BOTANY Higher Secondary Second Year - Textbooks Online

Point or gene mutation
Point mutation is sudden change in small segment of DNA either a
single nucleotide or a nucleotide pair. Gross mutation is a change involving
more than one or a few nucleotides of a DNA.
The gene mutation may be caused by loss or deletion of a nucleotide
pair. This is called deletion mutation and reported in some bacteriophages.
Addition of one or more nucleotides into a gene results in addition
mutation. Replacement of certain nitrogen bases by another base in the
structure of DNA results in substitution mutation. The deletion and addition
mutation alter the nucleotide sequence of genes and ultimately result in
the production of defective protein and this leads to the death of the
organism. The substitution mutations can alter the phenotype of the
organism and have great genetic significance.
There are two types of substitution mutations – transition and
transversion. When a purine or a pyrimidine is replaced by another purine
or pyrimidine respectively this kind of substitution is called transition.
When a mutation involves the replacement of a purine for pyrimidine or
viceversa this is called transversion.
Mutagenic agents
The chemical substances and environmental conditions which cause
mutations in the organisms are called mutagens or mutagenic agents.
There are two kinds of mutagenic agents – physical and chemical mutagenic
agents.
Physical mutagenic agents
Electromagnetic radiation, radiations like α, β and γ, ultraviolet rays,
temperature, etc. are some of the examples for physical mutagens. X-rays
and gamma rays are ionizing radiations which induce mutation in seeds.
UV rays are nonionizing radiations. Pollen can be treated with UV since
pollen has germinal nucleus in which mutation can be caused.
Chemical mutagenic agents
Chemicals can also be used for inducing mutations in the organisms.
Such chemicals are called chemical mutagenic agents.
eg. Nitrous acid, Methyl methane sulphonate (MMS) and
ethyl methane sulphonate (EMS). Ethyl methane sulphonate
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