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The DNA damage theory of aging proposes that aging is a consequence of unrepaired accumulation of naturally occurring DNA damages. Damage in this context is a DNA alteration that has an abnormal structure. Although both mitochondrial and nuclear DNA damage can contribute to aging, nuclear DNA is the main subject of this analysis. Nuclear DNA damage can contribute to aging either indirectly (by increasing apoptosis or cellular senescence) or directly (by increasing cell dysfunction). In humans and other mammals, DNA damage occurs frequently and DNA repair processes have evolved to compensate. In estimates made for mice, on average approximately 1,500 to 7,000 DNA lesions occur per hour in each mouse cell, or about 36,000 to 160,000 per cell per day. In any cell some DNA damage may remain despite the action of repair processes. The accumulation of unrepaired DNA damage is more prevalent in certain types of cells, particularly in non-replicating or slowly replicating cells, such as cells in the brain, skeletal and cardiac muscle. ==DNA damage and mutation== To understand the DNA damage theory of aging it is important to distinguish between DNA damage and mutation, the two major types of errors that occur in DNA. Damages and mutation are fundamentally different. DNA damages are physical abnormalities in the DNA, such as single and double strand breaks, 8-hydroxydeoxyguanosine residues and polycyclic aromatic hydrocarbon adducts. DNA damages can be recognized by enzymes, and thus they can be correctly repaired if redundant information, such as the undamaged sequence in the complementary DNA strand or in a homologous chromosome, is available for copying. If a cell retains DNA damage, transcription of a gene can be prevented and thus translation into a protein will also be blocked. Replication may also be blocked and/or the cell may die. Descriptions of decrements in function, characteristic of aging, associated with accumulation of DNA damages, are given later in this article. In contrast to DNA damage, a mutation is a change in the base sequence of the DNA. A mutation cannot be recognized by enzymes once the base change is present in both DNA strands, and thus a mutation cannot be repaired. At the cellular level, mutations can cause alterations in protein function and regulation. Mutations are replicated when the cell replicates. In a population of cells, mutant cells will increase or decrease in frequency according to the effects of the mutation on the ability of the cell to survive and reproduce. Although distinctly different from each other, DNA damages and mutations are related because DNA damages often cause errors of DNA synthesis during replication or repair and these errors are a major source of mutation. Given these properties of DNA damage and mutation, it can be seen that DNA damages are a special problem in non-dividing or slowly dividing cells, where unrepaired damages will tend to accumulate over time. On the other hand, in rapidly dividing cells, unrepaired DNA damages that do not kill the cell by blocking replication will tend to cause replication errors and thus mutation. The great majority of mutations that are not neutral in their effect are deleterious to a cell’s survival. Thus, in a population of cells comprising a tissue with replicating cells, mutant cells will tend to be lost. However, infrequent mutations that provide a survival advantage will tend to clonally expand at the expense of neighboring cells in the tissue. This advantage to the cell is disadvantageous to the whole organism, because such mutant cells can give rise to cancer. Thus DNA damages in frequently dividing cells, because they give rise to mutations, are a prominent cause of cancer. In contrast, DNA damages in infrequently dividing cells are likely a prominent cause of aging. The first person to suggest that DNA damage, as distinct from mutation, is the primary cause of aging was Alexander in 1967. By the early 1980s there was significant experimental support for this idea in the literature. By the early 1990s experimental support for this idea was substantial, and furthermore it had become increasingly evident that oxidative DNA damage, in particular, is a major cause of aging.〔Bernstein C, Bernstein H. (1991) Aging, Sex, and DNA Repair. Academic Press, San Diego. ISBN 978-0120928606 partly available at http://books.google.com/books?id=BaXYYUXy71cC&pg=PA3&lpg=PA3&dq=Aging,+Sex,+and+DNA+Repair&source=bl&ots=9E6VrRl7fJ&sig=kqUROJfBM6EZZeIrkuEFygsVVpo&hl=en&sa=X&ei=z8BqUpi7D4KQiALC54Ew&ved=0CFUQ6AEwBg#v=onepage&q=Aging%2C%20Sex%2C%20and%20DNA%20Repair&f=false〕 In a series of articles from 1970 to 1977, PV Narasimh Acharya, Phd. (1924–1993) theorized and scientifically proved that cells undergo "irreparable DNA damage," whereby DNA crosslinks occur when both normal cellular repair processes fail and cellular apoptosis does not occur. Specifically, PVN Acharya noted that double-strand breaks and a "cross-linkage joining both strands at the same point is irreparable because neither strand can then serve as a template for repair. The cell will die in the next mitosis or in some rare instances, mutate."〔Acharya, PV; Ashman, SM; Bjorksten, J; The isolation and partial characterization of age-correlated oligo-deoxyribo-ribo nucleo peptides. Finska Kemists Medd. 81 No. 3 (1972) Suomen Kemists. Tied. Chemical Abstacts, Vol 78, No. 19. May 14, 1973. Abs. N. 122001 g.〕〔Acharya, PVN. Isolation and Partial Characterization of Age-Correlated Oligo-nucleotides with Covalently Bound Peptides. 14th Nordic Congress, Umea, Sweden, June 19, 1971.〕〔Acharya, PVN. DNA-damage: The Cause of Aging. Ninth International Congress of Biochemistry: Stockholm. July 1–7, 1973 (Abs.3 m 12).〕 Acharya's research also showed how irreparable DNA damage is caused by environmental pollutants, low dose ionizing radiation and food additives, particularly nitrites and nitrates and such damage to the DNA is a causal factor for pre-mature aging and cancer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「DNA damage theory of aging」の詳細全文を読む スポンサード リンク
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