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8.7 – Mutations Key Concept Mutations are changes in DNA that may or may not affect phenotype. mutated base What Are Mutations? Changes in the nucleotide sequence of DNA May occur in somatic cells (aren’t passed to offspring) May occur in gametes (eggs & sperm) and be passed to offspring Are Mutations Helpful or Harmful? Mutations happen regularly Almost all mutations are neutral Chemicals & UV radiation cause mutations Mutagens are anything in the environment that can cause mutations Many mutations are repaired by enzymes Are Mutations Helpful or Harmful? Some types of skin cancers and leukemia result from somatic mutations Some mutations may improve an organism’s survival (beneficial) Types of Mutations Chromosome Mutations May Involve: Changing the structure of a chromosome Loss or gain of part of the chromosome Chromosome Mutations Five types exist: Deletion Inversion Translocation Nondisjunction Duplication Deletion Due to breakage A piece of a chromosome is lost Inversion Chromosome segment breaks off Segment flips around backwards Segment reattaches Duplication Occurs when a gene sequence is repeated Translocation Involves two chromosomes that aren‘t homologous Part of one chromosome is transferred to another chromosome Nondisjunction Failure of chromosomes to separate during meiosis Causes gamete to have too many or too few chromosomes Nondisjunction Can cause “Trisomy” (three copies of the same chromosome in an egg or sperm) Trisomy 21 (Down syndrome) Chromosome Mutation Animation Gene Mutations Change in the nucleotide sequence of a gene May only involve a single nucleotide May be due to copying errors, chemicals, viruses, etc. Types of Gene Mutations Include: Point Mutations Substitutions Insertions Deletions Inversions Frameshift Similar to the chromosomal versions of these mutations Point Mutation Change of a single nucleotide Includes the deletion, insertion, or substitution of ONE nucleotide in a gene Point Mutation Sickle Cell disease is the result of one nucleotide substitution Occurs in the hemoglobin gene Nonsense Mutation Type of point mutation Results in a premature stop codon and usually a nonfunctional protein Frameshift Mutation Inserting or deleting one or more nucleotides Changes the “reading frame” like changing a sentence Proteins built incorrectly Frameshift Mutation Original: The fat cat ate the wee rat. Frame Shift (“a” added): The fat caa tat eth ewe era t. Amino Acid Sequence Changed Gene Mutation Animation Silent Mutations Some mutations have no effect and are called “silent” Example: GUC changed to GUG Both code for the amino acid valine This would not affect the protein being made in any way Mutations may or may not affect phenotype. Chromosomal mutations tend to have a big effect. Some gene mutations change phenotype by: causing a premature stop codon. causing a change in protein shape or the active site. causing a change in gene regulation. Some gene mutations do not affect phenotype. A mutation may be silent because…. – It occurs in a non-coding region. – It may not affect protein folding or the active site.