Thymidine (the thymine nucleoside) in nucleic acids: properties and pairing behavior Which combination of statements best describes thymidine in DNA and its relation to RNA?

Introduction / Context:
Thymidine is the nucleoside composed of the base thymine attached to deoxyribose. Its chemical features determine base pairing, stacking, and differences between DNA and RNA. Recognizing these attributes helps explain DNA stability and the substitution of thymine with uracil in RNA.

Given Data / Assumptions:

• Thymine bears a methyl group at the 5-position (5-methyluracil).
• RNA uses uracil instead of thymine in most biological contexts.
• Canonical base pairing is A•T with two hydrogen bonds in DNA.

Concept / Approach:
The 5-methyl group on thymine increases hydrophobic character and modulates stacking interactions, contributing to duplex stability. During transcription and in RNA genomes, uracil replaces thymine, providing comparable pairing with adenine while altering repair recognition pathways. Thymine (in DNA) and uracil (in RNA) pair with adenine via two hydrogen bonds.

Step-by-Step Solution:
Evaluate hydrophobic contribution: the 5-methyl substituent enhances base stacking; statement (a) is correct.Compare DNA vs RNA: RNA uses uracil in place of thymine; statement (b) is correct.Check pairing: A•T pairs via two hydrogen bonds; statement (c) is correct.Therefore, the aggregate choice that includes all correct statements is “All of the above.”

Verification / Alternative check:
Thermodynamic measurements show sequence-dependent contributions from methylated bases to stacking; structural analyses (X-ray, NMR) confirm two H-bonds for A•T and replacement of T by U in RNA.

Why Other Options Are Wrong:
“Forms three hydrogen bonds with guanine” is incorrect; three H-bonds are characteristic of G•C, not T with any base.

Common Pitfalls:
Confusing nucleoside vs nucleotide terminology (thymidine vs thymidylate) and mixing DNA vs RNA base identities.

Final Answer:
All of the above.