MUTATION

MUTATION

Sudden heritable change in genetic material of an organism is called as Mutation.

Mutation are source of discontinuous variation.

Only those mutation are heritable which occur in germinal cell of an organism. While somatic mutations are non heritable. Somatic mutations are also heritable in vegatative propagated plants.

Mutation word was given by Hugo De Vries.

 De Vries studied mutations in the plant Oenothera lamarckiana (evening primrose).

Mutation was first observed by Seth Wright. He observed some short legged sheep (Ancon) variety in a population of long legged sheep.

Beadle and Tatum induced mutations in Neurospora by the help of U. V. rays. or X-rays.

Wild neurospora (PROTOTROPH)----U.V. Rays----Mutant Neurospora (AUXOTROPH)

Normal Neurospora can be grown in minimal medium because Neurospora can make all essential nutrients required for it. This is known as Prototroph.

Mutant Neurospora doesn't has capability to grow in minimal medium because due to mutation It loses those genes which codes for the enzyme that helps to prepare some special nutrients for it. They gave "one gene one enzyme" concept. This form is known as Auxotroph.

M.S. Swaminathan induced mutations in wheat by the help of γ-rays to obtain good varieties tor eg. Sharbati Sonora, Pusa Lerma. Swaminathan established     γ (gamma) garden in IARI-New Delhi (Pusa Institute).

Types of mutation :

i. CHROMOSOMAL MUTATION 

ii. GENE MUTATIONS

1. Chromosomal Mutations- Each species of organisms has constant number of chromosomes. There are two sets of homologous chromosomes in the somatic cells of an organisms. The number, type and sequences of genes on chromosomes is fixed and all the genes are oriented at their definite loci. The gene loci retain the same sequence they have had for many generations past.

Sometimes, during cell division, as a results of certain abnormalities, changes in structure and number of chromosomes are produced. Such type of change are called chromosomal aberration. These chromosomal aberration can be classified into following two classes:-

Types of chromosomal mutation :

1.Heteroploidy/Genomatic mutation-change in chromosome number.

2. Chromosomal aberration-change in structure of chromosome.

1. Heteroploidy / Genomatic mutationor ploidy- In different species of all the organism, the number of chromosomes remains fixed and stable throughout their life, but sometimes change in the number of chromosomes occurs. The Change in number of one or few chromosomes in a set or number of entire set of chromosome.

It is of two types:

Euploidy-Change in number of chromosome sits

Aneuploidy - Change in number of chromosome in a set

EUPLOIDY- Change in number of sets of chromosome  i.e. either loss or addition of sets of chromosomes.

Monoploidy (x) - Presence of one set of chromosomes.

Diploidy (2x) - Presence of two sets of chromosomes.

Polyploidy - Presence of more than two sets of chromosomes.

It may be : Triploidy (3x) Tetraploidy (4x) Pentaploidy (5x) Hexaploid (6x) Heptaploid (7x) octaploidy  (8x)

Polyploid plants with even number of sets are always fertile, reproduce sexually and form seeds.

Polyploid plants with odd number of sets are always sterile don't reproduce by sexual reproduction. They don't produce seeds but they may produce seedless fruits by parthenocarpy. eg. Banana and seedless grapes.

ANEUPLOIDY- Loss or addition of chromosomes in a set of chromosomes.

Types of Aneuploidy:-

1. Hypoaneuploidy (loss) 

2n-1=Monosomy - (loss of one chromosome in one set) 

2n-1-1=Double monosomy (loss of one chromosome from each set, but these are non homologous).

2n -2=Nullisomy (loss of two homologous chromosome) 

2. Hyperaneuploidy (addition) 

2n +1=Trisomy : addition of one chromosome in one set.

2n +1 +1=Double Trisomy : addition of one chromosome in each set

2n +2=Tetrasomy: addition of two chromosome in one set.

• Cause of aneuploidy is chromosomal nondisjunction means chromosomes fail to separate during meiosis.
• Chances of aneuploidy are more in higher age female due to less activity of oocyte, so chances of syndrome increase in children who are bom from higher age female.

2. Chromosomal Aberrations : Change in structure of chromosome. The change in the number or orientation of genes located on chromosomes is called structural chromosomal abberation. Such types of changes are produced mainly as a results of some irregularities occured during meiotic divisons. Such irregularities include breaking, reattachment, and rearrangement etc. Structural changes in the chromosomes are of following four types:-

(i) Deletion : Loss of a part or segment of chromosome which leads to loss of some gene is called as deletion.

It is of 2 types :

(A) Terminal deletion -Loss of chromosomal segment from one or both ends.Eg. The cry-du-chat syndrome is an example of terminal deletion in short arm of 5th chromosome.

(B) Intercalary deletion - Loss of chromosomal part between the ends.

(ii) Inversion : Breakage of chromosomal segment but reunion on same chromosome in reverse orders. It leads to change in distance between genes on chromosome or sequence of genes on chromosome so crossing over is affected

It is of 2 types:-

1. Paracentric -If inversion occur only in one arm and inverted segment does not include centromere.

2. Pericentric - In this type of inversion inverted segment include centromere.

(iii) Duplication : Occurence of a chromosomal segment twice on a chromosome.

Example : In drosophila "Bar eye character" is observed due to duplication in X-chromosome. Bar eye is a character whose eyes are narrower as compared to normal eye shape.

(iv) Translocation : In this, a part of the chromosome is broken and may be joined with non homologous chromosome. This is also known as illegitimate crossing over (illegal crossing over).

Types of translocation

(A) Simple Translocation- When a chromosomal segment breaks and attached to the terminal end of a non homologous chromosome.

(B) Reciprocal Translocation- exchange of segments between two non homologous chromosome.

In man, STRUCTURAL CHROMOSOMAL ABERRATIONS.

In man, many examples of chromosomes breakage with resulting aberration have been found. 

For example, a terminal deletion of a part of long arm of chromosome 21 in man causes Chronic granulocytic leujemia. Since this shortened chromosome was discovered in Philadelphia, it is known as the Philadelphia chromosome. Similarly deletion of a part of short arm of chromosome 5 in man causes undeveloped larynx  in children as a result of which they produce mewing sound. Because of this it is called as Cri-du-chat or cat cry syndrome.

Chronic myeloid leukemia (C M L) is a type of blood cancer. This disease is a result of reciprocal transiocation between 22 and 9 chromosome.

Note: If exchange of segments take place in between homologous chromosomes then it is called crossing over.

(2) GENE MUTATION OR POINT MUTATION

Two types:-

1. Substitution 

2. Frameshift mutation.

A. Substitution:

Replacement of one nitrogenous base by another nitrogenous base is called as substitution.

 It causes change in one codon in  genetic code which leads to change in one amino acid in structure of protein. eg. Sickel cell anaemia.

Change may not occur some time because for one amino acid more than one type of codons are present

Substitution is of two types

1. Transition- Replacement of one purine by another  purine or replacement pyrimidine by another pyrimidines.

2. Transversion- Replacement of purine by pyrimidine or pyrimidine by purine is called transversion.

B. Frame shift mutation/Gibberish mutation:

Loss or addition of one or rarely more than one nitrogenous bases in structure of DNA.

Frame shift mutation is of two types.

1. Addition-Addition of one or rarely  more than one nitrogenous bases in structure of DNA.

2. Deletion- loss of ons or rarely more than one nitrogenous bases in structure of DNA.

 Due to frame shift mutation complete reading  of genetic code changed. It leads  to change in all amino acids in structure of proterin so a new protein is formed which is completely different from previous protein.

 So frameshift mutations are more harmful as compared to substitution.

 eg: Thalassemia (lethal genetic disorder)

MUTAGENS- 

Mutagens are those substances which cause mutations.

Non ionising:- U.V. Rays.

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