Nucleotide and protein distribution in BrdU-labelled polytene chromosomes revealed by ion probe mass spectrometry

Detailed chemical maps of BrdU-labelled polytene chromosomes of Drosophila melanogaster, obtained by imaging secondary ion mass spectrometry, reveal separately the distribution of DNA and proteins in the chromosomes. The thymidine-analogue BrdU within the chromosomal DNA is localized by detecting the Br- secondary ion signal, while both nucleic acid and protein content are mapped through the abundantly emitted CN- signal. This novel approach supercedes, and helps explain the origin of, the banding patterns that are observed by conventional staining techniques. The high spatial resolution and chemical and isotopic sensitivity of this technique should enhance the localization of specific genes by in situ hybridization in mitotic chromosomes.