As I seem to recall from high school biology, the human aging process is the expression of a function related to cell growth. During youth the number of new cells exceeds the number of dying cells by the millions, per day. This implies physical growth. As puberty passes and our bodies enter adulthood the number of new cells produced by our bodies effectively equals that of the dying cells. While there is a constant overhaul of our cells (skin cells and their frequent "shedding" serves as an appropriate example) little form of the body is gained or lost. Once old age sets in, we find that our body is simply losing more cells than we can replace, and the number of dying cells exceeds the number of new ones. This leads directly to the physical deterioration of an individual and eventually to their death. And as we all know, death is bad, and living forever is cool.

Cytokinins are a growth hormone found primarily in plant life which promote cytokinesis (cytoplasmic division). Geneticists have found it possible to splice different genes into a complete genome (although few examples with humans are viable) through the use of restrictive enzymes and the such. It is also entirely plausible to create a gene with a built in inhibition feedback, so that the enzyme or hormone released is never on over-abundancy.

Here's the interesting part: If one were to splice such cytokinins as zeatin or kinetin into the human genome with aging as the activation switch that person would not age. Prerequisite technology would have to include a type of automated cell auditor to determine just when more cells were dying than being created, but if such a tool could be created and linked to the feedback of the hormone, cells would be forced to divide until the rates of death and creation were equal, at which point the hormone would be shut off.

The most practical applications which leap to mind include battling Alzheimer's Disease, promoting the quality of life as well as prolonging the quantity (most life extending medicines do not promote the quality of remaining years), and serving as a more controlled version of steroids for people recovering from surgery of major illness.

Sources
Advanced Placement Biology
Biology, Third Edition by Campbell

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