Point Mutation – Class 12 | Chapter – 5 | Biology Short Notes Series PDF

Point Mutation: It is a change in one base in the DNA sequence and can be caused by a variety of internal and external factors. Two types of point mutations can occur – transition and transversion mutations. Transition mutations occur when a purine is switched out with another purine, or when a pyrimidine is replaced with another pyrimidine. For example, if adenine is replaced with guanine. Transversion mutations on the other hand occur when a purine is replaced with a pyrimidine and vice versa.

Types of Point Mutation

It can cause a change in how DNA is read and thus impact protein synthesis. There are several ways in which DNA expression can be altered. Therefore, depending on how they do so, it is divided into three types:

  • Substitution Mutations: If the mutation occurs by the substitution of a nucleotide in the genome of an organism then it is known as a substitution mutation. It is further subdivided into three types:
    • Silent mutations: this occurs when the point mutation does not alter protein structure. This is because even though the point mutation persists, the protein synthesis process is undisturbed as the point mutation is still coded for the same protein structure.
    • Missense mutations: this occurs when the point mutation alters protein structure. This is because the point mutation resulted in coding for a different amino acid sequence in the protein to be expressed, causing a change in protein structure. An example of missense mutations is the manifestation of sickle cell disease, where the base “A” is replaced with a “T” causing abnormal protein synthesis.
    • Non-sense mutations: this occurs when the point mutation alters protein structure. This is because the point mutation resulted in the early termination of protein synthesis, leading to an unfinished product. A common example of nonsense mutations is cystic fibrosis, in which the CFTR gene does not produce a functional CFTR protein due to an early stop in protein production.
  • Insertion or Deletion Mutations: When an extra-base pair is added to the sequence of the amino acid then the insertion mutation occurs. If an extra-base pair is removed from the sequence of the amino acid then it is said to be a deletion mutation. These types of mutation are grouped together since they can affect the sequence of the amino acid drastically.

Diseases Caused by Point Mutations

  • Neurofibromatosis: Neurofibromatosis is caused by point mutations in the Neurofibromin 1 or Neurofibromin 2 gene.
  • Sickle-Cell Anaemia: Sickle-cell anemia is caused by a point mutation in the β-globin chain of hemoglobin, causing the hydrophilic amino acid glutamic acid to be replaced with the hydrophobic amino acid valine at the sixth position.
  • Tay–Sachs: The cause of Tay–Sachs disease is a genetic defect that is passed from parent to child. This genetic defect is located in the HEXA gene, which is found on chromosome 15.

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By Team Learning Mantras