Question

Thymine is (a) 5 -methyluracil (b) 4 -methyluracil (c) 3-methyluracil (d) 1-methyluracil

Short answer

Thymine is 5-methyluracil (option a).

Step-by-step solution

Understanding the Problem

We need to identify the correct structural name of thymine. Thymine is a pyrimidine base that pairs with adenine in DNA.

Thymine Structure Analysis

Thymine is known as 5-methyluracil because it arises from adding a methyl group to the 5th carbon of uracil, which is another pyrimidine base.

Evaluating Options

Review the options provided: (a) 5-methyluracil (b) 4-methyluracil (c) 3-methyluracil (d) 1-methyluracil Now, compare each option with the known structure of thymine as 5-methyluracil.

Select the Correct Answer

The correct option is 5-methyluracil, which aligns with the structural knowledge of thymine as a derivative of uracil with a methyl group at the 5th position.

Key concepts

Pyrimidine Bases

Pyrimidine bases are one of the two categories of nitrogenous bases found in nucleic acids. They are distinguished by their single ring structure, which is composed of six atoms: four carbon atoms and two nitrogen atoms. These bases are typically found in DNA and RNA and include cytosine, thymine, and uracil. Thymine and cytosine are found in DNA, whereas uracil replaces thymine in RNA. These bases are crucial for encoding genetic information and ensure the stability and integrity of the genetic sequences.

• Pyrimidine bases are smaller compared to their purine counterparts.
• They pair with purines in nucleic acids, for example, thymine pairs with adenine in DNA.
• The chemical modifications of pyrimidines, such as methylation, play important roles in gene expression and DNA stabilization.

Nucleic Acid Chemistry

Nucleic acid chemistry refers to the structure and function of nucleic acids, which are the building blocks of DNA and RNA. Each nucleic acid is composed of long chains of nucleotides, which themselves consist of a sugar, a phosphate group, and a nitrogenous base. These nucleotides join together via phosphodiester bonds, forming a sugar-phosphate backbone. The nitrogenous bases in nucleic acids—adenine, guanine, cytosine, thymine (in DNA), and uracil (in RNA)—attach to the sugar molecules. Their specific sequence encodes genetic information. Additionally, these bases engage in specific pairings, fundamental for the double-helix structure of DNA:

• Adenine (A) pairs with thymine (T) in DNA, and with uracil (U) in RNA.
• Guanine (G) pairs with cytosine (C) in both DNA and RNA.

This precise pairing is vital for the processes of replication and transcription, where genetic information is copied and transferred.

DNA Structure

DNA structure is famously represented by its double-helix formation. This was first elucidated by James Watson and Francis Crick in 1953. The double-helix is composed of two complementary strands running in opposite directions, held together by hydrogen bonds between paired nitrogenous bases. The standout feature of DNA structure is its ability to store and replicate genetic information. Key features include:

• The sugar in DNA is deoxyribose, distinct from the ribose sugar in RNA.
• The presence of four nitrogenous bases: adenine, thymine, guanine, and cytosine.
• Hydrogen bonds between adenine and thymine (two hydrogen bonds) and between guanine and cytosine (three hydrogen bonds).

The stability provided by the DNA structure is essential for maintaining genetic integrity across generations.

Uracil Derivatives

Uracil derivatives originate from modifications to the uracil base, a pyrimidine found primarily in RNA. These modifications can occur at various positions on the uracil molecule, leading to diverse biochemical roles. Thymine, the focus of our exercise, is a well-known uracil derivative, where a methyl group is added to the 5th carbon. Thymine's role in DNA is crucial:

• By replacing uracil with thymine, DNA gains increased stability, which is less prone to mutation, thanks to thymine's extra methyl group. This methylation prevents spontaneous deamination, which can convert cytosine into uracil incorrectly.
• Thymine is found only in DNA, providing a specific identifier for DNA as distinct from RNA.

Modifications like methylation are natural processes that regulate gene expression and protect DNA integrity.