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A brominated thymidine analogue incorporated during DNA synthesis. Substitution changes buoyant density, allowing copied strands to be distinguished from unsubstituted strands.

Base composition changes both the initial buoyant density and the size of the BUdR-induced density increment. The fraction-number axis runs toward lower density. The unlabelled viral peak lies at a lower fraction number than the corresponding host peak, consistent with viral DNA having a higher GC content. BUdR replaces thymidine in newly synthesized strands, so relatively AT-rich DNA incorporates more brominated nucleoside per unit length and undergoes a larger density increase.

Consequently the more AT-rich host DNA can overtake the viral DNA in the fully substituted class: the host fully heavy peak is slightly farther toward the dense end, although host unlabelled DNA was less dense. The hybrid peaks are intermediate. A useful qualitative model is ${\rho }_{LL}={\rho }_0(g)$, ${\rho }_{LH}={\rho }_0(g)+\Delta \rho (g)/2$ and ${\rho }_{HH}={\rho }_0(g)+\Delta \rho (g)$, where $g$ is GC fraction, ${\rho }_0$ increases with $g$, and $\Delta \rho$ decreases as the thymidine fraction decreases. Unequal substitution efficiencies could contribute, but different replication mechanisms are unnecessary. These are equilibrium density positions, not molecular-length measurements.

Thymidine conventionally means the nucleoside containing thymine and deoxyribose. Its phosphorylated derivatives supply thymidine nucleotides for DNA metabolism.

The sugar identity matters: ribothymidine contains ribose instead. Distinguishing base, nucleoside, and nucleotide prevents a change in phosphate content from being hidden by an imprecise name.

Thymine is a pyrimidine nucleotide base, structurally 5-methyluracil. It is a principal base in DNA and ordinarily pairs with adenine.

Its methyl group distinguishes it from uracil, the corresponding common RNA base. Thymidine adds a deoxyribose sugar, so a base and its nucleoside are different molecular species.