Orientation of separated DNA strands of coliphage 186 relative to its genetic map

The 3′-terminus of the $\text{l}$ strand of phage 186 double-stranded DNA was radioactively labeled by the reaction catalysed by AMV reverse transcriptase, under conditions where the $\text{r}$ strand remained unlabeled. Upon strand separation in a poly(U, G)-CsCl equilibrium density gradient, radioactivity was found to be associated with the peak of lower buoyant density (L), thereby specifying the $\text{l}$ strand. The peak with the higher buoyant density (H) contains the $\text{r}$ strand.