Replication timing of DNA sequences associated with human centromeres and telomeres

The timing of replication of centromere-associated human alpha satellite DNA from chromosomes X, 17, and 7 as well as of human telomeric sequences was determined by using density-labeling methods and fluorescence-activated cell sorting. Alpha satellite sequences replicated late in S phase; however, the alpha satellite sequences of the three chromosomes studied replicated at slightly different times. Human telomeres were found to replicate throughout most of S phase. These results are consistent with a model in which multiple initiations of replication occur at a characteristic time within the alpha satellite repeats of a particular chromosome, while the replication timing of telomeric sequences is determined by either telomeric origins that can initiate at different times during S phase or by replication origins within the flanking chromosomal DNA sequences.