Mutations are changes in the DNA. There are three main causes of mutations: duplication and transcription errors, damage to the DNA by chemicals, and damage by radiation. Although mutations can affect any cell in the body, it is mutations to sex cells, sperm and eggs, that can be passed on to the next generation. Damage to body cells can harm the individual but will not be passed on to offspring.
Whenever a cell divides, the DNA must be copied so that both cells have complete sets of DNA. This process is called mitosis. When sex cells are produced, which contain only half the DNA of the parent, a more complicated process called meiosis occurs. In either process, mistakes can be made. A piece of DNA may be inverted, deleted or changed. In each case, the result is a cell with a different set of instructions than the original cell, in other words, a mutation. In most cases, these changes will mean that instead of making a protein correctly, the instructions are now wrong and either nothing will be made or a different protein will be constructed. Since each protein has specific uses in the cell and the body, these changes can be deleterious but occasionally a new product is formed that has some new use and thus these mutations, if they occur in the sex cells, can lead to evolutionary change. Luckily most mutations have no effect because they occur in parts of the DNA that do not code for anything or they occur in a duplicate set of instructions so the correct protein can still be constructed elsewhere.
Transcription errors occur when the DNA instructions are passed on to the messenger RNA molecules. If the code is incorrectly transmitted because of the substitution of an incorrect base in the newly constructed RNA molecule, the mistake can have similar effects: either the protein is not made or a different protein is constructed and a mutation has occurred. As with DNA duplication errors, most of these are harmless or harmful and very, very few lead to a positive change.
Although mutations can happen spontaneously because of mistakes in DNA duplication and transmission, not all mutations occur this way. Both chemicals and radiation can cause mutations. Again, for these to affect future generations, they must occur in the sex cells, but both chemicals and radiation can harm the individual by affecting other cells in the body. A good example is skin cancer, which is caused by excessive exposure to the sun's ultraviolet radiation. This causes changes in the DNA of skin cells which can lead to skin cancers. Human-generated radiation can also cause cancers due to mutations. Many people exposed to nuclear explosions, whether tests or the bombs dropped on Nagasaki and Hiroshima, developed cancers that were caused by radiation-induced mutations to their cells. As the radiation passes through the body, it damages the DNA by breaking strands and destroying individual bases in the strands. Then when the DNA reproduces, it passes on these mistakes, which are for the most part harmful, to the next generation of cells. If the sex cells are affected, the harmful changes can be passed on to the offspring as well.
Some chemicals are also known to be mutagenic, including nitrous acid, benzene and bromine. DNA is made up of bases and some chemicals such as 5-bromouracil are similar enough to bases to be inserted into the DNA, turning normal code into meaningless gobbledygook that does not produce the correct protein needed. In this case, the 5-bromouracil, when inserted in the DNA, pairs with an adenine base instead of the normal guanine and this produces a meaningless message. These mutagens are known as base analogs. Nitrous acid on the other hand acts as a deaminating agent and removes an amine sequence from the DNA. Benzene is a carcinogen and by damaging DNA in bone marrow, liver and other tissues can cause leukemia or other cancers.
Mutations are changes to the DNA codes in our cells. They can occur naturally as duplication and transcription errors or they can occur because of exposure to radiation or harmful chemicals. Cigarette smoke is full of harmful chemicals and that is why tobacco smokers and passive smokers have much higher rates of cancer than the general population. We cannot protect ourselves from normal transcription errors, which increase with age and are part of the aging process. But we can protect ourselves from radiation and harmful chemicals. We need to be especially careful in childhood and our reproductive years in order to protect our children. In our post-reproductive years, we can enhance our chances of living longer by avoiding mutagenic chemicals and radiation.
http://learn.genetics.utah.edu/archive/sloozeworm/mutationbg.html http://evolution.berkeley.edu/evolibrary/article/mutations_04 http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20laboratory/mitosis/mitosis.htm