Mutation and its types

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Mutation: Chromosomal aberration

Hello readers, another article is ready for you.Now it is on an important topic of zoology, Mutation and its types.

In this article I will cover:

  • Brief history of Mutation
  • Father of Mutation
  • Definition
  • Cause
  • Types
  • Turners Syndrome
  • Cri-du-chat Syndrome
  • Down Syndrome

History :

Earlier in 1869,in Darwin’s Theory,sudden appearance of new heriditory character recorded by Plant and animal breeders and referred to them as ‘sports’.

The earliest record of point mutation dates that to 1791,when Seth Wright noticed a lamb with usually short legs in his flock of ship.In the successive generations this trait was transferred and a line was developed,where all ship had short leg.This character resulted from successive mutation.

Hugo De Vries(1901) proposed Mutation Theory to explain the origin of new species in his book ‘Mutation Theory‘ vol-1, originally written in German.

Father of Mutation Theory:

Hugo De Vries is considered to be the father of Mutation Theory.

What is mutaton?:

Mutation is a abrupt, stable and inheritable change in the nuclear DNA leading to the change in phenotype. But now it is used to cover only those changes which alter the chemical structure of the gene at the molecular level.

Causes of Mutation:

Mutation may be caused by two ways.

  1. Spontaneous
  2. Induced

Spontaneous Mutation occurs due to error during DNA replication.

Induced Mutation is caused by some factors listed below:

  • Radiations like UV,X-Ray,Gamma rays,alpha particles etc.
  • Chemical mutagen like Nitrous acid, Hydroxylamine,Nitrosamine on tobacco,Acridine dye,benzene etc
  • High temperature is also capable of causing mutation.

Types of Mutation:

Generally mutation is of two types;

  1. Gene mutation
  2. Genome mutation

Point Mutation:

Mutation that occurs within a single gene is called point or gene mutation.

For this,changes happened in a single locus of a chromosome. This is cytologically invisible.

Gene mutation is generally caused by two ways:

  • Base substitution
  • Frameshift mutation

Base Substitution:

When one base pair is replaced by another base pair,the changes are referred to as base substitution. Two(2) formal terms are often used to describe nucleotide substitution….

  • Transition mutation
  • Transversion mutation

Transition Mutation:

Transition mutation involving a change from one purine to another purine or one pyrimidine to another pyrimidine.

The four types of transition are: A<->G or C<->T

Transversion Mutation:

When the base pair substitution involves a change from a purine to pyrimidine or a pyrimidine to purine,it is called Transversion . The eight types of transversion are : A<->T, G<->C, A<->C, G<->T.

According to change in DNA sequence there are many types of mutations. These types of mutations are described below:

Missense Mutation:

A base replacement aur substitution that alters the sense of a codon resulting replacement of one amino acid with another.

Example: AAG(Lysine)-> GAG(Glutamic Acid)

Nonsense Mutation:

A base substitution that changes the sense of a codon into a termination or stop codon.

Example: AAA(Lysine)-> UAA(Stop codon)

Silent Mutation:

A Mutational change in codon that results into the change of an amino acid by same amino acid.

Example: AAA(Lysine)-> AAG(Lysine)

Frameshift Mutation:

Mutations that result from the addition or deletion of one or more base pairs in a gene are called frameshift mutations.

such a change will cause all the subsequent three letter words (codon) to be changed, i.e. the frame of reading in mRNAs will be altered. A frameshift mutation will occur when any number of bases are added or deleted except in multiple of 3, which could re establish the initial frame of reading.

Genome Mutation:

A change in the organisation of a chromosome i.e. changes either in the number or morphology of chromosome by deletion or duplication of genes or segments of chromosome and rearrangements of genetic material either within or among chromosomes are called chromosomal or genome mutation or aberrations.

Genome mutation takes place by two ways: I) Numerical changes II) Structural changes.

Different types of Genome Mutation
Different types of Genome Mutation.

Numerical Changes:

It is of two types:

  • Aneuploidy
  • Euploidy

Aneuploidy:

Mutations which involve changes in the number of any single chromosome within a set is known as aneuploidy.

It is of 4 types:

1) Monosomy:

The loss of a single chromosome from diploid genome is called monosomy. It produces a (2n-1) complement.

Partial Monosomy:

When only part of one chromosome is lost it is said to be partial monosomy or segmental deletions.

Double monosomy:

When two different or non-homologus chromosomes are lacking from the diploid set of chromosome, it is called double monosomy. The genomic formula will be (2n-1-1).

2)Nullisomy:

When a pair of homologous chromosomes are missing from the diploid set of chromosome it is called nullisomy. The gametic formula will be (2n – 2).

3)Trisomy:

When an extra chromosome is present in addition to the normal diploid condition, trisomy is formed.The genomic formula will be (2n+1).

Double Trisomy:

When two non-homologous chromosomes are added to the normal diploid set of chromosome, double trisomy results.The genomic formula will be (2n+1+1).

4) Tetrasomy:

When a pair of homologous chromosome is added to the normal diploid set of chromosome it is called tetrasomy.The genomic formula will be (2n+2).

Euploidy:

Mutations that involve changes in the number of entire set of chromosome are called euploidy. It is of three types: a) Haploidy b) Diploidy c) polyploidy.

polyploidy can be classified once again into a) autopolyploidy and b)allopolyploidy.

Autopolyploidy:

When the sets of chromosome in a polyploid are derived from the same species it is called autopolyploidy.

Allopolyploidy:

When a polyploid is produced from two different species it is called allopolyploidy.

Structural Changes:

Variation in the structure of chromosomes include

  1. Deletion
  2. Duplication
  3. Inversion
  4. Translocation

1) Deletion:

If a chromosome breaks in one or more places and a portion is lost,it is termed as deletion or deficiency.

It is of two types_ I) Deletion occuring near one end of the chromosome is known as Terminal Deletion. II) Deletion occuring in the interior part of the chromosome is called Intercalary Deletion.

2) Duplication:

A chromosomal changes that results in the doubling of a segment of a chromosome is called duplication.

3) Inversion:

A chromosomal aberration in which a segment of a chromosome is excised and then reintegrated in an orientation 180 degree from the original orientation,is known as inversion.

It is of two types: pericentric and paracentric.

4) Translocation:

A chromosomal aberration involving a change in the position of a chromosome segment and the gene sequence it contains, is called translocation.

Turner’s Syndrome: Monosomy of one X chromosome from female produces Turner’s syndrome.

Signs:

  • Turner females have female like external genitalia but are nevertheless chromatinegative. Their ovaries do not develop properly and represented by fibrous streaks. Primordial follicles are usually absent. Uterus is small
  • Underdeveloped breast consisting mostly of fat.
  • Short stature; broad, shield like chest with widely spaced nipples.
  • Colour blindness, diabetes mellitus and congenital pyloric stenosis are higher in Turner female than normal women.
  • Webbed neck and a low posterior hairline is seen.

Cri-du-chat Syndrome: in man partial monosomy of one X chromosome producers Cri-du-chat syndrome.

Signs:

  • Individuals with cri-do-chat syndrome show abnormal development of glottis and larynx. As a result the infant has a cry similar to the meowing of cat, thus giving the syndrome its name.
  • Malformed anatomy with gastrointestinal and cardiac complication.
  • Mental retardation are common in the affected individuals.
  • Unique facial features include widely spaced eyes with epicanthic folds.

Down’s Syndrome: in man trisomy of chromosome 21th produces mongolism or down syndrome.

Signs:

  • Individuals with down syndrome have a prominent epicanthic fold in the corner of each eye.
  • They are short with dull and happy looking.
  • They have flat, round heads; protruding furrowed tongues which cause the mouth to remain partially open.
  • Both physical and mental development is retarded.
  • No sexual maturity. Testes are small and undescended in male and in females labia majora tend to be large while labia minora are small or absent.
  • Down children are prone to respiratory disease and have malformed heart.

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