Problem 49 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

The nucleic acid base having two possible binding sites is: [AIIMS 2004] (a) thymine (b) cytosine (c) guanine (d) all of these

Step-by-Step Solution

Answer

The nucleic acid base with two possible binding sites is thymine.

Step 1: Understanding the Context

In nucleic acids (DNA and RNA), bases pair through hydrogen bonding. Each base can have specific sites that allow it to bind to other bases. We need to identify which base among thymine, cytosine, or guanine has exactly two possible binding sites.

Step 2: Analyzing Thymine

Thymine pairs with adenine in DNA. It forms two hydrogen bonds with adenine, meaning it offers two binding sites.

Step 3: Analyzing Cytosine

Cytosine pairs with guanine in DNA. It forms three hydrogen bonds with guanine, indicating it utilizes three binding sites, not two.

Step 4: Analyzing Guanine

Guanine pairs with cytosine in DNA. It forms three hydrogen bonds with cytosine, which also shows it has three binding sites.

Step 5: Conclusion from Analysis

From our analysis, thymine has exactly two binding sites. Cytosine and guanine both have three. Therefore, thymine is the nucleic acid base with two possible binding sites.

Key Concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in the structure and function of nucleic acids. It is a type of weak chemical bond that occurs when a hydrogen atom, covalently bonded to a more electronegative atom like nitrogen or oxygen, interacts with another electronegative atom. In the context of DNA and RNA, hydrogen bonds are essential for the stability and specificity of base pairing, ensuring that the long strands of nucleotides properly pair.

Some key aspects of hydrogen bonding in nucleic acids include:

• Hydrogen bonds are relatively weak compared to covalent bonds, but their collective strength is significant in stabilizing the double helix structure of DNA.
• They occur between complementary bases on opposing strands, helping to hold the two DNA or RNA strands together in a stable configuration.
• They are highly specific, with each base pairing having a characteristic number of hydrogen bonds; for example, adenine pairs with thymine through two hydrogen bonds.

Hydrogen bonds are dynamic and can be broken and reformed. This property is particularly important during biological processes like DNA replication and transcription.

DNA Base Pairing

DNA base pairing is a fundamental concept that describes how nucleotides pair with each other on opposite strands of the DNA double helix. This principle ensures that genetic information can be accurately replicated and transferred, as the bases always pair in a specific way.

The rules of base pairing were established through the research of scientists like Watson and Crick:

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

The exact base pairing is crucial because it allows DNA to maintain a uniform width and structure. Each pairing mechanism relies on the complementary shapes and chemical structures of the bases. This means that only specific pairs fit snugly into the DNA double helix, much like pieces of a puzzle.
Understanding DNA base pairing is vital for grasping how genetic information is stored, replicated, and expressed. It is the reason DNA can accurately pass on traits from one generation to the next.

Thymine

Thymine is one of the four nucleobases in the nucleic acid of DNA. Paired specifically with adenine, thymine plays a crucial role in stabilizing DNA structure.

Here are some highlights about thymine:

• Thymine is a pyrimidine base, characterized by a single-ring structure, which it shares with cytosine and uracil.
• It pairs with adenine through two hydrogen bonds, providing a precise mechanism for DNA's replication and stability.
• In the context of RNA, thymine is replaced by uracil. This is an essential distinction between DNA and RNA.

Thymine's role in DNA is not only structural but also fundamental in preserving the integrity of genetic information. Any mutation or alteration in the thymine base can lead to significant genetic disorders.
Through understanding thymine, we gain insight into the mechanisms of DNA stability and the processes of genetic transcription and replication.