In the yeast Saccharomyces cerevisiae, origins of replication (autonomously 
replicating sequences; ARSs), centromeres, and telomeres have been isolated and 
characterized. The identification of these structures allows the construction of 
artificial chromosomes in which the architecture of eukaryotic chromosomes may 
be studied. A common feature of most, and possibly all, natural yeast 
chromosomes is that they have an ARS within 2 kilobases of their physical ends. 
To study the effects of such telomeric ARSs on chromosome maintenance, we 
introduced artificial chromosomes of approximately 15 and 60 kilobases into 
yeast cells and analyzed the requirements for telomeric ARSs and the effects of 
ARS-free chromosomal arms on the stability of these molecules. We find that 
terminal blocks of telomeric repeats are sufficient to be recognized as 
telomeres. Moreover, artificial chromosomes containing telomere-associated Y' 
sequences and telomeric ARSs were no more stable during both mitosis and meiosis 
than artificial chromosomes lacking terminal ARSs, indicating that 
yeast-specific blocks of telomeric sequences are the only cis-acting requirement 
for a functional telomere during both mitotic growth and meiosis. The results 
also show that there is no requirement for an origin of replication on each arm 
of the artificial chromosomes, indicating that a replication fork may 
efficiently move through a functional centromere region.