Immortality

====Life-extending substances====
Some{{who|date=July 2021}} scientists believe that boosting the amount or proportion of [[telomerase]] in the body, a naturally forming enzyme that helps maintain the protective caps at the ends of [[chromosome]]s, could prevent cells from dying and so may ultimately lead to extended, healthier lifespans. A team of researchers at the Spanish National Cancer Centre ([[Madrid]]) tested the hypothesis on mice. It was found that those mice which were "[[genetic engineering|genetically engineered]] to produce 10 times the normal levels of telomerase lived 50% longer than normal mice".<ref>{{cite news |url=https://www.telegraph.co.uk/health/healthnews/3489881/Scientists-take-a-step-closer-to-an-elixir-of-youth.html |archive-url=https://web.archive.org/web/20081201081233/http://www.telegraph.co.uk/health/healthnews/3489881/Scientists-take-a-step-closer-to-an-elixir-of-youth.html |url-status=dead |archive-date=1 December 2008 |work=The Daily Telegraph |location=London |title=Scientists take a step closer to an elixir of youth |first=Richard |last=Alleyne |date=20 November 2008 |access-date=5 May 2010}}</ref>

In normal circumstances, without the presence of telomerase, if a cell divides repeatedly, at some point all the progeny will reach their [[Hayflick limit]]. With the presence of telomerase, each dividing cell can replace the lost bit of [[DNA]], and any single cell can then divide unbounded. While this unbounded growth property has excited many researchers, caution is warranted in exploiting this property, as exactly this same unbounded growth is a crucial step in enabling cancerous growth. If an organism can replicate its body cells faster, then it would theoretically stop aging.

[[Embryonic stem cells]] express telomerase, which allows them to divide repeatedly and form the individual. In adults, telomerase is highly expressed in cells that need to divide regularly (e.g., in the immune system), whereas most [[Somatic (biology)|somatic]] cells express it only at very low levels in a cell-cycle dependent manner.