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Birth; Growth; Aging; Death
Various body-functions gradually begin to deteriorate and senescence (aging) sets in and ultimately results in death.
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The end of the life is by a definite process of senescence, which should be distinguished from disease (even though some of its manifestations lead to disease). It has been recently shown that cells from older men are capable of fewer divisions than those from younger ones. This proves that senescence itself is a programmed terminus and death is a part of the program of life. Actually, death occurs at each moment throughout life-thousands of blood cells and cells of the intestine die every second, but they are replaced and repaired by programmed mechanisms that allow the attainment of the predetermined age. There are enzymatic mechanisms by which even DNA can be repaired. If a piece of one of the double chains of DNA dysfunctions, it is removed and a new stretch of DNA is synthesized. Specific enzymes the ligases · then join the new pieces into the old chain and more and more such repairs have to be made in older organisms. The problem is that repair mechanisms themselves need repairing and the regress of what repairs the repairer and the repairer of the repairer cannot be indefinitely avoided. Thus, there is a limit on the possible length of survival of any individual homeostatic device. This fact is, sometimes, expressed in a different way in terms of cell-division. There is an upper maximum limit to the number of divisions of a cell called 'Hayflick limit' after which they cease dividing and die. In a life time of 100 years, the number of divisions is about 50. Thus senescence or aging is a complex process with many contributing factors not necessarily exclusive. "Hayflick limit" of cell division agrees with the concept of an inherently programmed finite life-span of about 110 or 120 years. This does not mean that there is no possibility of improving repair processes. Though the actual life-span could never be exceeded, and the idea of much longer life is not altogether pleasing, particularly to those of us who are already very old, knowledge of some of the factors, would not only reduce the possibility of dying in younger age, but also some of the miseries of old age.
