Chapter 6: Mutation

Concept:

• Mutant: An organism that carries a mutated gene and expresses it as a phenotype.

• Gene mutation: A change in the structure of a gene, involving one or a few nucleotide pairs.

Point mutation:

• Loss or addition of nucleotides: Causes a frameshift in the reading frame of the genetic code, especially severely affecting if it occurs near the 3′ end of the template strand.

• Nucleotide substitution:

• May not alter the polypeptide chain due to the degeneracy of the genetic code.

• Or change an amino acid in the polypeptide chain.

• Cause premature termination of translation, resulting in a shorter polypeptide chain.

Mechanism of occurrence:

• Mutagenic agents:

• UV rays: Cause cross-linking between two thymines on the same DNA strand.

• Chemicals:

• 5-bromouracil: Replaces adenine with 5-bromouracil, which then pairs with guanine instead of adenine.

• Alkylating agents: Replace guanine with 6-methylguanine, leading to pairing with thymine instead of cytosine.

• Rare bases: Cause changes in the pairing position with the gene, leading to mispairing and nucleotide substitution.

Consequences of gene mutation:

• Can be beneficial, harmful, or neutral.

• The level of harm depends on environmental conditions and gene combinations.

Role:

• Forms new alleles.

• Is the primary fuel for evolution.

Somatic mutation:

• Occurs in somatic cells.

• Not inherited to offspring through sexual reproduction.