Mutations

Mutations

• any change in genetic info

• can occur due to:

  • mistakes in DNA replication before mitosis or meiosis

  • mistakes in meiosis with chromosome separation (nondisjunction) or with crossing over

  • mistakes in transcription, RNA processing, or translation

  • exposure to environmental mutagens that disrupt DNA’s structure

    • UV radiation causing thymine dimers (covalent bonds b/t adjacent T bases)

    • chemicals that covalently bond another molecule to DNA

      • DNA adducts

    • are problematic b/c they can disrupt proper binding of proteins during DNA replication/transcription

• are essential for evolution

  • can create new genes + protiens

  • but only matter evolutionarily if occur in gametes → can be inherited

  • positive? Usually become more common in the species

  • negative? usually become less common in the species

Gene Mutations

• affect the nucleotide sequence of protein coding DNA

• can produce an abnormal or absent protein

• Types of mutation

  • point mutation: changes only 1 base (AKA substitution)

    • silent: does not chnage the resulting amino acid → no noticeable effect

    • missense: changes only 1 amino acid → produces a protein w/ a different function than original

    • nonsense: causes an early stop codon, shortents the protein → no function

  • frameshift mutation: addition or removal of 1+ bases

    • changes the “reading frame” of the mRNA

    • alters every amino acid after the change (downstream)

    • produces a faulty protein

Chromosomal Mutations

• May affect a whole chromosome

  • leads to offspring with monosomy or trisomy who have an imbalance in “gene dosage”

  • essentially produce too little or too much of mRNA/proteins from specific genes

    • monosomy → too little → insufficient genetic instructions

    • trisomy → too much → additional “stress” on protein synthesis machinery

• May also affect only part of a chromosome

  • Types:

    • that lead to an imbalance in “gene dosage”

      • Deletion: section not replicated or broken off during crossing over → lose genetic info

      • Duplication: section gets copied twice → additional genetic info

    • that leads to incorrect proteins

      • Inversions: section gets reversed

        • caused by mutagen exposure/incorrect repair after DNA damage repair

        • genes read backwards or not under control of the same regulatory sequences

      • that can interrupt other genes

        • Translocation: parts of 2 non-homologous chromosomes are swapped

          • can be cause by errors in crossing over

        • Insertion: part if 1 chromosome is added to another

          • occur when chromosomes break and are improperly repaired.

          • or from “jumping genes” (transposons)