**7. Aging process**

Aging is a natural process in various changes occurs in normal physiological process and ultimately increases the risk of disease and death of cell, tissue and organ (**Table 1**).

 One of the major causes of aging process is the cellular damage that causes the shortening of DNA, leading to the process called apoptosis. Apoptosis is a process in which cell is programmed to be death in given life spam. This process is important when we realized that each cell contain genetic material and mitochondria or power house of cell. As we know that mitochondria serves as an energy generator during normal cell process and free radicals are produced a byproduct and these free radicals damage DNA and creating DNA fragments and triggers cell to apoptosis.

 As time passes the more free radicals damages more DNA and fragments increase the process of cell apoptosis and our body cannot generate cells faster enough to maintain or compensate loss, so in older age we have very thin skin in comparison to young age.

In another process, the cellular down-regulation of enzymes such as superoxide dismutase, catalase and glutathione peroxide which are natural oxidative enzymes which making our antioxidant defense lesser efficient with age.

 As we grow with age, the process of cellular reproduction increases and body creates non-functional cells along with functional cells, leading to rapid deterioration of the body's function. As we grow with age, the more numbers of useless cells are formed which interfere with normal cellular processes and leading to the aging process.

*Free radicals*: Free radicals produced by mitochondria as a byproduct and are oxygen containing highly reactive molecules with single electron in the outermost orbital that are very eager to pair up with anything else that has electrons. The life span of free radical is one-millionth of a second. Free radicals attack on cell's DNA and destroy cell prematurely and during young age out antioxidant defense system protect the cell from free radicals but as we grow this system weekend and not be able to work efficiently [48, 49].

#### **7.1 Theories of aging**


#### *7.1.1 Genetic theory*

 This theory demonstrates that gene of an animal or human being contains a 'program' of life span. The genetic theory of aging focuses on telomeres which are repeated units deoxyribonucleic acid and are present at the end of chromosome. The number of repeats in a telomere defines the life span of a cell, and multiple repeats are lost when each time a cell divides. When telomere has been reduced to a certain size, the cell reaches at a stage where it is prevented from further dividing, at this stage cell die.

#### *7.1.2 Non-genetic cellular theory*

According to this theory accumulation of harmful substance in the cells lead to aging process. Lipofuscin a dark colored insoluble substance that is accumulates day to day. This cellular garbage or cellular waste interferes with the physiology of cells and ultimately lead to death of the cell.

#### *7.1.3 Error theory*

 Sometime during RNA transmission process a mutant protein or enzyme is produced which is not the exact same copy of original then it is not be able to work well in maintaining life, as a result cell grows and die. As we know that RNA are unstable molecules and formed continuously while DNA are stable molecules and maintained throughout the lifespan of cell and tissue.

#### *7.1.4 Wear and tear theory*

Changes in the internal and external environment of the cell cause cell damage and cell will not work their function efficiently. As time passes, changes in the more numbers of the cell functioning lead to change the normal physiology of body and lead to aging and death.

#### *7.1.5 Cross linkage theory*

 As time passes deoxyribonucleic acid, different molecules and structural molecules develop a cross-linking with each other. These unwanted links/bonding decrease the mobility and elasticity of proteins and other molecules as a result molecule will not work efficiently. This cellular damage stick surround and can cause problems and cross linking appears when weakened immune system is unable to clean up the unwanted glucose in the blood. These sugar molecules react with adjacent neighbor and can cause cross-linking and lead to formation of free radicals.
