The Hayflick Limit
In the early 60's, Dr. Leonard Hayflick carried out research at the
Wistar Institute in Philadelphia which led to the discovery of the
Hayflick found that lung tissue appeared to die out after the cells
had divided a certain number of times (roughly 50). Cell cultures
were also frozen after dividing 25 times. On revival, the cells
would continue until the 50 division limit was reached, then die.
As the cells approached the end of their division limit, the cells
would take on the appearance of old tissue. This appearance
included age pigments (lipofuscin) which is also found in aged
hearts and brain cells.
The discovery of the Hayflick Limit led to theories speculating on
the existence of a cellular "clock". These clocks could be
internally regulated for each individual cell (accounting for
variations below and above the 50 limit) or controlled systemically
by the hypothalamus of the brain.
Another possibility is that control is exerted by the DNA since DNA
is known to contain our genetic blueprints. Dr. James Fries and
Dr. Lawrence Crapo write,
"Probably, aging just happens, as the result of cumulative,
random, and inevitable errors in translation of DNA into
protein. The errors may even be a crucial part of a process
that allows variation among individuals and thus allows
Mistakes in cell division tend to accumulate from the actions of
viruses, free radicals, radiation and chemicals to affect the
healthy replication of DNA. As the system ages and the DNA becomes
more damaged, the DNA repair mechanism can no longer perform to its
Organisms and animals which have better DNA repair mechanisms tend
to live longer than those who don't.
One technique used by lower life forms is to minimize cell
replication during daylight hours to prevent radiation damage from
ionizing rays such as UV.
Cell division is encouraged during the night to allow the best
possible replication with a minimum of interfering radiation.