Peter Hemmerich

(A) Transcription can be visualized by exposure of living cells to the nucleotide analog Fluoro-Uridine (Fl-U) for short periods of times. Fl-U is incorporated into nascent RNA-transcripts and can be detected by immunostaining of the Fl-U epitope. Depending on the imaging technique (confocal or electron microscopy), several hundreds to more than 10,000 discrete transcription sites can be detected throughout the entire nuclear volume.

(B) If Bromo-desoxy-Uridine (Br-dU) is used in an incorporation assay, nascent DNA can be visualized in cells that are in S-phase. The pattern of Br-dU incorporation is changing throughout S-phase with dynamics strikingly similar in all mammalian cell nuclei.

(C) Genotoxic stress, such as irradiation or certain drugs, can induce a variety of DNA damages that need to be repaired. Common to all lesions within the genomic DNA is an immediate phosphorylation of the histome variant H2AX. Using antibodies against this phosphorylated epitope (γ-H2AX) the sites of DNA damage can be identified. During these processes, many specific factors are recruited to theses factories in a timely organized fashion.