Replication Banding Patterns in Human Chromosomes Detected Using 5-ethynyl-2'-deoxyuridine Incorporation

A novel technique using the incorporation of 5-ethynyl-2'-deoxyuridine (EdU) into replicating DNA is described for the analysis of replicating banding patterns of human metaphase chromosomes. Human lymphocytes were synchronized with excess thymidine and treated with EdU during the late S phase of the cell cycle. The incorporated EdU was then detected in metaphase chromosomes using Alexa Fluor® 488 azides, through the 1,3-dipolar cycloaddition reaction of organic azides with the terminal acetylene group of EdU. Chromosomes with incorporated EdU showed a banding pattern similar to G-banding of normal human chromosomes. Imaging by atomic force microscopy (AFM) in liquid conditions showed that the structure of the chromosomes was well preserved even after EdU treatment. Comparison between fluorescence microscopy and AFM images of the same chromosome 1 indicated the presence of ridges and grooves in the chromatid arm, features that have been previously reported in relation to G-banding. These results suggest an intimate relationship between EdU-induced replication bands and G- or R-bands in human chromosomes. This technique is thus useful for analyzing the structure of chromosomes in relation to their banding patterns following DNA replication in the S phase.