Sunday, September 27, 2009

Mutation, Gene Mutation, Chemical Mutagen and Recombination

MUTATION

Genes along with their chromosomes are fairly stable and transmit heredity more or less uncharged. They sometimes undergo changes called ‘Mutation’. Mutation is any change in the amount, organization or content of genetic material. It does not include exchange as a result of recombination between homologous chromosomes. Mutations may be obvious and observable by cytological techniques. Some changes in chromosomes are called chromosomal aberrations. They may be however invisible when they are in genes. Both types of mutations when passed from adult to the offspring alter the hereditary instructions.

GENE MUTATION

A change in genetic massage of a cell is called Mutation. Mutational changes that affect the message itself producing alterations in the sequence are called ‘point mutations’; since they usually involve only one or a few nucleotide. In both bacteria and eukaryotic individual genes may move from one place on the chromosome to another by a process called ‘transposition’. When a particular gene moves to different location there is often alternation in its expression or in that of neighbouring genes. In eukaryote large segments of chromosomes may change their relative location or undergo duplication. Such chromosomal rearrangement often has drastic effects on the expression of the genetic message.

Point mutations involving only one or few nucleotide, result either from chemical or physical damage to the DNA or from spontaneous pairing errors that occur during DNA replication. First class of mutation is of particular practical importance because modern industrial societies produce and release into the environment many chemicals capable of damaging DNA. These chemicals are called Mutagens.

CHEMICAL MUTAGENS

Many mutations result from the direct chemical modification of DNA bases. The chemicals that act on DNA fall into three classes (i) Chemicals that look like DNA nucleotides but pair incorrectly when they are incorporated into DNA (ii) chemicals that remove the amino group from adenine or cytosine, causing them to mispair and (iii) chemicals that add hydrocarbon groups to nucleotide bases, also causing them to mispair.

RECOMBINATION

Scientists J. Lederberg and E. L. Talum discovered specific type of sexual reproduction in E. coli bacteria. Two mutant kinds were found in E. coli bacteria. One mutant strain was named as Y10 and other mutant strain was known as Y24. Scientists prepared mixed cultures of Y10 and Y24 by providing all six additional substances which were necessary for their growth. They also discovered various recombinations of six mutations. Because the rate of back mutation was already determined in these bacteria which were very low rate therefore it was explained that the discovery of wild type bacteria and other recombinations of mutations could be the explanation that the exchange of hereditary material might have occurred during a process that was named as conjugation.

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