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6.7 Mutations

Keywords

English Term 中文翻译 Definition & Explanation
Mutation 突变 A permanent alteration in the DNA sequence that makes up a gene.
Point Mutation 点突变 A change in a single nucleotide pair of a gene.
Frameshift Mutation 移码突变 An insertion or deletion of nucleotides that alters the reading frame of the genetic message.
Nondisjunction 染色体不分离 Failure of chromosomes to separate properly during meiosis or mitosis.
Aneuploidy 非整倍性 A condition in which a cell has an abnormal number of chromosomes (e.g., trisomy).
Horizontal Gene Transfer 水平基因转移 The movement of genetic material between unicellular and/or multicellular organisms other than by vertical transmission.

1. Types of DNA Mutations

Alterations in a DNA sequence can cause changes in the type or amount of protein produced, which in turn affects the phenotype.

Small-Scale Mutations

  1. Point Mutations (Substitutions): One nucleotide is swapped for another.
    • Silent Mutation: Changes the codon but results in the same amino acid (due to the redundancy of the genetic code). No phenotypic change.
    • Missense Mutation: Results in a different amino acid. This may or may not affect protein function depending on the chemical properties of the new amino acid.
    • Nonsense Mutation: Changes an amino acid codon into a premature stop codon, usually leading to a nonfunctional, truncated protein.
  2. Frameshift Mutations: Caused by the insertion or deletion of nucleotides (not in multiples of three). This shifts the entire "reading frame" downstream, typically changing every amino acid follow-on and often creating a premature stop.

[Image of point vs frameshift mutations]

2. Causes and Environmental Context

Mutations are random and can be caused by: * Errors in DNA replication or DNA repair mechanisms. * External factors (mutagens) such as radiation (UV, X-rays) and reactive chemicals.

The Role of the Environment

Whether a mutation is beneficial, detrimental, or neutral depends entirely on the environmental context.

  • A mutation might be harmful in one environment but provide a selective advantage in another (e.g., antibiotic resistance in bacteria).
  • Mutations are the primary source of genetic variation, which is the raw material for natural selection.

3. Chromosomal Mutations

Errors during mitosis or meiosis can result in large-scale changes in phenotype.

  1. Nondisjunction: Occurs when homologous chromosomes (Meiosis I) or sister chromatids (Meiosis II/Mitosis) fail to separate. This leads to aneuploidy (e.g., trisomy or triploidy).
  2. Structural Alterations: Deletions, duplications, inversions, or translocations of chromosome segments often lead to significant genetic disorders.

[Image of nondisjunction during meiosis]

AP Exam Note: Exclusion Statement

  • Knowledge of specific mutations and their biochemical effects is beyond the scope of the exam.
  • Knowledge of specific chromosomal disorders (e.g., Down Syndrome, Turner Syndrome) by name is beyond the scope. Focus on the mechanisms (nondisjunction).

4. Increasing Genetic Variation in Prokaryotes

Prokaryotes do not undergo meiosis, yet they maintain high genetic diversity through Horizontal Gene Transfer (HGT):

  • Transformation: The uptake of "naked" foreign DNA from the surrounding environment.
  • Transduction: The transfer of bacterial genes from one host to another via a virus (bacteriophage).
  • Conjugation: The direct cell-to-cell transfer of DNA (usually plasmids) through a cytoplasmic bridge (pilus).
  • Transposition: The movement of DNA segments (transposons) within and between DNA molecules.

[Image of transformation, transduction, and conjugation in bacteria]

5. Illustrative Examples

  • CFTR Gene: Mutations in this gene disrupt chloride ion transport, resulting in Cystic Fibrosis.
  • MC1R Gene: Mutations in this gene provide adaptive melanism (dark fur) in rock pocket mice living on dark lava flows, illustrating environmental context.
  • Sickle Cell Anemia: A single point mutation in the hemoglobin gene that provides resistance to malaria (heterozygote advantage).

Quiz

Campbell Biology Chapter 17 Practice Test: Gene Expression and Mutations

Click the link above to practice related multiple-choice questions (opens in a new tab).