11.5 Mutations

Mutations are the driving force behind genetic diversity and evolution. From tiny point mutations to massive frameshift changes, these DNA alterations can have profound effects on organisms. Understanding how mutations occur and their impacts is crucial for grasping the fundamentals of genetics and heredity.

Detecting mutagens and carcinogens is vital for protecting human health. The Ames test, using specially engineered bacteria, helps identify potential DNA-damaging agents. This knowledge allows us to better understand the causes of genetic diseases and cancer, paving the way for prevention and treatment strategies.

Mutations and Their Effects

Point vs frameshift mutations

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• Point mutations involve a single nucleotide change can be substitution (one nucleotide replaced by another), insertion (one nucleotide added), or deletion (one nucleotide removed) may or may not alter the amino acid sequence of the protein
• Frameshift mutations involve the insertion or deletion of a number of nucleotides not divisible by three cause a shift in the reading frame of the genetic code altering the amino acid sequence from the point of the mutation onward often resulting in a truncated or nonfunctional protein

Effects of mutation types

• Missense mutations are point mutations that result in a different amino acid being incorporated into the protein potentially altering protein structure and function
• Nonsense mutations are point mutations that create a premature stop codon (UAA, UAG, or UGA) resulting in a truncated protein that may be nonfunctional
• Silent mutations are point mutations that do not change the amino acid sequence of the protein due to the redundancy of the genetic code (multiple codons code for the same amino acid) generally having no effect on protein structure or function

Light vs dark repair mechanisms

• Light repair (photoreactivation) requires visible light involves the enzyme photolyase repairs specific UV-induced DNA damage (thymine dimers) by binding to the damaged DNA and using light energy to break the bonds between the thymine dimers
• Dark repair (excision repair) does not require light involves multiple enzymes repairs various types of DNA damage through the following steps:
  1. Recognition and removal of the damaged DNA segment
  2. Synthesis of a new DNA strand using the undamaged strand as a template
  3. Ligation of the newly synthesized strand to the existing DNA
• DNA repair mechanisms play a crucial role in maintaining genetic stability and preventing mutations

Mutagens and Carcinogen Detection

Mutagens and DNA damage

• Physical mutagens include ionizing radiation (X-rays, gamma rays) causing double-strand breaks and oxidative damage to DNA and non-ionizing radiation (UV light) inducing the formation of pyrimidine dimers (thymine dimers)
• Chemical mutagens include alkylating agents (ethylmethanesulfonate, nitrosoguanidine) adding alkyl groups to DNA bases causing mispairing during replication, base analogs (5-bromouracil) incorporating into DNA in place of normal bases causing mispairing, and intercalating agents (ethidium bromide) inserting between base pairs causing frameshift mutations
• Biological mutagens include viruses (human papillomavirus) integrating their DNA into the host genome disrupting normal gene function and transposons (Tn elements) as mobile genetic elements that can insert into genes causing mutations
• Mutagenesis can be induced by exposure to these various types of mutagens

Principle of Ames test

• The Ames test is a bacterial reverse mutation assay using Salmonella typhimurium strains with preexisting mutations in the histidine biosynthesis pathway
• Principle: Potential carcinogens (mutagens) will cause reverse mutations restoring the bacteria's ability to synthesize histidine and grow on histidine-deficient media
• Procedure involves exposing the Salmonella strains to the potential carcinogen, plating the bacteria on histidine-deficient media, counting the number of revertant colonies that grow, and comparing the number of revertant colonies to a control (no exposure to the potential carcinogen)
• A significant increase in the number of revertant colonies indicates that the substance is a mutagen and potential carcinogen
• The mutation rate can be calculated by comparing the number of revertant colonies to the total number of cells plated

Classification of mutation types

• Comparing a mutated DNA sequence to the wild-type (normal) sequence allows for identifying the type of mutation:
  • Point mutations include substitution (a single nucleotide replaced by another), insertion (a single nucleotide added), and deletion (a single nucleotide removed)
  • Frameshift mutations include insertion (a number of nucleotides not divisible by three added) and deletion (a number of nucleotides not divisible by three removed)
• Determining the effect of the mutation on the amino acid sequence:
  • Silent mutation: The amino acid sequence is unchanged
  • Missense mutation: A different amino acid is incorporated into the protein
  • Nonsense mutation: A premature stop codon is created resulting in a truncated protein

Genetic Variation and Heredity

• Mutations are a source of genetic variation in populations
• Genetic variation is essential for evolution and adaptation to environmental changes
• Heredity involves the transmission of genetic information, including mutations, from parents to offspring