Biologic Theories

1. Cellular theory :  Cells have been subjects of  much scientific inquiry in exploring aging phenomena, Cell has three distinct components:
  a. cells that can reproduce
  b. cells that cannot reproduce
  c. inter cellular substance/materials

Cells that can reproduce : Reproduction of cells occur. Some new cells become non-functioning or less effective than the others that are replaced. Three systems in human body are continuously replaced . With the progress in the aging process there is accumulation of these inefficient  and non-functioning cells. Organism functional ability becomes apparent. Visible changes occur in the aging process.
Cells that cannot reproduce : Eg. CNS kidney. With age cells progressively wear and tear out and or destroyed. Develop an accumulation of non-functioning cells. Systems became less efficient and difficult to handle.
Inter cellular substance/materials :  Gradual deterioration of inter cellular material with aging reduces the ability of the cells to provide necessary nutrients and oxygen for respective tissue and directly interfere the functioning abilities of each system.

2. Programmed aging theory : Aging and death, according to this theory, are not a result of wear and tear or exposure, but are a programmed, natural and necessary part of genetics. In short, we are programmed to age and die.

3. Error Theory :
Miss-transcription and miss-translation of certain genes products.
Results in self amplifying error producing derangement
Initial error will, most likely result in further errors of similar types
The productions of faulty products of other genes which accumulate with aging. eg. Enzyme with decrease catalytic activity in the later life.
4. Somatic mutation theory (similar to error theory)
Cells exposed to x-ray radiation or chemicals
Cell by cell alteration in DNA occurs
Increase the incidence of chromosomal abnormalities
Occurs more at youth and its deleterious effect are seen in later life
5. Wear and Tear Theory
Postulates that an organisms wears out with use
Damage begins at the level of molecules in our cells
DNA that makes up our genes sustains repeated damage from toxins
Bodies have the capacity to repair the damage but not all are accurately corrected
Damage cells progressively accumulates
6. Auto immune theory :  Postulates that with age, the immune system produces auto-anti-bodies that causes cell death or cell changes that fosters the aging process

7. Free radical theory
Free radical is a molecule with an unpaired, highly reactive electron. One type of free radical generated in our body is oxygen free radical
This free radical grabs the molecule from any other molecule and damage the other molecule
Molecule that are damaged by free radicals are Fat , Proteins and DNA. (both of nucleus and mitochondria)
Under normal condition natural defense mechanism prevent most of the oxidation damage
This theory purpose that little by little small damage accumulates and contributes to deterioration of tissues and organs
 8. Cross linkage theory :  Also called as collagen theory.
With age, our proteins, DNA and other inappropriate cross links to one another
This unnecessary links decreases the elasticity of proteins and other molecules
Proteins that are damaged are no longer needed and are broken down by pro tease enzyme
Presence of cross linkage inhibits the activity of pro tease.
Damaged and unneeded proteins stick around and can cause problem. Eg: wrinkling of skin at aging, age related cataract formation.