Problem 145 - Objective Biology for NEET

Question

Assertion: Watson-Crick model of DNA is the secondary structure of DNA. Reason: It contains covalent as well as hydrogen bond.

Step-by-Step Solution

Answer

Both the assertion and the reason are correct, and the reason is the correct explanation for the assertion.

Step 1: Verify the Assertion

The Watson-Crick model indeed represents the secondary structure of DNA. The primary structure is the sequence of bases, while the secondary structure refers to the geometric shape of the molecule, most significantly the double-helical structure.

Step 2: Understand Covalent and Hydrogen Bonds

In this context, understand that covalent bonds are the bonds which connect the sugar and phosphate to form the 'backbone' of the DNA molecule, while the hydrogen bonds are the bonds which connect the two strands of the DNA molecule together by forming between the complementary base pairs.

Step 3: Evaluate the Reason

Given reason states that the Watson-Crick model of DNA contains covalent as well as hydrogen bonds. This statement is true. According to the Watson-Crick model of the DNA structure, the DNA backbone is composed of alternating sugars and phosphate groups, linked by covalent bonds. The two strands of DNA are held together by hydrogen bonds formed between the complementary bases.

Step 4: Conclude the Exercise

Considering the points mentioned, it can be concluded both the assertion and reason are correct and the reason is the correct explanation for the assertion

Key Concepts

Understanding the Secondary Structure of DNA

DNA's secondary structure is a fundamental concept in molecular biology, representing the geometric configuration taken by a biological macromolecule. The iconic double helix of DNA discovered by James Watson and Francis Crick, and known as the Watson-Crick model, serves as the ideal portrayal of this secondary structure.The double helix is the result of the specific way in which the bases pair and the strands twist around each other, resembling a twisted ladder. Such a structure allows DNA to store genetic information securely and supports the precise replication during cell division because it is stable and predictable. The ability to fit so much information in such a compact space is largely due to this intricate and elegant structure.

Role of Covalent Bonds in DNA

Covalent bonds are the unyielding connections between atoms that provide the structural backbone for DNA. In the context of the Watson-Crick model, these bonds manifest between the sugar of one nucleotide and the phosphate group of the next. This linkage forms a robust backbone that is crucial for the molecule's stability and integrity.

These sugar-phosphate backbones run on the outside of the double helix, and their construction through covalent bonding ensures that the DNA can withstand the cellular processes it must participate in. Without these strong covalent bonds, DNA strands would lack the necessary support to maintain their function and structure, leading to a collapse of the genetic framework.

Significance of Hydrogen Bonds in DNA

The hydrogen bonds within DNA are just as essential but carry a different role than covalent bonds. While covalent bonds create the backbone, hydrogen bonds are responsible for the pairing between specific complementary bases (adenine with thymine and guanine with cytosine) that form the rungs of the DNA's 'ladder'.

The pairing via hydrogen bonds is specific, following the base-pairing rule, which is foundational for genetic fidelity during DNA replication and transcription. These bonds are weaker than covalent bonds, which is beneficial because it allows the two strands of DNA to separate when needed, such as during replication or protein synthesis, without breaking the structural integrity of the molecule.