How Mutations Occur

Most of the mutation occurs at the time of DNA replication. Some mutation occur as the damage done by exposure to UV light or X rays. Since these grants are inescapable part of the environment, they probably account for many spontaneous mutations. However, mutation rate can be increased substantially by deliberately increasing the exposure to such radiation. Any agent that increases the rate mutation rate is called a mutagen. Mutation occur due to the use of mutagen, are called induced mutation rather than spontaneous mutation. UV light causes mutation in both natural and laboratory conditions. In laboratory conditions, rate if mutation by UV light is higher than that in natural conditions, because of prolong exposure to UV light.

The major effect of UV light us to cause the formation of dimer by crossing between adjacent pyrimidine, especially thymine, residues in DNA. These cross linked residues disrupt the normal process of replication by preventing the various polymerase from functioning. When x – ray interact with DNA, the result is usually a break in the phosphodiester backbone of the nucleic acid.

Studies of the mutagenic effect of various chemicals have most revealing finding about mutation. There are three main type of mutagenic chemicals. The first consist of compounds that can react chemically with DNA. Since specificity of DNA replication depends upon purine – pyrimidine bonding, which result from hydrogen bonding between the amino and hydroxyl groups of the purines and pyrimidines, chemical modification of these amino and hydroxyl group van cause mutation. Nitric acid which can remove amino groups from purines and pyrimidines, is such a mutagen. Another type of mutagenic chemicals are consists of base analog. These are chemical sufficiently similar in structure to normal DNA bases to be substituted for them during DNA replication. Although similar in structure, base analog do not have the same hydrogen bonding properties as the normal bases. They can therefore causes in errors during replication and causes mutation. A third type of mutagenic chemicals is intercalating agents. These are flat molecules that are intercalate (slip in) between base pairs in the central stack of the DNA helix. By this means they distort the structure and causes replication errors. Examples of such agents are acridine orange, proflavin, and nitrogen mustereds.

Recently it has been proven that mutation can also occur by transposons. Transposons are units of DNA which move from one DNA molecule to another, inserting themselves nearly at random. They are also capable of causing DNA rearrangements such as deletion or inversions. For example, one such transposon is from the bacterial virus called Mu, which may be considered a mutagen.

How Mutations Are Repaired

In the above text we have said that DNA damage can occur by UV radiation, x ray, and certain other chemicals. Fortunately cells contain specific enzyme those can check DNA for defects and can repair them. Because of this some infected cells can function normal in effected conditions.

Many kind of bacteria and yeasts cells have been shown to posses an effective photoreactivating mechanisms to repair damage caused by UV radiation. This photoreactivation occur when cell exposed to lethal dose of UV light are immediately exposed to visible light. A special enzyme designated PRE, induced by visible light, splits or unlink the dimers formed because of UV light and restore the DNA to its original form.

Some bacteria have enzyme, called endonucleases and exonucleases that exercise or cut out a damaged segment of DNA. Then the other enzymes, polymerase and ligase, repair the resulting break by filling in the gap and joining the fragment together.

The process by which E. Coli repair large amount of DNA damage inducible or SOS repair.

Reference: Microbiology Pelczar