Telomerase is comprised of a protein and a RNA subunit. The protein is Reverse transcriptase Enzyme called TERT which uses the RNA subunit TERC as a template for the addition of nucleotides in repeating patterns to the overhang telomere ends of chromosome on the lagging strand of DNA to allow replication to finish and the cell to continue with division. The size of the RNA subunit varies between species depending on the telomere sequence needed. For example the Tetrahymena thermophila has 159 nucleotide bases in its RNA subunit, whereas budding yeast has an RNA subunit of 1167 nucleotides.
In human somatic cells the telomerase enzyme is switched off. This means that with every generation of cell division the chromosome becomes progressively shorter as the telomeres are reduced on the lagging strand. As the lagging strand starts eating into the coding-DNA the cell is no longer able to survive and therfore is unable to survive. This allows the body to control the lenght of cell life.
In many cancer cells the telomerase enzyme is switched on, which means that cells can divide indefinitely as the DNA does not become damaged. Telomerase plays an important part in ageing, and the prevention of ageing, as the telomerase enzyme is also switched on in Germ lineand Stem cells, which allows them to divide continuously without any loss of DNA so the the cell life is longer.