The DNA copying enzymes have a hard time working to the end of a
chromosome. For circular chromosomes this is not a problem,
since there is not a sharp 'end'. However, for a linear
chromosome, without extra mechanisms in place, a bit of DNA is
lost off the end of the chromosome after each replication.
Because of this, eukaryotes have a telomere to cap off their
chromosomes. In most cells of a mutli-cellular organism, this
telomere is slowly worn away after each reproduction leading to
apoptosis. Cells that need to reproduce indefinitely such as
germ and stem cells have to invest in extra mechanisms to
replenish the telomere. For multi-cellular eukaryotes I can see
how this might be usefull (for instance as a cancer counter
mechanism). However, multi-cellular organisms evolved from
single-cell eukaryotes. I cannot see a reason for wanting
apoptosis in a single-cell organism. However, single cell
eukaryotes (say yeast) still have linear chromosomes with
telomere caps. What advantage did linear chromosomes provi