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Mutation

by Ramneet Kaur
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Mutation

Mutation is a sudden inheritable variation.

  • Term given by Hugo de Vries while working on the plant Oenothera lamarckiana (Evening Primrose).

  • 1st experimental evidence on mutation was given by Morgan while working on eye color in Drosophila. White eye color is due to a recessive mutation having occurred on the X-chromosome.

Mutations are of 2 types: – Chromosomal mutations and Gene mutations.

Chromosomal mutations:

It includes chromosomal aberrations and genomic mutations.

Chromosomal aberrations change in chromosome structure.

They are 4 types: Deficiency, Duplication, Translocation, and Inversion.

Deficiency:

It is the loss of certain genes from a haploid genome. It may be terminal or interstitial. E.g.,

  • Notched wing in Drosophila is due to deletion of a segment in X-chromosome.

  • Cri-du-chat syndrome in humans is due to deletion of a part of the short arm of chromosome 5.

    deletion

Duplication:

Certain genes are present twice in a haploid genome.

  • The duplicated genes are attached to its homologous chromosome or to a non-homologous chromosome. E.g.,

  • Bar eye Drosophila is due to B gene for the normal eye being present in duplicates in the X-chromosome.

    duplication

Translocation:

Means position of certain genes has been changed in a haploid genome.

It is of 2 types: simple and reciprocal.

  • In reciprocal translocation exchange of segments occur between non-homologous chromosomes.

  • Also referred as illegitimate crossing over.

  • Complications occur during meiotic pairing.

  • E.g., Chronic myeloid leukemia is caused due to translocation of a part of chromosome 22 to chromosome 9.

    translocation

Inversion:

An in-between part of a chromosome is attached in reverse order.

  • Inversion prevents crossing over.

    inversion

Genomic mutations change in chromosome number.

It is of 2 types: Aneuploidy and Euploidy.

Aneuploidy is addition or loss of chromosomes.

It occurs due to non-disjunction of chromosomes.

  • Monosomic: (2n-1) one chromosome is without its homologue. E.g., Turner’s Syndrome in human female   (44 + X).

  • Nullisomic: (2n-2) complete pair is missing. They do not survive.

  • Trisomic: (2n +1) one chromosome is in triplicate.

     E.g., Down’s Syndrome in man, chromosome 21 in triplicate.

      Klinefelter’s Syndrome in human male (44 + XXY).

       Patau’s Syndrome in man, chromosome 13 in triplicate.

Euploidy: increase or decrease in the number of genomes.

It is of 2 types haploidy and polyploidy.

  • Haploidy: only one genome instead of two.

  • Polyploidy: more than 2 genomes are present. It is very common in plants. It is of 2 types: autopolyploid and allopolyploid.

Autopolyploid: it is increasing in the number of the same genome.

 E.g., banana is Autotriploid (AAA)

Allopolyploid: done through hybridization between two species followed by doubling of chromosomes.

E.g., Triticum aestivum (AABBDD),

Triticale produced from wheat and rye.

Gene mutations:

Changes occurring within a gene.

It includes substitution mutation and frame shift mutation.

Substitution: one nitrogenous base replaces the other.

It is of 2 types:

  • Transition: purine replaces purine and pyrimidine replace pyrimidine.

  • Transversion: purine replaces pyrimidine and pyrimidine replace purine.

  • e.g., the gene for Sickle Cell Anaemia in which 6th amino acid of β-chain of Haemoglobin. Glutamic acid is replaced by valine. As GAG (glutamic acid) is changed into GUG (valine) due to substitution.

    SUBSTITUTION

Frameshift mutation:

Addition/deletion of one or two bases shifts the reading of the genetic code from the point of addition /deletion.

  • Due to which the codon sequence changes and hence complete sequence of amino acid in a polypeptide chain changes.

  • E.g., Cystic fibrosis has occurred due to deletion of 3 nucleotides.

frame shift mutation

  • Missense mutation: any mutation due to which a single amino acid change occurs.

  • Nonsense mutation: any mutation due to which a codon coding for an amino acid changes into termination codon.

  • Silent mutation: any mutation that does not have any effect on the organism.

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