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Life and Health |
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Why not all smokers get cancer? |
Not all heavy smokers get cancer. They give a false sense of security to many smokers. There are several reasons why this is so. The two most important reasons are: 1. Carcinogenesis is an extremely complex process, taking place through a series of steps, being mediated at each step by a variety of factors (enzymes and genes). 2. At the same time, the host defence mechanisms and protective agents in the diet are actively engaged in preventing the onset of cancer.
The final emergence of a full-blown tumour, i.e. lung cancer in the case of smokers, represents the ultimate triumph of the carcinogenic processes. If a heavy smoker does not get cancer, he has to be extremely thankful to his inherent body resistance and defence mechanisms, and to the protective agents he may be consuming, even without knowing.
Carcinogenesis is broadly divided in to three major stages - initiation, promotion and progression. In the first stage, the carcinogen transforms a normal cell (or a group of normal cells), into the malignant phenotype, i.e. confers on it the potentiality to become a malignant cell and later, a malignant tumour.
Initiation involves a mutation, i.e. a small, but significant change in the DNA of the cell, brought about by reaction of the carcinogen with the DNA (Deoxyribonucleic acid present in the chromosomes inside the cell).
Most of the carcinogens, including the polycyclic aromatic hydrocarbons present in the cigarette smoke, cannot react directly with DNA. They have to be converted into more reactive forms, "the ultimate carcinogens", by a super family of enzymes, known as "Mixed function oxidases", in the liver.
Only the ultimate carcinogen can combine with the DNA in the normal cell, forming an "adduct", i.e. an addition compound, initially, and subsequently mutating it into the malignant phenotype, if the cell divides and replicates, before the DNA is repaired. It is at this stage, the body's defence mechanisms come into play.
The Hindu, Chennai
February 23, 2000
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