The classic example has to do with telomeres. At the end of all DNA strands are bits of repetitive sequences called telomeres that act like end caps for the individual strands. We all start out with very long telomeres because the cellular machinery that replicates DNA is imperfect and can’t replicate the entire strand—a small bit at the end gets cut off each time. If we were constantly cutting off bits of DNA that coded for important proteins, our cells couldn’t function for more than a few replication cycles. Instead, we cut off our telomeres, which don’t code for anything. The problem, of course, is that you can’t grow them back, and eventually your telomeres get so short that the cell can’t keep dividing. This seems to be a significant contributor to the process of growing old, so lot of anti-aging research centers around how to either slow telomere decline or regenerate them after they're gone.