The answer is telomeres. These are the regions at the end of the chromosomes that are composed of random repeats of nucleotide sequences. When the DNA is duplicated entirely by DNA polymerases such as during mitosis, the lengths of chromosomes reduce slightly by every duplication. This is due to the aspect of the lagging strands replication that is unable to be completed to the end of the chromosome. Therefore telomeres protect the chromosome from losing critical genes since mutations on telomeres will not affect the cell functionality.