@umbra21 - They take advantage of that function in cancer cells when they grow human cells for laboratory testing.
Most of the cells used in lab testing were originally samples from cancer patients, where it was discovered that the sample just continues to grow without the DNA replication process eventually breaking down, like it does with normal cells.
I know there's one particular strain of cells that's been going since the 1960's. The woman who was the original donor of the cells is long gone, but because her DNA was faulty her cells continue to live on today.
It's a bit creepy on the one hand, but on the other hand, you're right in that it could eventually lead to humans being able to live for a very long time.