While alien genomes may be constructed by completely different molecules (for example, silicon based life forms are a possibility rather than carbon) it is logical to assume that all life forms would have some sort of protective mechanism to prevent the degradation of DNA during replication. Essentially, to prevent mutations and rapid aging, DNA, alien or otherwise, would have some types of telomeres. They might either be long chains of repeated or irrelevant code or molecules that would not be easily corrupted.
