During the process of DNA replications errors occasionally occur in the polymerization of second strand. These errors are known as mutations. These can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence to gene. Error rates are usally very low- l error in every 10-100 million bases- due to the proofreading ability of DNA polymerases.
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Processes that increase the rate of changes in DNA are called mutagenic. These chemicals promote errors in DNA replication, often by interferring with the structure of base pairing while U V radiation induces mutations by causing damage to DNA structure.
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Mutation: Sudden heritable changes in the characters of origansm.
Mutants: Individual showing mutational changes.
Mutant allele: The allele producing the changed pheno-type variant. Mutation is the ultimate source of all the Genetic variation.
Historical:
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Seth Wright (1791): Male lamb with unusually short legs.
Hugo de vries (1900): The term mutation was introduced sudden heritable changes observed by him is another contribution.
Morgan (1910): White eye mutant of drosophila.
H. J. Muller (1927): Mutagenic action of X-rays in drosophila.
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Characteristics:
i. Generally mutant alleles are recessive.
ii. Random events.
iii. Most have harmful effects.
iv. Recurrent in nature.
v. Occurs at very low frequencies.
vi. Mutable genes.
vii. Mutaor genes.
viii. Highly mutable sites within a gene – hot spots.
ix. Rate of induced mutation varies from one gene to the other.
x. Most Mutant alleles are pleiotropic.
xi. Occurs in both forward and reverse directions.
Classification:
a. The direction of Mutation
1. Forward 2. Reverse
b. Cause of Mutation
1. Spontaneous 2. Induced
c. Dominance Relationship
1. Dominance Mutation
2. Recessive Mutation
3. Co dominant (blood group)
4. Partially dominant.
d. Tissue of Origin
1. Somatic Mutation (bud mutation)
2. Germinal Mutation
e. Effect on survival
1. Lethal 2. Sublethal 3. Subvital
4. Vital 5. Super vital
In crop improvement → Vital and Super Vital
f. Type of Trait Affected
1. Visible Mutation 2. Character Mutation
g. Intensity of Character Expression
1. Amorphic 2. Hypomorphic 3. Isoallelic 4. Hyper morphic
h. Quantum of Morphological Effect Produced
1. Macro mutation 2. Micro Mutation
i. Effect on the Expression of Neighbouring genes
1. Polar Mutation 2. Non polar Mutation
j. Cytological Basis
1. Chromosomal Mutation
2. Gene Mutation
3. Cytoplasmic Mutation
k. Molecular basis
1. Base substitution
2. Deletion
3. Addition
l. Type of Amino Acid Replacement in the polypeptide
1. Missense 2. Non sense 3. Frame shift
Change in base sequence of DNA molecule occurs in two ways:
1. Base substitution
2. Base addition and deletion
Base Substitution:
When one base in DNA molecule is replaced by another one.
2 types: (i) Transition (ii) Transversion
(i) Transition:
Purine is replaced by another purine e.g. Adenine is replaced by Guanine.
A <————— >G
One base substitution affects the base sequence of only one codon as a result only amino acid is altered in the concerned protein.
(ii) Transversion:
Pyrimidine (Thymine and Cytosine) is replaced by a purine (adenine and guanine) A <——> T are base substitution affects the base sequence of only one codon as a result, only one amino acid is altered in the concerned protein.
Base Addition and Deletion
Base Addition:
Insertion of one or more bases in a DNA molecule.
Base Deletion:
Loss of one or more bases in a DNA molecule.
Mutator Gene:
Some genes increase the frequency of spontaneous mutation of other genes located in the same cell.
Anti Mutator Gene:
Some genes decrease the frequency of spontaneous mutation of other genes located in the same cell.
Mutagens:
Also, Refer to Chapter 4, Article No. 4.3.4
Applications in Crop Improvement:
1. Development of Improved Varieties:
Wheat – Sarbati Sonora, Pusa Lerma
Rice – Jagannath
Tomato – Pusa lal, Meeruti
Cotton -MCU7, MCU10
Sugarcane – Co8152, 8153
2. Induction of Male Sterility:
Genetic male sterility has been induced in durum wheat using radiations.
3. Production of haploids:
Use of X rays radiated pollen in production of haploids in many crops.