Shova Baral
Mutation
Mutation is defined as the sudden and heritable change in the genotype of an organism. The
change includes change in the chromosomal structure, number or genetic sequence. Mutation
was first studied by Hugo de Vries in 1901. The factors that cause mutation are called
mutagens. Mutagens may be radiations (uv ray, x-ray, gamma ray etc) or chemical substances
(nitrous oxide, ethylmethyl sulphonate, colchicine, hydrogen peroxide etc)
Types of mutation
On the basis of presence or absence of mutagens mutation is of two types
Spontaneous mutation: Mutation that occur naturally are called spontaneous mutation.
Induced mutation: Mutation that occur artificially by physical and chemical agents or
mutagens is induced mutation.
On the basis of origin mutation is of two types:
Micro mutation
Macro mutation
Micro mutation
The change in the nature of gene at its molecular level is called micro mutation. It is the result
of change in the nucleotide sequence of the DNA molecule in a particular region of the
chromosome. It is also known as point mutation or gene mutation. It is of following types.
o Frameshift mutation
o Substitution mutation
Frameshift mutation
The mutation caused by addition or deletion of nitrogenous bases in the DNA or mRNA is
frameshift mutation. It is of two types:
Addition/Insertion: It is a type of point mutation caused by addition of one or more
nitrogenous bases in the DNA sequence. All the triplets after insertion are affected.
Deletion: it is a type of point mutation caused by deletion of one or more nitrogenous
bases in the DNA sequence. All the triplets after deletion are affected.
Substitution mutation
The mutation caused by the replacement of one nitrogenous base by another nitrogenous base
is substitution mutation. It is of two types:
Transition: The substitution mutation in which one purine base is replaced by other
purine base and one pyrimidine base is
replaced by other pyrimidine base is
AUA CGA UUA UAC GGG AAA
transition.
AUA CGG UUA UAC GGG AAA
Transversion: The substitution mutation in which one purine base is replaced by another
pyrimidine base and vice-versa is transversion.
AUA CGA UUA UAC GGG AAA
AUA CGU UUA UAC GGG AAA
Chromosomal mutation/Macro mutation
The change in the structure and number of chromosomes is called chromosomal mutation. It is
also known as macro mutation. It is of two types:
Chromosomal aberrations
Ploidy
Chromosomal aberrations
The change in the morphological structure of chromosome is called chromosomal aberration. It
is of following types:
Deficiency: The loss of terminal
segment from a chromosome is
called deficiency.
Deletion: The loss of intercalary
segment of the chromosome is
called deletion.
Duplication: The repetition
of a segment or part of the
chromosome is called
duplication.
Inversion: A segment of
chromosome breaks, rotates
through 180 and rejoins. This
phenomena is called
inversion.
Translocation: The exchange of segment of chromosome between non homologous
chromosomes is called translocation. It is of two types:
o Simple translocation: It is a type
of translocation in which a
segment of a chromosome
attach to another chromosome
to form a new combination.
o Reciprocal translocation: It is a type
of translocation in which mutual
exchange of chromosome segment
occur between the non-homologous
chromosomes.
Ploidy
The mutation that takes place due to change in the number of chromosome is called ploidy. It is
of two types:
Aneuploidy
Euploidy
Aneuploidy
The addition or loss of one or more chromosome from the complete diploid set is called
aneuploidy. It is of following types:
Monosomics: The loss of one chromosome from a complete set is called monosomics. It
is represented by 2n-1.
Nullisomics: The loss of a single pair of chromosome from a complete set is called
nullisomics. It is represented by 2n-2.
Polysomics: The addition of one or more chromosome in a complete set is called
polysomics. It is of following types:
o Trisomic: The addition of one chromosome in a compete set (2n) is called
trisomic. It is represented by (2n+1).
o Double trisomic: The addition of two different chromosomes in a complete set
(2n) is called double trisomic. It is represented by (2n+1+1).
o Tetrasomic: The addition of a pair of chromosome in a complete set (2n) is called
tetrasomic. It is represented by (2n+2).
o Pentasomic: The addition of 3 similar chromosome in a complete set (2n) is
called pentasomic. It is represented by (2n+3).
o Hexasomic: The addition of 4 similar chromosome in a complete set (2n) is called
hexasomic. It is represented by (2n+4).
Euploidy
The addition or loss of complete set of chromosomes from a diploid set of chromosome is
called euploidy. It is of following types:
Monoploidy: The loss of complete set of chromosomes from a diploid set is called
monoploidy. It is also known as haploidy. It is represented by 2n-n.
Polyploidy: The addition of one or more set of chromosomes in a diploid set of
chromosome is called polyploidy. The organism with more than two set of chromosome
(diploid set) is called polyploid. The polyploidy may be triploid (3n), tetraploid (4n),
pentalploid (5n), hexaploid (6n) etc. Polyploidy is more common in plants than in
animals. It is of two types:
o Autopolylploidy
o Allopolyploidy
Autopolyploidy
Polyploidy that arise due to multiplication of chromosome set of a same species is
autopolyloidy and resultant organism is called autopolyploid. Autopolyploid may be
autotriploid, autotetraploid etc. If a diploid organism has two set of chromosomes (AA),
chromosomes present in autotriploid is AAA, autotetraploid is AAAA and so on.
Autopolyploids are vigour and large sized. Autotriploids are sterile and does not produce seeds.
Allopolylploidy
The polyploidy that arise due to multiplication of chromosome number in a hybrid is called
allopolyploidy and the resultant species is called allopolyploid. The F1 hybrid produced from
different species are sterile (hybrid sterility). The sterile hybrid become fertile when the
chromosome number is doubled after colchicine treatment and a new species is produced. Eg.
Raphanobrassica, Triticum aestivum etc.
Raphanobrassica
Karpechenko (1926) crossed radish (Raphanus sativus, 2n=18) and cabbage (Brassica oleraceae,
2n=18) and obtained diploid sterile hybrids. But some plants produced diploid male and female
gametes that fused to form fertile plants Raphanobrassica. On cytological analysis the fertile
plants were found to have allotetraploids (2n=36).
Role of polyploidy
Polyploidy is the source of variation and help in evolution.
Polyploidy results in hybrid vigour (heterosis- improved quality of hybrids).
Seedless varieties of tomatoes, grapes etc have been produced by triploids.
Disease resistant and high yielding varieties of wheat, paddy are produced by
allopolyploidy.
Polyploidy is used in obtaining fodder plants as the leaves, flowers and fruits are large in
size.
Role of Mutation
Mutation causes change in structure of an organism. Such changes are undesirable and non-
functional. Mutation result the change in sequence of nitrogenous bases in DNA which result
change in codon. Change in codon leads to the formation of different polypeptide chain which
may or may not have desired functions.
