A defining characteristic of cancer cells is their immortality. Usually, normal cells are limited in the number of times they can divide before they stop growing. Cancer cells, however, can overcome this limitation to form tumors and bypass "mortality" by continuing to replicate.
Telomeres play an essential role in determining how many times a cell can divide. These repetitive sequences of DNA are located at the ends of chromosomes, structures that contain genetic information. In normal cells, continued rounds of replication shorten telomeres until they become so short that they eventually trigger the cell to stop replicating. In contrast, tumor cells can maintain the lengths of their telomeres by activating an enzyme called telomerase that rebuilds telomeres during each replication.
