6.7 Mutations

Describe the various types of mutation.

• Alterations in a DNA sequence are mutations that can cause changes in the type or amount of the protein produced and the consequent phenotype. DNA mutations can be beneficial, detrimental, or neutral based on the effect or the lack of effect they have on the resulting nucleic acid or protein and the phenotypes that are conferred by the protein.
  • i. Point mutations occur when one nucleotide has been substituted for a diff erent nucleotide.
  • ii. Frameshift mutations occur when one or more nucleotides are ins erted or deleted, causing the reading frame to be shifted.
  • iii. Nonsense mutations occur when there is a point mutation that causes a pr emature stop.
  • iv. Silent mutations occur when the change in the nucleotide sequence has no eff ect on the amino acid sequence.
  • Exclusion: Knowledge of specific mutations and their effects is beyond the scope of the AP Exam.

Explain how changes in genotype may result in changes in phenotype.

• Errors in DNA replication or DNA repair mechanisms as well as external factors, including radiation and reactive chemicals, can cause random mutations in the DNA.
  • i. Whether a mutation is beneficial, detrimental, or neutral depends on the environmental context.
  • ii. Mutations are a source of genetic variation.
• Errors in mitosis or meiosis can result in changes in phenotype.
  • i. Changes in chromosome number resulting from nondisjunction often result in new phenotypes caused by triploidy (aneuploidy).
  • ii. Changes in chromosome number often result in disorders with developmental limitations.
  • iii. Alterations in chromosome structure lead to genetic disorders.
  • Exclusion: Knowledge of specific disorders related to changes in chromosome number is beyond the scope of the AP Exam.

Explain how alterations in DNA sequences contribute to variation that can be subject to natural selection.

• Changes in genotype may affect phenotypes that are subject to natural selection. Genetic changes that enhance survival and reproduction can be selected for by environmental conditions.
  • i. The horizontal acquisitions of genetic information in prokaryotes via transformation (uptake of DNA), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer of DNA), and transposition (movement of DNA segments within and between DNA molecules) increase genetic variation.
  • ii. Related viruses can recombine genetic information if they infect the same host cell.
  • iii. Reproductive processes that increase genetic variation are evolutionarily conserved and are shared by various organisms.