oteins, 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 a...
