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The theory of ageing that ageing is due to
A. Random mutation in DNA of somatic cells.
B. Increased cross-linkage of collagen and other proteins.
C. Cumulative result of damage to tissues by free radical.
D. All of the above.

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Last updated date: 04th Mar 2024
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Hint: Ageing is the process during which structural and functional changes accumulate in an organism with time. Ageing is an unavoidable process occurring in almost every living organism and it is important to understand this process.

Complete Answer:
The modern theory of ageing in humans falls into two main categories: programmed and damage or error theories. The programmed theories suggested that ageing follows a biological clock and the damage or error theories imply that environmental factors induce damage to the living organism at various levels as the cause of ageing.

The programmed theory:
- Programmed Longevity suggests that ageing is the result of a sequential switching on and off of some genes and senescence is defined as the time when age-associated deficits are manifested.
- The endocrine theory says that biological clocks act through hormones to control the rate of ageing.
- The immunological theory states that our immune system is programmed to weaken over time which leads to an increased vulnerability to infectious disease and thus ageing and death.

The damage and error theory:
- Wear and tear theory, where the vital parts in our cells and tissues lose their function or wear out leading to ageing.
- The rate of living theory keeps up with the theory that the greater is an organism’s rate of oxygen basal metabolism, the shorter will be the life span.
- Cross-linking theory states that the accumulation of cross-linked proteins damages cells and tissues which slows down bodily functions and results in ageing.
- Free radicals theory states that superoxide and other free radicals cause injuries to the macromolecular components of the ell, damaging various tissues, and eventually the organs cease to function.
- DNA damage theory states that genetic damage, mutations, etc. can cause abnormal gene expression which causes DNA damage and induces cells to undergo apoptosis.

Hence, the correct option is D, ‘all of the above’.

Note: The actual cause of ageing is still uncertain, current theories shed light on some of the many causes leading to ageing.