Statement I: Purine bases present in DNA are adenine and guanine

Statement II: The base thymine is present in RNA whereas base uracil is present in DNA

Statement I is correct but Statement II is false

The correct answer is option 3. Statement I is correct, but Statement II is false.

Statement I: Purine bases present in DNA are adenine and guanine.

In DNA (deoxyribonucleic acid), the nucleotide bases are classified into two main types: purines and pyrimidines.

Purines: Purine bases have a double-ring structure. The purine bases found in DNA are:

Adenine (A): Adenine forms two hydrogen bonds with thymine (T) in DNA.

Guanine (G): Guanine forms three hydrogen bonds with cytosine (C) in DNA.

These base pairs (A-T and G-C) are essential for the stability and complementary pairing of DNA strands in the double helix structure. Adenine and guanine are examples of purine bases because of their double-ring structure, consisting of fused pyrimidine and imidazole rings.

Statement II: The base thymine is present in RNA whereas base uracil is present in DNA.

Thymine (T): Thymine is a pyrimidine base found exclusively in DNA. It pairs with adenine (A) via two hydrogen bonds in DNA replication and transcription processes. Thymine is derived from uracil through methylation, which enhances DNA stability and integrity.

Uracil (U): Uracil is a pyrimidine base found exclusively in RNA. It replaces thymine in RNA, where it pairs with adenine (A) via two hydrogen bonds. This pairing is crucial for the synthesis of RNA molecules during transcription processes.

Detailed Explanation:

Purine Bases in DNA: Adenine (A) and guanine (G) are classified as purine bases due to their double-ring structure. Adenine pairs with thymine (T) and guanine pairs with cytosine (C) in DNA through specific hydrogen bonding patterns. These base pairs are fundamental for maintaining the genetic information and stability of DNA.

Presence of Thymine and Uracil:

Thymine (T) is unique to DNA and is not found in RNA. It is synthesized from uracil during DNA replication and serves to stabilize DNA through enhanced hydrogen bonding with adenine. Uracil (U) is found exclusively in RNA. It pairs with adenine during RNA synthesis, forming base pairs that are critical for the synthesis of RNA molecules.

Conclusion:

Statement I is correct because it accurately identifies adenine and guanine as the purine bases present in DNA, which form complementary base pairs with thymine and cytosine, respectively.

Statement II is incorrect because it incorrectly states the presence of thymine in RNA and uracil in DNA. Thymine is specific to DNA, while uracil is specific to RNA.