Essential Genetics 6th Edition by Daniel Hartl

Exercise 17

The first evidence for semiconservative replication of DNA in eukaryotic chromosomes made use of a synthetic analog of thymidine called bromodeoxyuridine (BUdR) in which the methyl group in thymine is replaced with an atom of bromine. When chromatids whose DNA contains BUdR are stained with certain fluorescent dyes, the chromatids with one strand labeled and one unlabeled fluoresce very brightly (light blue in the accompanying illustration), whereas those with both strands labeled fluoresce dully (dark blue). The illustration depicts the fluorescence patterns of chromosomes in mitotic metaphase after one and two rounds of DNA replication in the presence of BUdR, and the dotted lines represent the DNA strands in the DNA duplex present in each chromatid. Depict the BUdR labeling of each chromatid by (1) making the line solid if the strand is fully labeled with BUdR or (2) leaving it dashed if it is half labeled with BUdR.