Problem 69 - Objective Biology for NEET

Question

The Watson-Crick Structure of DNA is (a) \(1^{\circ}\) structure (b) \(2^{\circ}\) structure (c) \(3^{\circ}\) structure (d) \(4^{\circ}\) Structure

Step-by-Step Solution

Answer

The Watson-Crick Structure of DNA is a secondary (\(2^{\circ}\)) structure. So, the correct answer is (b) \(2^{\circ}\) structure.

Step 1: Understanding Watson-Crick Structure

The Watson-Crick Structure of DNA describes its double helix structure. The structure is also constituted by a sugar-phosphate backbone on the outer sides and the nitrogenous bases on the inside.

Step 2: Identifying primary, secondary, tertiary, and quaternary structure

The primary structure of a biological molecule is the exact specification of its atomic composition and the chemical bonds connecting those atoms, including stereochemistry. The secondary structure is the pattern of hydrogen bonds in a biopolymer, causing it to fold into a repeating pattern. The tertiary structure refers to the overall folding of the entire polymeric chain, while the quaternary structure is attributed to the assembly of polypeptides or subunits into a complex.

Step 3: Matching Watson-Crick Structure of DNA with its correct form

In the case of DNA, the primary structure is defined by the sequence of nucleotides. The secondary structure of DNA is its Watson-Crick double helix. There is usually no further structural hierarchy beyond the secondary structure, so tertiary and quaternary structures do not apply to DNA.

Key Concepts

Watson-Crick model

The Watson-Crick model revolutionized our understanding of DNA back in 1953. This model explains how DNA molecules are structured as a double helix. The double-helix shape resembles a twisted ladder, which perfectly encapsulates the elegant nature of DNA's structure. At the heart of this model is the pairing mechanism of nitrogenous bases. These bases include adenine, thymine, cytosine, and guanine.

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

These base pairs are held together by hydrogen bonds. The importance of the Watson-Crick model lies in explaining how genetic information can be stored and replicated within living organisms. It also highlights the complementary nature of the DNA strands, which is fundamental to many biological processes.

secondary structure

When discussing biological molecules like DNA, the term "secondary structure" describes the specific way the molecule folds due to interactions such as hydrogen bonding. In the case of DNA, the secondary structure is its iconic double helix.

• DNA's secondary structure is stabilized by hydrogen bonds between complementary nitrogenous bases.
• The structure alternates with sugar-phosphate backbones running along the outer spiral edges.

The secondary structure is crucial because it provides the necessary stability and flexibility for DNA's role in storing genetic information. Though secondary structures can vary in other biomolecules, DNA's helical form remains consistent and reliable across various environments.

double helix

The double helix is perhaps the most recognizable structure in the field of genetics. This spatial configuration of DNA is like a twisted ladder where the sides represent the sugar-phosphate backbone and the rungs represent base pairs.
The shape of the double helix allows DNA to compactly store vast amounts of genetic information while still providing mechanisms for easy access during replication and transcription. Its helical nature is not just ideal for storage but also plays a vital role in the molecule's interactions with proteins and enzymes critical for various cellular functions.
The double helix is symmetrical, with uniform spacing between base pairs due to precision in base pairing. This regular spacing is crucial for DNA replication as it ensures exact duplication of genetic information.

nucleotide sequence

A nucleotide sequence is essentially the blueprint of the genetic information contained within DNA. Each nucleotide is composed of three components:

• A nitrogenous base (adenine, cytosine, guanine, or thymine)
• A sugar molecule (deoxyribose in DNA)
• A phosphate group

The specific order of these nucleotides along DNA strands determines genetic coding for proteins, directly influencing biological functions and traits. Sequencing helps scientists and researchers pinpoint genetic variations that can lead to differences in health and development. Understanding the nucleotide sequence is important because it acts as the underlying code for life, guiding cellular processes and regulation in all living organisms.