Mutation

Mutation

The term 'mutation' was introduced by Hugo De Vries in 1901 for the remarkable variation in the plant called Oenothera lamarckianai:e Evening primrose.

The sudden heritable change in amount or content and organization of genetic material of organism that results in the change in appearance is called mutation. The smallest segment of DNA that helps in mutation is called muton.

Mutagenic agents-

Any factors, elements or chemicals or agents which can cause mutation or increase the frequency of mutation in an organism are called muta-genes or mutagenic agents. On the basis of mutagens, mutation is of two types:

• Spontaneous mutation- The mutation that occurs naturally due to intracellular factors like errors in the replication, different chemicals produced inside the cell, is called spontaneous mutation.
• Induced mutation- The mutation the occurs by different environmental factors like biological (eg-viruses), physical mutagens (eg- radiations like U.V rays, X-rays e.t.c and temperature) and chemicals mutagens (eg- Nitrous acid, ethyl methyl sulphonate, hydrogen peroxide, mustard gas e.t.c) is called induced mutation.

Types of mutations-

Broadly mutation is classified into two types:

• Gene Mutation ( Micro mutation or point mutation)
• Chromosomal mutation ( Macromutation)

A) Gene mutation-

The sudden heritable changes occurring in the genes or the genetical constituent of an organism is called gene mutation. It usually occurs during DNA replication, so it is also called error mutation. It is of three types:

1. Substitution mutation
2. Frameshift mutation
3. Nonsense mutation

1) Substitution mutation- In this mutation, a nitrogen base of a nucleotide is changed with another nucleotide. It is of two types-

⇒ Transition:- When purine base [A] is replaced by another purine base [G] or a pyrimidine base [T] with a pyrimidine base [C], it is called transition mutation.

A↔ G

T↔ C

⇒ Transversion:- This is a type of substitution mutation in which a purine base is replaced by a pyrimidine base or vice-versa.

A ↔C

T ↔ G

2) Frameshift mutation- The mutation which is caused by the addition or deletion of nitrogen bases in the DNA or mRNA is called frameshift mutation. This mutation changes the sequence of amino acids, as a result, new proteins are formed. It is of two types-

⇒ Addition mutation:- The point mutation which is caused by the addition of one base pair to a gene is called addition mutation. All the triplets after the insertion are affected. The protein after this change formed is completely different and will not work as normal.

Normal chromosome - GAT GAT GAT GAT

↑G added

New message - GAT {GGA TGA TGA} new triplets

⇒Deletion mutation:- The point mutation caused by the loss of one base pair in a codon of a gene is called deletion mutation. All the codons after the deletion are affected. The protein so formed will not work like the original.

Normal chromosome - GAT GAT GAT GAT

↓G deleted

New message - GAT {ATG ATG ATG} new triplets

3) Nonsense mutation-It takes place due to the conversion of sense codon into nonsense codon which stops the synthesis of protein. Eg: AUG (sense codon)↔ UGA (nonsense codon).

B) Chromosomal mutation-

This mutation is caused by the change in structure and number of chromosomes, correspondingly it is of following two types:

1. Structural changes in chromosome (Chromosomal abrration]
2. Genomic mutation (change in number of chromosomes)

I) Structural changes in chromosomes-This type of mutation are caused by either: a) intrachromosomal or

b) interchromosomal modification.

a) Intrachromosomal modification- It occurs by the change in the structure of homologous chromosomes. This type of modification is called intrachromosomal mutation. It is taken place by following:

⇒Deficiency:- The loss of terminal or intercalary segment from a chromosome is called deficiency. It is caused by a single break or double break and reunion of the segments of the chromosome. This carries out the mutation in the organism.

↓ E, F-Deletion

New-chromosome

⇒Duplication:- This is the addition of an extra piece of a chromosome in one of the homologous pair of chromosome. Duplication is less harmful to individuals but meiosis is abnormal and abnormalities are developed in the body of organisms.

↑D Added

⇒Inversion:-A chromosome segment becomes inverted in this mutation. Inversion is caused by the breakage at the point of intersection of a chromosome loop and reunion with two segments. It produces abnormal meiosis and abnormal gametes.

b) Interchromosomal mutation- This mutation takes place by changing the structure of two non-homologous chromosomes by translocation. The exchange of chromosome segment between non-homologous chromosomes is called translocation. It is of two types-

⇒Recriprocal translocation:-

There is an exchange of chromosome segments between two non-homologous chromosomes in reciprocal translocation.

⇒Non-recriprocal translocation:- A part of the chromosome is translocated from one non-homologous chromosome to other so that one chromosome becomes deficient and another non-homologous chromosome gains the piece of the chromosome and becomes long.

II) Genomic mutation-The chromosome number is one of the most important characters that differentiates one species from another. The genomic mutation is defined as the process of changing the number of chromosomes. It is also called ploidy. It is of two types-

a)Aneuploidy:- It is the process

b)Euploidy

a) Aneuploidy:- It is the process of loss of one or more chromosome from a diploid set of chromosome. Such organisms in which aneuploidy phenomena takes place are called aneuploids. They arise due to the failure of separation of the homologous chromosome during meiosis and results the formation of abnormal gametes. They are two types-

⇒Hypoploidy:- It is the process of loss of one or more chromosome from the diploid set of chromosome. They are:

2n-1= monosomic

2n-1-1= double monosomic

2n-2= nullisomic

⇒Hyperploidy:- It is the process of addition of one or more chromosome in the diploid set of chromosome. It is of different types:

2n+1= Trisomic

2n+1+1= Double trisomic

2n+2= Tetrasomic

Aneuploidy causes high mortality rate, reduces the fertility power, mental retardation, swelling of tongue and eyelids (Turner's syndrome), abnormal body growth, reduction of immunity system e.t.c.

b) Euploidy:- The process in which the loss or addition of the complete set of the chromosome from a diploid set of the chromosome is called euploidy. It is of two types:

⇒Haploidy-Haploidy or monoploidy is defined as the process of loss of one complete set of the chromosome from the diploid set of the chromosome, as a result, the offspring comes to have the half set of the chromosome (2n-n=n). It is responsible for the smaller size of the body of the organism. Example- Male wasp or male bee.

⇒Polyploidy:-It involves the addition of one or more sets of chromosomes in diploid sets of chromosomes. Depending upon the number of sets of chromosome present in the polyploids, they are known as-

3n=Triploid

4n=Tetraploid

5n=Pentaploid e.t.c.

Significance of mutation

1) It generates the variation in a natural population of the organisms.

2) It helps in genetic analysis in T₄ bacteriophage.

3) It is a raw material for evolution.

4) Inversion mutation suppresses the process of crossing over.

5) Reciprocal, translocation helps to prevent the expansion of lethal gene.

6) Induced mutation is used in plant breeding.

7) New plants can be created by mutation.

8) It can be useful to the organism in order to adjust the changed environment.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.