6.3 Mutations

Mutations

• Definition: Permanent changes in the nucleotide sequence of a cell’s DNA.

• Transmission:

  • All mutations are copied during DNA replication and passed to daughter cells.

  • Mutations in reproductive cells can be inherited by future generations, while mutations in somatic cells do not affect offspring.

Categories of Mutations

• Single-gene mutations (focus of discussion).

• Chromosomal mutations.

Effects on Nucleotide Sequence of DNA

1. Point mutations: A mutation involving a single base pair substitution, insertion, or deletion.

   • Substitution: One base pair is replaced with another.

   • Insertion: One or more base pairs are added into the DNA sequence.

   • Deletion: One or more base pairs are removed from the DNA sequence.

Effects on Protein

1. Substitutions lead to:

   • (a) Silent mutation:

     • No change in the amino acid sequence.

     • No effect on the protein or the cell.

   • (b) Missense mutation:

     • Changes a codon, incorporating the wrong amino acid, which may alter protein function.

     • Does not always have a negative effect.

   • (c) Nonsense mutation:

     • Converts a codon into a stop codon, producing a shortened protein.

2. Deletion and Insertions lead to:

   • (d/e) Frameshift mutation:

     • Changes the entire reading frame and the amino acid sequence of the protein, generally resulting in catastrophic effects on protein function.

Causes of Mutations

• Spontaneous mutations:

  • Errors occurring during DNA replication.

  • Transposons: Short DNA segments capable of moving within the genome, often referred to as "jumping genes."

• Physical Mutagens:

  • X-rays: Physically change the structure of DNA.

  • UV light: Distorts DNA and affects replication.

• Chemical Mutagens:

  • Alter DNA through chemical processes (e.g., nucleotide substitution, frameshift mutations, insertions).

  • Examples include nitrites, gasoline fumes, and cigarette smoke.

DNA Repair

• Mechanisms to correct mutations and maintain DNA integrity, details not provided in the content.