The sudden and heritable change in the DNA sequence is known as mutation. The organism in which mutation occurs is known as the mutant. Mutation plays an important role in the formation of the allele. Hence, it is the raw material of evolution. Mutations have allowed organisms to adopt to their environment.
Mutation is the causes of certain disorders like cancer, diabetes as well as genetic disorders like down syndrome.
The genetic change or mutation in any gene is known as gene mutation. The causes of gene mutations are ploidy, chromosomal aberration or change at single nucleotide are known as point mutations. It may be because of addition and deletion or substitution of single base pair in sequence.
Mutations and nucleotide sequence in nucleic acid:-
The alterations in the base sequence are of following types:-
    (i)    deletion
    (ii)    insertion
    (iii)    inversion 
    (iv)    substitution
Deletion, insertion and inversion are due to breakage and reunion of DNA segments. However, the substitution may occur during replication of DNA. These can be of two types: transitions and transversions. 
18.1.    Transition 
When a purine is replaced by purine and a pyrimidine is replaced by pyrimidine is known as transition. For example A:T is replaced by G:C.
18.2.    Transversion
When a purine is replaced by a pyrimidine or vice versa is known as transversion. For example, C:G can be replaced by G:C or A:T can be replaced by T:A.

(1)    G:C to A:T transition

(2)    A:T to G:C base pair transition

(3)    C:G to T:A base pair transitions mutations.

The tautomeric shift is a phenomenon in which there is a proton shift in the nitrogenous base, resulting into formation of rare tautomeric form. The stable keto forms of thymine and guanine undergo a tautomeric shift to less stable enol form. The stable amino forms of adenine and cytosine undergo a tautomeric shift to less stable imino form.
For a very short period of time, the base pairs remain in less stable tautomeric form. But if they exist during replication, the mutation may result.

18.3.    Mutagens :
Mutagens are mutation causing physical or chemical agents.
18.3.1.    Physical Mutagens:-
Physical mutagens include low pH, high temperature, ionizing radiations and non-ionizing radiation.  Low or acidic pH:-
At low or acidic pH, there is complete removal of purine and pyrimidine which is known as depurination and depyrimidination respectively. In this process, the breaking of the glycosidic bond occurs. High temperature:-
At low pH and high temperature, there is a loss of exocyclic amino group and the process is known as delamination. Cytosine, adenine, guanine and 5 methylcytosines undergo deamination and get converted into uracil, hypoxanthine, xanthine and thymine respectively.  Ionizing radiations:- 
The ionizing radiation like X-rays, gamma (g) rays, alpha (a) particles have a direct effect on the chromosome. They may directly break chromosomes or alter the DNA bases depending upon the intensity of radiation and type of cells.
It may be possible that radiation in the presence of O2, form peroxide radicals. These radicals causes break in DNA and prevent from rejoining. Thus, increases the frequency of mutation.  Non-ionizing radiations:-
UV radiation is strong mutagen which has a more specific effect. UV act on pyrimidine bases and form dimer. For example, two molecules of thymine get connected and form thymine dimer.
The DNA damage is regularly repaired by the DNA repair system. But this repair system is impaired in a person who is suffering from xeroderma pigmentosum. It is a skin prone disease. The patient does not have the functional enzyme to remove thymine dimer produced due to UV radiation.

18.3.2.    Chemical mutagens:-
Chemical mutagens are the chemical compounds that affect nucleotide sequence either during replication or in the resting stage. Source of chemical mutagens are food, air and water. Effect of radiation is localized, while chemical mutagens spread in the complete body through blood circulation.
Changes during DNA replication:-
Incorporation of base analogues in the DNA causes pairing mismatch. Hence, creates the mutation.  5 - Bromo uracil (5-Bu):- 5 Bromo uracil is just like thymine pair with adenine. Thus 5 Bromo uracil can also pair with adenine. When 5 Bromo uracil is given to replicating cell. It incorporated in the DNA. In the next round of replication, 5 Bromo uracil pair with Guanine. This results in point mutation.  2 - Amino purine
A.    2 Amino purine form two hydrogen bond with thymine and it form single hydrogen bond with cytosine. Incorporation of amino purine at G and A  give rise to Amino Purine: Cytosine base pairing and Amino purine: T base pairing respectively. This leads to mutation.
(b)    Changes during the resting stage. Nitrous acid:-
In the presence of nitrous acid, (HNO2), guanine, cytosine and adenine undergo deamination. The adenine deaminates into hypoxanthine which pairs with cytosine. Cytosine deaminates into uracil which pairs with adenine. Guanine deaminates into xanthine which pairs with cytosine. The changes are A:T to G:C and G:C to A:T because of adenine to hypoxanthine and cytosine to uracil respectively.

Likewise, guanine deaminates and converted into xanthine.  Alkylating agent:-
Chemical agents having one or more alkyl group are known as the alkylating agent. These agents add alkyl group to the DNA.
Examples -    diethyl sulphate (DES)
                dimethyl sulphate (DMS)
                methyl methanesulphonate (MMS)
                Ethylethane sulphonate (EES)
                Ethyl methane sulphonate (EMS)  Intercalating agent or dyes :
Proflavin and acridine orange are examples of an intercalating agent. These agents directly bind to the nucleic acid. There is a deletion or insertion of a single base pair in the DNA sequence which leads to a frameshift mutation.
18.4.    Types of mutation:-
18.4.1.    Somatic mutation:-

Those mutations which take place in the somatic cells are known as somatic mutation. They are passed to daughter cells during mitosis but do not pass to the next generation.
18.4.2.    Germinal mutation:-
Those mutations which take place in the germ cell line are known as germinal mutations. These mutations occur in the reproductive organ which forms gametes and passed from one generation to the next generation and so on.
18.4.3.    Condition mutation:-
Those mutations which show their effect at a particular condition. These mutants remain unaffected at the permissive condition but show effects under restrictive condition.
18.4.4.    Forward mutation:-
The mutation which brings changes in phenotype of wild type to mutant is known as forwarding mutation.
18.4.5.    Backward mutation:-
The mutation which brings changes in phenotype of mutant to wild type is known as backward mutation.
18.4.6.    Non-sense mutation:-
When a nucleotide change in one codon causes premature termination of protein synthesis by introducing nonsense codon in a polypeptide chain.
18.4.7.    Mis-sense mutation:-
When a nucleotide change in genetic code causes the change in one amino acid of a polypeptide chain which can lead to alteration or loss of protein function.
18.4.8.    Neutral Mutation:-
A change in one nucleotide in codon does not change the amino acid in the polypeptide chain because of both codon codes for the same amino acid.
18.4.9.    Suppressor mutation:-
Those mutations which suppress the effect of the first mutation and restores the function are known as suppressor mutation. These are the two types of suppressor mutation.
(i)    Intragenic suppressor mutation:- The mutation which takes place at a different site in the same gene.
(ii)    Intergenic suppressor mutation:- The mutation which takes place in a different gene.
18.5.    Ames test :
The Ames test was described by Bruce Ames and his group. It is a widely employed method that uses bacteria for identifying mutagenic chemical and physical agents. It is the Biological assay to detect whether any chemical compound can cause mutations in the DNA. The Ames test uses bacterium Salmonella typhimurium that carry mutations in genes involved in histidine synthesis. These strains are auxotrophic mutants, they require histidine for growth but cannot produce it. This method tests the capability of the certain chemical in creating a mutation that results in a return to the proto atrophic state so that cells can grow on a histidine-free medium. The bacteria are spread on an agar plate with a small amount of histidine. This small amount of histidine in the growth medium allows the bacteria to grow for an initial time and have the opportunity to be mutated. When the histidine is depleted only bacteria that have mutated to gain the ability to produce its own histidine will survive.
In humans, many chemicals are present in the form of protomutagens. These protomutagens must be converted into active mutagens for this activity. But in bacteria, the enzymes are absent which convert proto-mutagens into active mutagens. So, the chemical or compound to be tested, first mixed with enzymes extracted from rodent liver. This enzyme activates the mutagens. Now activated mutagens are used in Ames test.
A positive test indicates that the chemical is mutagenic or not and may be carcinogenic or not.  

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