Standard canonical Watson-Crick base pairing ($A=T$ with 2 hydrogen bonds, $G\equiv C$ with 3 hydrogen bonds) forms the structural foundation of the canonical double-helical structure in duplex DNA molecules, such as eukaryotic genomic DNA.

Transfer RNA (tRNA) contains the highest proportion of post-transcriptionally enzymatically modified bases (up to 10–15% of all residues), including pseudouridine, dihydrouridine, inosine, and ribothymidine, crucial for stability and codon recognition.

The melting temperature ($T_m$) of DNA is influenced by:
GC content (higher GC increases $T_m$ due to 3 hydrogen bonds and stacking).
AT content (higher AT lowers $T_m$).
Divalent cation ($\text{Mg}^{2+}$) concentration (shields negative phosphate repulsion, increasing stability and $T_m$).
pH (extreme acidic or basic pH denatures duplex by ionizing base pairing groups, altering $T_m$).

Hyperchromicity of nucleic acids upon thermal denaturation:

• Assertion (A) is true: Thermal denaturation (strand unwinding/melting) of double-stranded DNA produces an increase in absorbance at $260\text{ nm}$ by approximately $30\text{--}40\%$, termed the hyperchromic effect.
• Reason (R) is true: In native dsDNA, aromatic purine and pyrimidine ring $\pi$-electron systems are closely stacked parallel to each other inside the double helix, which quenches their UV dipole absorption. Upon denaturation, the complementary strands separate and the stacked bases unstack and become fully solvent-exposed, unshielding the transition dipoles.
• Reason (R) provides the direct physical mechanism and explanation for Assertion (A).