To use an analogy, the telomeres are the cellular equivalent of phone cards. Each cell starts with a fixed amount on the card, and each time it divides an amount is deducted. When there is no more time left on the card, the cell can't divide anymore. If a cell tries to divide with an empty card, the penalty is stiff. An empty card, or a missing telomere, will cause the chromosome to unravel, and the cell's DNA may be lost.
Most cancer cells have a high level of the enzyme telomerase.
This enzyme, which is expressed in 80 percent to 90 percent of all tumor cells, is lacking in most normal cells. Telomerase has the ability to add DNA back to the telomeres, like a vending machine that adds time back to a phone card. Because they express telomerase, cancer cells can divide indefinitely and always have some time left on their card. DNA consists of four basic building blocks. The last few G's at the end of each telomere can fold into a sort of box, called a G-quadruplex.