is crucial because:

• It strengthens the linkage between the two DNA strands, ensuring stability.
• It allows for the specific pairing between the complementary bases.

The hydrogen bonds work like tiny molecular velcro, securely holding the DNA strands together while still allowing them to unwind and separate during processes like replication and transcription.
Their presence is essential because they maintain just the right amount of stability and flexibility crucial for DNA's biological functions.

Base Pairing

Base pairing is a mechanism in DNA where specific nitrogenous bases pair together due to their structural compatibility, further enhanced by hydrogen bonding. In the double helix, two types of base pairs are formed:

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

This precise base pairing plays a vital role because:

• It ensures that genetic information is accurately copied during DNA replication.
• It maintains the consistent width of the DNA helix.

The specific pairing also allows DNA to carry vast amounts of genetic information in a way that can be faithfully transmitted across generations. This code guides the manufacture of proteins, which are essential for life functions.

Molecular Interactions in DNA

Molecular interactions in DNA involve not only hydrogen bonding and base pairing but also other forces. These interactions work in harmony to maintain the stability and function of the DNA molecule.

• **Hydrogen bonds**, as previously described, create connections between the bases, maintaining the helix's structural integrity.
• **Van der Waals forces** play a less direct, yet complementary role by stabilizing the stacked base pairs through subtle, cumulative attractions.
• **Electrostatic interactions**, primarily involving phosphate groups in the DNA backbone, help in maintaining the molecule's shape and interaction with histones and other DNA-binding proteins.

Understanding these interactions helps us appreciate why DNA is such a robust molecule, capable of storing genetic information securely yet accessible for biological processes.
This intricate web of interactions is what facilitates the unique properties of DNA, enabling it to function as the molecule of life.