sentences:  Thus, ATR-Chk1 activity further prevents potential replication problems at the level of single replication origins by inhibiting initiation of replication throughout the genome, until the signaling cascade maintaining cell-cycle arrest is turned off.Eukaryotic DNA must be tightly compacted in order to fit within the confined space of the nucleus.Chromosomes are packaged by wrapping 147 nucleotides around an octamer of histone proteins, forming a nucleosome.