Aging Theories Essays - Molecular Biology, Senescence, Mutation

Aging Theories This report outlines the main theories of how the process of aging works. Since researchers have not discovered a universally-accepted theory of aging, the theories discussed are potential explanations of how we age. The likelihood of each hypothesis is considered roughly equal. The different theories discussed focus on the workings of different parts of the body, from the molecular level of DNA mutations and replication, to the organism level of becoming "worn out." Aging is a very complex and gradual process, and its ongoing operation is present to some degree in all individuals. It is a journey to the maturity, as well as to the degeneration of the body. Because aging affects every part of the body, many different steps are involved and various types of reactions occur. Changes in DNA take place, which can and often do affect the way the body functions; harmful genes are sometimes activated, and necessary ones deactivated. A decrease in important body proteins like hormones and certain types of body cells is almost inevitable. These, among many, are characteristic changes that take place in our bodies as time moves on and aging continues. At present, a universal explanation for how we age or why we age does not exist, but there are many theories to explain this puzzle, and they are supported by continuous research. In this report, some of the how theories of aging will be examined. Among them are theories concerning spontaneous mutations, damage from free radicals, the clock gene, cellular aging, a weakened immune system, wear and tear, and hormonal and neuroendocrinous changes. Spontaneous Mutations The spontaneous mutations theory, also known as the somatic mutation hypothesis, states that the crucial events that cause aging are mutations. These are changes in a cell=s DNA, which are passed on to daughter cells during mitosis. Since genes on DNA code for specific proteins, mutated genes may produce defective proteins, which do not work properly. Many proteins can be affected, such as enzymes, proteins comprising muscle tissue, and a recently discovered type of protein called transcription factors, which bind to DNA and regulate the individual activities of genes themselves. Physical mutagens are substances that increase the chance of mutation and include such physical phenomena as x-rays and radioactivity from radium. The atomic bombs dropped on Hiroshima and Nagasaki in Japan are examples of physical mutagens that caused an increase in the number of cases of leukemia. Certain chemicals and radiation cause mutations to occur in DNA by giving off high energy particles. These particles collide with the DNA and knock off atoms of the DNA randomly, damaging it. DNA consists of sequences of four possible nitrogenous bases: adenine, guanine, cytosine, and thymine, paired so that adenine always pairs with thymine, and guanine always pairs with cytosine. As cells repair the damaged DNA, a different DNA base is often substituted. This base-substitution is known as a point mutation and can cause the production of a defective or damaged protein. Apart from being caused by radiation or chemicals, mutations also occur spontaneously but at lower rates. Physicist Leo Szilard and biochemist Denham Harmon proposed that because most mutations are harmful, the more spontaneous mutations that arise, the more abnormalities that arise as defective proteins are produced. These could ultimately kill an individual (Ricklefs and Finch, 1995, 20). Although it has been proven that many proteins undergo alterations during aging, the spontaneous mutations theory is not the cause (Ricklefs and Finch, 1995, 21). It has, however, been proven that DNA is chemically altered during aging. Modifications in DNA bases, called I-spots, have been found to increase in number during aging. Besides I-spots, another modified base known as 8-hydroxyguanine, the DNA base guanine with an added OH group, has also been found to increase during aging. It is unclear how changes such as these arise, but similar changes seem to be caused be exposure to mutation-causing chemicals, some of which are found in tobacco smoke (Ricklefs and Finch, 1995, 21). Another factor supporting the spontaneous mutations theory may lie in the temporal occurrence of genetic mutations. Certain cancers and abnormal growths seem to appear more frequently as the process of aging continues. Two tumour suppressor genes called p16 and p53 are responsible for slowing cell proliferation, and therefore keep certain cells from becoming cancerous. However, if they become mutated, they do not carry out their function properly so cells with these mutations begin to grow and divide quickly, causing cancer and other growths (Ricklefs and Finch, 1995, 22). Werner's syndrome is a disorder that significantly accelerates the aging process starting at