A method for the selective irradiation of part of a genome

We developed a method for partial irradiation of cell nuclei and for highlighting the irradiated chromatin domain(s) in both interphase nuclei and metaphase chromosomes. The method involves the use of the replication program of chromosomes and consists of three major steps: I) selection of a suitable chromatin domain, II) damage induction by 125I, and III) visualization of the domain. Here, the first step of the method, applied to Chinese hamster HA-1 cells, is described. Using pulse labelling with the replication marker IUdR, it was shown that Xq does not replicate at early S-phase and that the replication timing of Xq can be highly effectively synchronized with hydroxyurea in a whole cell population. Thus, the replication timing of Xq may be used to exclude or to incorporate 125I into the Xq. Other chromatin can be selected and targeted with 125I in a similar way. Examples of possible applications of the method are given.