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I.1.7, 9 Mutation

Mutation

Sudden Heritable change in genetic material of an organism is called as Mutation. Mutation are discontinuous source of variation. Mutation word was given by Hugo De Vries. Frequency of mutation at present is 1 cell in : 1 million-cell. De Vries gave (proposed) mutation theory of evolution. Credit of discovery of mutation is given to Morgan.

According to De Vries there are two types of variations in evolutions.

  1. Continuous variations :
    1. The variations are develop in every generation of an organism.
    2. These variations are developed by crossing over/meiosis/sexual reproduction.
    3. Only minor variation are developed by crossing over.
  2. Discontinuous variations :
    1. These variations are developed by mutations
    2. They are sudden in nature
    3. Can occur in any generation

Mutator gene and Mutable gene :

  • Gene which induce mutation in another gene is called mutator gene and in which mutation is induced is called as mutable gene.

Neutral Mutation\Suppression :

  • Mutation in one gene is neutralized by mutation in another gene called as neutral mutation. So it means mutation without effect and in neutralization two mutations are required.

Hot Spot :

  • Place on DNA or gene where frequency of mutation is high.

Muton (unit of mutation) :

  • Smallest part of DNA which undergoes mutation, It is one nucleotide.

Types of mutation :

  • I. Chromosomal Mutation
  • II. Gene Mutation

I. CHROMOSOMAL MUTATIONS- Change in number or structure of chromosome.

Types of chromosomal mutation :

  1. Heteroploidy/Genomatic mutation – change in chromosome number
  2. Chromosomal aberration – change in structure of chromosome.
  1. Heteroploidy/Genomatic mutation : Change in number of sets or chromosomes in sets. Two types-
    • a. Euploidy – Change in number of sets.
    • b. Aneuploidy -Change in number of chromosome in set.
  • a. Euploidy : Change in number of sets/loss or addition of sets of chromosomes.
      • i. monoploidy- Loss of one set (2n – n = n)
      • ii. polyploidy- Addition of set

        • Addition of one set – Triploidy
        • Addition of two set – Tetraploidy
        • Ad dition of three set – Pentaploidy
        • Addition of four set – Hexaploidy
      • Polyploidy is of two types :
        1. Autopolyploidy :It is repetition of same set of chromosomes.
        2. Allopolyploidy : It is repetition of  more than one set of chromosomes.
  • b. Aneuploidy : Loss or addition of chromosomes in sets of chromosomes.
    • i. Hypoaneuploidy -Loss of chromosomes in sets of chromosomes.
      1. monosomy:- (loss of one chromosome in one set) 2n – 1
      2. double monosomy (loss of one chromosome from each set, but these are non homologus.) 2n – 1 – 1
      3. Nullisomy (loss of two homologus chromosome)2n – 2
    • ii. Hyperaneuploidy- Addition of chromosomes in sets of chromosomes.
      1. Trisomy: addition of one chromosome in one set. 2n + 1
      2. Double Trisomy: addition of one chromosome in each set. 2n + 1 + 1
      3. Tetrasomy: addition of two chromosome in one set. 2n + 2 =

Cause of aneuploidy is chromosomal non disjunction 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 born from higher age female.

2. Chromosomal Aberrations :

a. 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:-

i. Terminal deletion- Loss of chromosomal segment from one or both ends. e.g., The cry-du-chat syndrome is an example of terminal deletion in 5th chromosome.

ii. Intercalary deletion- Loss of chromosomal part between the ends.

b. 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 :

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

ii. Pericentric – In this type of inversion inverted segment include centromere.

c.Duplication : Occurrence of a chromosomal segment twice on a chromosome. If in this segment any recessive gene is present, then it given it’s expression due to homozygous condition. If in this segment any recessive but lethal gene is present, it lead to death of organism.

d. Translocation : A part of the chromosome is broken and may be joined with non homologous chromosome. This is also known as Illegitimate crossing over (illegeal crossing over). Three types of translocation-

      • i.Simple Translocation : When a chromosomal segment breaks and attached to the terminal end of a non-homologous chromosome.
      • ii. Interstitial or shift translocation : If a segment of chromosome breaks and gets inserted in interstitial position of a non homologous chromosome.
      • iii. Reciprocal Translocation : Exchange of segments between two non-homologous chromosome. e.g., Chronic myloid leukemia [C M L] is a type of blood cancer. This disease is a result of reciprocal transiocation between 22 and 9 chromosome.

II. GENE MUTATION OR POINT MUTATION- Two types : –

  1. Substitution
  2. Frame shift mutation

1. 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. e.g., Sickle cell anaemia.

Change may not occur because for one animo acid more than one type of codons are present. Substitution is of two types :

    • (a) Transition
    • (b) Transversion.
    • a. Transiton : Replacement of one purine by another purine or replacement of pyrimidine by another pyrimidine.

Methods of Transition :- 1 . By Tautomerisation 2. By Ionisation 3.By Base Analogues

    • b. Transversion : Replacement of purine by pyrimidine or pyrimidine by purine is called transversion.

EMS , Ethyl methane sulphonate, MMS , Methyl methane sulphonate causes depurination means they remove one purine from structure of DNA. So a gap is formed. If this gap is filled by another purine then it is called as transition. But if this gap is filled by pyrimidine then it is called as transversion.

2. Frame shift mutation/Gibberish mutation : Arredine, proflavin causes loss or addition of one or two nitrogenous bases in structure of DNA so complete reading of genetic code is changed. It leads to change in all animo acids in structure of protein so a new protein is formed which is completely different from previous protein. So frame shift mutations are more harmful as compared to substitution. eg. Thallesemia (lethal genetic disorder)

Mutagens : Mutagens are those substance which cause mutations : –

1. Radiation : –  Radiations mainly cause chromosomal aberrations which cause major change in organisms. So chromosomal mutations are more important in evolution.

    1. Ionising:-  X-ray
    2. Non ionizing:- U.V. rays

2. Chemical mutagens : Chemical mutagens are more harmful than radiations because body is not protected against chemicals. Source of chemical mutagens are food, air and water. Effect of radiation is localized, while chemical mutagens spread in complete body through blood circulation and when they reaching gonads they cause germinal mutation. Chemicals also cause chromosomal mutations.eg.  Carbon tetra sulphide, Nitrous acid (HNO2) Organic peroxide, Ethyl urethane, Pesticides etc. DDT (Dichloro Diphenyl Trichloro Ethane), LSD (Lysergic acid diethylamide), Mustard gas (first identified Chemical Mutagens)

3. Antibiotics :Also causes  mutations.

 

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