The enzyme telomerase is able to extend telomeres by adding nucleotides to the ends of the chromosome. It is present in gametes to ensure that newly formed embryos have long telomeres.
Telomeres are reduced after each replication of DNA because of shortening on the lagging strand. Telomerase adds telomere DNA back on after replication.
In most cells, this enzyme is switched off after differentiation. This means that the cell can only replicate a limited number of times before dying.
Telomerase is sometimes turned back on in late-stage cancers. This explains how a cancer cell can divide many times without dying.
Cells called stem cells also have telomerase activated. These are cells that have not yet differentiated - they have the potential to become any type of cell within the body.