MUTATION mutants 1. Point mutants are leaky mutants.

MUTATION –
PART II

 

NONSENSE MUTATION:
Premature chain termination of the protein is caused by the non-sense mutation.
The nonsense mutant codons are- UAG (amber); UAA (ochure) and UGA (opal). No
such corresponding species of tRNA is observed in this case. The binding of any
tRNA is not stimulated by the UAG; UAA and UGA.

The prematured terminated protein or peptide fragment would
completely lose its activity in the nonsense mutation. After that, it leads to
the lethal mutation. The mutation due to which the survival of organisms is
prevented is known as the lethal mutation.

 

Properties of Point mutants

1.        
Point mutants are leaky mutants. Leaky mutants
are the type of mutants which possess some residual level of gene expression or
function.

2.        
Nonsense, missense and silent mutation are given
rise by these mutants.

3.        
They possess a much higher reversion rate than
the insertion or deletion mutants.

 

Difference between the point mutation and frame shift
mutation

POINT MUTATION

FRAME SHIFT
MUTATION

1.        
Occur due to the substitution of base pair.

1.        
The insertion or deletion of single base or
more than one base gives rise to the frame shift mutation.

2.        
Reversion rate is high.

2.        
Rate of reversion is low.

3.        
Chemical mutagens are able to increase the
frequency of the point mutation.

3.        
Chemical mutagens do not effect the frame
shift mutation.

4.        
Examples are base analogs like 5-bromo uracil
and chemical mutagens like HNO2. These cause the point mutation.

4.        
Frame shift mutation is caused by the
intercalating agents like acridine orange.

 

Spontaneous mutation: Spontaneous mutation can be
observed in all organisms. It is a certain type of mutation which is obtained
as a result of some normal cellular operations like replication or
recombination or by some random interactions with the environment.

It can be caused by UV irradiation. Covalent bonds between
two adjacent base can be introduced by this UV irradiation and results in the
formation of intrastrand thymine dimer in DNA.

 

Induced mutation: Treatment with some chemical agents
or compounds can increase the rate of the induced mutation. These chemical
compounds are known as mutagens and the changes which are caused by them is
known as the induced mutations.

 

Mutagens: They can be classified in different groups.
These are as follows,

A.       
Deaminating agents like HNO2.

B.       
Alkylating agents like dimethyl sulphate;
nitrogen mustard and ethylnitrosourea.

C.       
Acridine dyes like actinomycin.

D.       
Base analogs like 2-amino purine; 5-bromo
uracil.