Mutations and Their Freatures - Practice Questions & MCQ

• Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. 
• In addition to recombination, mutation is another phenomenon that leads to variation in DNA.
• According to the origin, mutations can be:
  • Spontaneous mutations: These occur randomly, naturally and automatically due to internal factors
  • Induced mutation: These are produced in response to specific external factors and chemicals
• The smallest part of a gene that can undergo mutation is known as muton. A muton can be as small as a nucleotide.
• Most of the gene mutations involve a change only a single nucleotide or nitrogenous base. These mutations are called point mutations.

Features of Mutations:

• It is a change in the number or arrangement of the nucleotide sequence of a gene.
• It is a heritable change in DNA sequence.
• Mutations can be harmful, beneficial, or have no effect.
• A mutation results in the appearance of a new heritable characteristic in an individual.

• Mutagenesis is the process of inducing mutations through a number of physical, chemical or biological agents.
• The agents that cause mutations are called mutagens.

Types of Mutagens:

• Physical Mutagens:
  • X-rays, gamma rays and cosmic rays are ionizing radiation.
  • They ionize the water of the cells to release free hydroxyl radicals.
  • The hydroxyl radical is a powerful oxidizing agent. It oxidizes the phosphodiester bond of DNA and induces mutations.
  • It can cause break in one or both the strands of DNA; loss of section of chromosome and fatality if remain unrepaired.
  • UV rays are non-ionizing radiation. These induce the formation of thymine dimer.
  • Formation of thymine dimer changes conformation of DNA.
• Chemical Mutagens:
  • Intercalating agents: 
    • The chemical intercalates or slips in between the two base pairs in double stranded DNA helix. 
    • It alters the morphology of DNA at the point of insertion, thereby increasing the chances of mutations during DNA replication. 
    • For example, ethidium bromide.
  • Nitrous Acid:
    • It is a deaminating agent which changes cytosine to uracil, guanine to xanthine and adenine to hypoxanthine. 
    • Hypoxanthine mis pairs with cytosine. Similarly, C-G is replaced by U-A.
  • Base analogues:
    • These resemble the normal bases of DNA and get incorporated into DNA during DNA replication.
    • Common base analogues are 5-bromouracil and 5-fluorouracil, that are substitutes for thymine.
    • 2-aminopurine is an analogue to adenine.
• Biological Mutagens:
  • Transposons and insertion sequences are biological mutagens.
  • Viruses can cause mutations in the DNA of host cells, resulting in cancer.