Mutations

Unit : Gene expression and regulation

Chapter: Mutations

Reference: Mutation, Types of Mutations, The Type of Cell Involved, Chromosomal Mutation and Types

Learning objectives

  • To understand the types of mutation

Mutation

Mutation is a process that produces a gene or chromosome that differs from the wild type. The mutation may result due to changes either on the gene or the chromosome itself.

Broadly mutation is classified into:

  • Gene mutation where the allele of a gene changes.
  • Chromosome mutation where segments of chromosomes, whole chromosomes, or entire sets of chromosomes change.

Types of Mutations

There is various classification of different kind of mutations. Depending on:

The Type of Cell Involved

1. Somatic mutations

  • Mutations that are in the somatic tissues of the body.
  • Mutations are not transmitted to progeny.

2. Germinal mutations

  • Mutations that are in the germ tissues of the body.
  • Mutations may be transmitted to progeny

Mode of Origin

(1) Spontaneous mutations-The spontaneous mutations occur suddenly in the nature and their origin is unknown.

(2) Induced Mutations-Besides naturally occurring spontaneous mutations, the mutations can be induced artificially in the living organisms by exposing them to abnormal environment such as radiation, certain physical conditions (i.e., temperature) and chemicals.

Direction of Mutation

According to their mode of direction following types of mutations have been recognised:

1. Forward mutations

In an organism when mutations create a change from wild type to abnormal phenotype, then that type of mutations are known as forward mutations. Most mutations are forward type.

2. Reverse or back mutations

The forward mutations are often corrected by error correcting mechanism, so that an abnormal phenotype changes into wild type phenotype.

Size and Quality

According to size following two types of mutations have been recognized:

1. Point mutation

When heritable alterations occur in a very small segment of DNA molecule, i.e., a single nucleotide or nucleotide pair, then this type of mutations are called “point mutations”.

Deletion mutations– The point mutation which is caused due to loss or deletion of some portion (single nucleotide pair) in a triplet codon of a cistron or gene is called deletion mutation.

Insertion or addition mutation- The point mutations which occur due to addition of one or more extra nucleotides to a gene or cistron are called insertion mutations.

Frame shift mutations-The mutations which arise from the insertion or deletion of individual nucleotides and cause the rest of the message downstream of the mutation to be read out of phase, are called frameshift mutations.

Substitution mutation– A point mutation in which a nucleotide of a triplet is replaced by another nucleotide, is called substitution mutation.

Multiple mutations or gross mutations.

When changes involving more than one nucleotide pair, or entire gene, then such mutations are called gross mutations.

Chromosomal Mutation and Types

The changes in the genome involving chromosome parts, whole chromosomes, or whole chromosome sets are called chromosome aberrations or chromosome mutations

Structural changes in chromosomes:

1. Changes in number of genes

(a) Loss: Deletion which involves loss of a broken part of a chromosome.

(b) Addition: Duplication which involves addition of a part of chromosome.

2. Changes in gene arrangement:

(a) Rotation of a group of genes 1800 within one chromosome: Inversion in which broken segment reattached to original chromosome in reverse order.

(b) Exchange of parts between chromosomes of different pairs: Translocation in which the broken segment becomes attached to a non- homologous chromosome resulting in new linkage relations.

3. Changes in number of chromosomes:

A. Euploidy

  • It involves the loss, or gain, of whole chromosome set.
  • The term euploidy (Gr., eu = even or true; ploid = unit) designates genomes containing chromosomes that are multiples of some basic number (x).
  • The number of chromosomes in a basic set is called the monoploid number, x.
  • Those euploid types whose number of sets is greater than two are called polyploid.
  •  
  1. Polyploidy-Addition of one or more sets of chromosomes. They may be further:
  • Auto polyploidy. The auto polyploidy involves polyploidy, in which the same basic set of chromosomes are multiplied.
  • Allopolyploidy. The polyploidy results due the doubling of chromosome number in a F1 hybrid which is derived from two distinctly different species. The resultant species is called an allopolyploid
  1. Aneuploidy
  • It involves the loss, or gain, of a part of the chromosome set.
  • It refers to a condition in which one or a few chromosomes are added or deleted from the normal chromosome number. Hence, the number of chromosomes in aneuploidy can be greater or smaller than the number of chromosomes in the wild type.
  • Various types of aneuploidies can be identified as: nullisomy, monosomy, and trisomy.
  • Nullisomy (2n-2) is the loss of both chromosomes of the homologous pair. This condition may be lethal in most organisms.
  • Monosomy (2n-1) is the loss of a single chromosome of the homologous pair.
  • Trisomy is the gain of an extra chromosome (2n+1). Klinefelter syndrome (44+XXY/XYY) and Down syndrome are examples of trisomy

Solved examples

Example 1. Which of the following would result in a frameshift mutation?

     a) Insertions only b) Substitution only c) Deletion only d) Insertions and Deletions

Solution 1: a. Insertions and deletions would result in a frameshift mutation.

Example 2. What mutation has occurred here?

  T-G-A-C-C-A

  T-G-A-G-C-A

  1. Substitution b) Deletion c) Insertion   d) Frameshift

Solution 2: a. Here substitution mutation has occurred.

Summary  

  • Mutation is a process that produces a gene or chromosome that differs from the wild type. The mutation may result due to changes either on the gene or the chromosome itself.
  • Point mutations occur when heritable changes occur in a very tiny piece of DNA, such as a single nucleotide or nucleotide pair.
  • The addition of an additional chromosome (2n 1) is known as trisomy. Trisomy includes conditions such as Klinefelter syndrome (44 XXY/XYY) and Down syndrome.

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