As a female child ages into a woman, the hips gradually become broader to permit the mature female to carry the growing fetus in her womb. The breasts become well-developed to allow her to provide the best nutrition possible once she delivers the conceived child. And since each multicellular organism is a composite of many cells, what we see happening to it is a reflection of what is happening inside the body, within the cells.
The aging process of a multicellular organism such as a human is a reflection of division, development, and programmed cell death of its somatic cells. For a cell, the earliest physiological process to disappear once it is developing is its ability to produce daughter cells. Because once a cell is developing, it steps out of the cell cycle and enter a nondiving state, the Go phase. Most of the cells in a developed human body are in Go. Most common examples of these are the specialized neurons, which do not divide (Campbell and Reece, 2002, p. 226). So on the onset of life, a fertilized egg cell divides into many, and will be grouped according to what their functions will be once they develop defining characteristics that make them different from others.
However, normal cells cannot multiply forever. Not only will development not permit them, as is the case once a cell dies before the organism, but mortality is engraved in the DNA. Since DNA replication mechanism only makes a daughter strand by adding nucleotides at the 3’ end, its 5’ end, which an RNA primer filled during the course of DNA replication, is left unfilled once the primer is removed, and the resulting DNA is shorter after every replication. The good thing is, the 5’ ends of DNA strands, called telomeres, do not actually code for a gene and are thus dispensable. But since telomeres are of defined length, DNA replication and cell division can only go through a defined number of cycles as well (Campbell