Genetics And Evolution Molecular Basis Of Inheritance 11

Definition and Importance of mutation#

• Mutation: A permanent alteration in the DNA sequence that makes up a gene, ranging from a single DNA base change to a large segment of a chromosome.
• Impact: Mutations can lead to changes in the protein sequence, potentially affecting the organism’s phenotype. They are a key source of genetic variation, crucial for evolution.

Mutation Based on Occurrence#

1. Depurination:

   • The most frequent spontaneous lesion, involving the loss of a purine base (A or G).
   • Can result in a nucleotide being replaced by another during DNA replication, leading to a point mutation.

2. Deamination:

   • Involves the removal of an amino group from a nucleotide, like cytosine converting to uracil.
   • If not repaired, it can cause a C-G base pair to become a U-A pair after replication.

3. Induced Mutation:

   • Result from exposure to environmental factors like ultraviolet light, radiation, or certain chemicals.

Types of Induced Mutations#

1. Base Analogs:

   • Chemicals that resemble natural DNA bases and can be incorporated into DNA during replication.
   • Their pairing properties may differ, leading to mutations in subsequent DNA replications.

2. Base Modifying Agents:

   • Includes alkylating agents that add alkyl groups to bases, altering their pairing properties and causing mispairing during replication.

3. Intercalating Agents:

   • Molecules that slide between DNA bases, distorting the DNA helix and causing frame-shift mutations, which are insertions or deletions of bases.

Chromosomal Mutation#

• Large-Scale Changes: Involving deletions, duplications, inversions, translocations of chromosome segments, or changes in whole chromosome numbers (aneuploidy).
• Consequences: Can lead to diseases or developmental issues; also a driving force in evolution and species divergence.

DNA Repair Systems#

1. Direct Repair System:

   • Photoreactivation: Direct reversal of UV-induced thymine dimers using light energy.
   • Alkyltransferase Mechanisms: Remove alkyl groups added by alkylating agents.

2. Excision Repair System:

   • Base Excision Repair (BER): Targets and removes small base lesions from DNA, followed by gap filling and ligation.
   • Nucleotide Excision Repair (NER): Removes bulky DNA lesions that distort the helix. It’s crucial for repairing UV-induced damage like thymine dimers.