Genetics: Mutations and Their Types

Mutations Defined

• A mutation is a change in a DNA base-pair or a chromosome.
• There are two types of mutations: somatic mutations (affect only the individual) and germ-line mutations (alter gametes, affecting the next generation).

Types of Mutations

• Point mutations: a change in a single DNA base-pair.
  • Base pair substitutions: silent, missense (new protein), or nonsense (stop codon).
  • Base pair insertions and deletions: frameshift mutations.
  • Triplet repeats: deletion or duplication of a codon.

Examples of Mutations

• Sickle cell disease: a missense mutation that changes the DNA sequence from CTC to CAC, encoding valine instead of glutamic acid.
• Factor XI deficiency: a nonsense mutation that changes the codon from GAA to UAA, signifying "stop".
• Gaucher disease: an insertion mutation that adds a single DNA base, preventing production of an enzyme.
• Cystic fibrosis: a triplet repeat mutation that deletes a codon, causing a deletion of phenylalanine.

Classification of Point Mutations

• Forward mutations: change the genotype from wild-type to mutant.
• Reverse mutations (reversions or back mutations): change the genotype from mutant to wild-type or partially wild-type.

Spontaneous and Induced Mutations

• Spontaneous mutations: occur naturally, without an external cause.
• Induced mutations: occur due to intentional or accidental exposure to mutagens (chemicals or radiation).

Induced Mutations

• Intentional use of mutagens: chemicals or radiation are used to induce mutations.
• Accidental exposures to mutagens: exposure to mutagens through accident or environmental factors.
• Natural exposure to mutagens: exposure to ionizing radiation from natural sources (e.g. uranium, radium).

Ionizing Radiation

• Alpha radiation: least energetic and most short-lived, absorbed by the skin.
• Beta radiation: more penetrating, emitters include tritium, carbon-14, and strontium-70.
• Gamma radiation: most penetrating, emitters include plutonium and cesium isotopes, used to kill cancer cells.

How Mutations Cause Diseases

• Familial hypercholesterolemia (FH): an example of incomplete dominance in humans, caused by a mutation in the LDL receptor gene.
• The severity of the disease depends on the number of disease-causing alleles.

Exploring Mutations

• Visible mutations: affect the appearance of an organism, such as human phenotypes, Drosophila eyes or wing-shape, coat color in animals, colony size in yeast, or plaque morphology of phages.

This quiz covers the concept of mutations in DNA, including somatic and germ-line mutations, point mutations, and their effects on the genetic code.