Mutation -types, chemical and physical mutagens

Mutation:-

•     Definition:- When there is an accidental and heritable change in a character of a plant, it is called mutation.

•    This occurs due to 2 reasons -

i. Change in structure of gene

ii. Chromosomal aberrations

•     Word “Mutation”:- Word "Mutation" was given by Hugo de Vries in 1910.

•     Discovery:- The mutation was discovered by T. H. Morgan in 1910 when he observe the White Eye mutant of Drosophila.

Characteristics:-

•      Mutated alleles are recessive.

•     Mutation is a random event.

•     It depends on coincidence.

•     Most mutations are harmful. But only 0.1% mutations are beneficial which can be used in plant breeding.

Types:-

i. Spontaneous mutation:-

Ø  In nature, spontaneous mutations occur automatically.

Ø  Different genes exhibit different rates of mutation.

Ø  Some genes exhibit high mutation rates known as mutable genes.

ii. Induced mutation:-

Ø  Mutations can be induced by treating cells with mutagens during the S-phase of the cell cycle.

Ø  The environment also affects the rate of induced mutation.

Point Mutation:- A change of a nucleotide in the coding part of DNA is called point mutation. There are two types of nitrogenous bases found in DNA - purines (A, G) and pyrimidines (C, T).

It is of 6 types:-

a. Silent mutation:-

b. Neutral mutation:-

b. Missense mutation:-

c. Nonsense mutation:-

d. Read through mutation:-

e. Frame Shift mutation:- Insertion (n+1) or Deletion (n-1)

i. Pre-insertion or Pre-deletion:- It occurs  in the starting base sequence of the gene. It is more lethal because it change almost all the amino acids in the protein.

ii. Post-insertion or Post-deletion:- It occurs in the last base sequence of the gene. It is less lethal because it changes only a few amino acids in the protein.

Mutagens:-

•    Definition:- Physical or chemical factors that induce mutation are called mutagens.

•      Types:- There are 2 types of mutagens -

a. Physical mutagens

b. Chemical mutagens

a. Physical mutagens:- These are of 2 types -

i. Ionizing radiations:- Which convert atoms into cations or anions. These reactive ions damage the bases and sugars.

Examples: - X - rays, gamma rays, neutrons, α - particles, β - rays.

ii. Non – ionizing radiations:- Which do not ionize atoms. They form thymidine dimers.

Example: - UV rays

b. Chemical mutagens:- These are of many types -

i. Alkylating agents:- These chemicals transfer the alkyl group to DNA. This transfer occurs at the N-ring of Guanin.

Examples:-

EMS (Ethyl Methane Sulfonate),

MMS (Methyl Methane Sulfonate),

EI (Ethylene Imine)

ii. Intercalating agents:- The molecules of these chemicals enter the space between the any two bases of DNA. This process is called intercalation.

Examples:- Acriflavin, Proflavin, Acridine orange, EtBr (Ethidium Bromide)

iii. Base analogues:- The molecules of these chemicals are very similar in structure to DNA bases. Thus, they are easily incorporated into DNA during DNA replication.

Examples:-

5 – BU (5 – Bromo Uracil),

5 – CU (5 - Chloro Uracil)

iv. Other Chemicals:- Some other chemicals are also used as mutagens.

Examples:-

Nitrous Acid = HNO₂

Sodium Azide = NaN₃

Hydroxyl Amine = NH₂OH

Note:- Nitrites are used as a preservative in European countries. It is sprayed on meat. But they can convert to HNO2 which causes mutations.

Optimum Dose:- The amount of mutagen that produces the lowest mortality and maximum mutation rate in plants is called the optimum dose.

Example:- LD50 = It produces 50% mortality in plants. (LD = Lethal Dose)