ANALYSIS OF REPLICATION TIMING OF RIBOSOMAL-RNA GENES BY FLUORESCENCEIN-SITU HYBRIDIZATION

Fluorescence in situ hybridization has been used to study the replicat ion timing of various repeat DNA families in the short arms of human a crocentric chromosomes. In interphase nuclei, unreplicated DNA segment s show singlet hybridization signals whereas replicated loci have doub let signals, The distribution of these two patterns in unsynchronized cell cultures revealed that the rRNA gene clusters replicate earlier t han the closely juxtaposed alpha- and beta-satellite DNA sequences. Wi thin the rDNA repeat unit, replication of the intergenic spacer appear s to precede that of the transcribed rDNA.