Loss of compaction is specific to satellite DNA and is not caused by increased nuclear size. (A and B) DNA FISH to Chr. 17 α-sat (green) shows compact satellite signals in a cycling cell (A) and distended satellites in a senescent cell (B) as determined by the presence of SAHF (DAPI). The size of the corresponding signal is shown above the brackets. (C) The mean length of Chr. 17 α-sat in senescent cells is six times the mean length in cycling cells. (D) The shortest Chr. 17 α-sat measured in senescent cells is longer than the largest satellite measured in cycling cells. (E–H) Differences in satellite length cannot be explained by the twofold increase in nuclear diameter (E). The mean size of the Chr. 21 BAC signal (red) did not change (F) in cycling (G) or senescent (H) cells. (I–L) Telomere signal size (red) in cycling (I and J) or SMURF2-induced senescent (K and L) LF1 cells did not change. Senescence is judged by distension of the α-sat (green) and presence of SAHF (DAPI; error bars represent standard error).