Longevity
How long a cell may live in a multicellular organism has a lot to do with its function and criticality.
Red blood cells have a 4-month lifespan. They are relatively long-lived service providers.
In contrast, white blood cells – warriors of the immune system – are lucky to last a month. The longest-lived white blood cells are lymphocytes, which retain the memories of past infections, to advise in future conflicts.
Turnover varies tremendously among proteins. Most are replaced several times during a cell’s lifespan. The average protein half-life in a yeast cell is 90 minutes; in mammals, 1–2 days. Only a few proteins last the life of a cell.
There is an intricate accounting system for proteins within cells. Proteins are stochastically festooned with sequential age markers, indicating their service state and degradation to date.
An exception is DNA, which owes its long life to dedicated repair mechanisms that remedy damage. Ordinary proteins lack such support.
The histones that bind DNA are extraordinarily long-lived. These proteins are essential to DNA function, as they act as the spool around which the genetic codes are wound.
