Gene Mutation -Point Mutation
Now a days the word mutations is related to the gene mutations, If a nucleotide sequence of DNA molecule undergoes a change, it is called mutation. That gene is called mutant gene. In 1909 Morgan observed white eyed Drosophila. This white eye color is a mutant character. The mutant gene and original gene will be present at the same locus of the chromosome. The mutant gene Is always recessive. Only in homozygous condition the mutant gene will be expressed. Gene mutations are of two types. In nature mutations arise by themselves. They are called spontaneous mutations.
Induced Mutations: By the applications of mutagenic agents mutations are brought forward. This l& called Induced mutation. In 1927 Muller brought’ induced mutations by X-rays in Drosophila. For this he awarded noble prize in 1928 Stadler induced mutations in Barley plant by X-rays. Auerbach used mustard gas to bring mutations. Many chemicals will cause mutations.
Reverse Mutation: A normal gene will undergo mutation and become a mutant gene. After some time this mutant gene will undergo mutation and becomes original gene. This is called reverse mutation.
Eg: in a a wild pea plant white flower is because of original gene. Because of mutations it becomes purple. Because of reverse mutation it becomes white
Characters of Gene Mutation:
- Mutation occurs at random. A direction cannot be predicted.
- It will not occur to full fill a specific requirement.
- The mutation is because of change.
- Usually the mutant genes are recessive.
- Usually the mutations are harmful.
- Mutations will form a raw material for evolution.
- DNA is the basic material of a gene. DNA is responsible for hereditary characters. In a DNA molecule a specific base sequence is present. It will not change through many generations.
A DNA molecule is made by two polynucleotide chains. In each strand four kinds of nucleotides are present. The genetic property of particular genetic molecules of DNA is determined by the specific sequence of its nucleotide bases. Changes In this sequence or on base pair is changed, mutation is caused. Watson and Crick predicted that base pair change at any point But it is very rare. This Is represented in the following way. AT, is changed into G.C. pair.
This type of change is called mutations. This is carried on to a point. Hence it’s called Point mutation. Because of this it is invisible, it is called Invisible mutation. This is also called Gene mutation. This is brought forward by following methods.
In DNA the base pairs are A-T and 0 - C in each of the four base molecules various atoms are arranged in a certain manner. A single shift in this arrangement as a result of hydrogen atom moving from one position to another position, within molecule will cause the change in the form of base. This process i called Tautomerism. The two forms of a base are called tautomeres common tautomeric forms of 0 and T are “keto” compounds The tautomeric forms of A and C are “amino” compounds. Now and then “ionyl” forms, “amino” forms may occur. These are called abnormal tautomers. Because of these abnormal tautomers, gene mutations are brought forward.
In these tautomers the mobile hydrogen atom Is changed in their position cause of which the tautorneric form show abnormal behavior. The normal pairing is A, T and T, A and G, C and C, 0
The tautomeric pairing shows A-C and G-T etc.
Unusual pairing during replication: Transition G-C to A-T pair as a result of in-corporation of rare tautomer A’ during replication of DNA, will cause point mutation. -
During replication of DNA C, C will separate, G combines with C. But C will combine with rare tautomer At This will bring a mutation.
This change is seen in the following way.
Some unnatural bases are chemically related to natural bases. Their chemical structure is related. But their behaviour is different.
Ex Thymine and 5 bromo uracil substances are chemically related. They are called analogs. Thymin combines with A but its analog 5 bromo uracil will combine with 0. In this way of the function they are differ. Hence by using base analogs we can bring mutations, A, I pair can be changed into G, C pair by using 5 bromouracil. When bacteria are grown on 5bromouracii medium mutations have come, Bromo uracil are Incorporated in place of T. Because of which in the replication A, T pair change into G, C pair.
Some chemical substances will bring mutations. They are nitrous acid hydroxyl amine.
When a bacterium is treated with nitrous acid A, Twill change into G,C pair. Nitrous acid will act on adenine and converted into xanthine. This xanthine will combine with Cytosine. This brings point mutations.
During replication of DNA molecule A,T G, C pair will produce. Now and then A may combine with G which is very rare. In the next replication this is corrected. This simple mistake in first pairing will convert A, T pair into (3, C pair.
Two bases of a pair will break their bond with sugar. When they unite with sugar they will show 180 degrees rotation. This cause C, C pair becomes C, C pair or A, T pair will become T, A. This is called transversion.
In this way point mutations will occur in organisms.