Chapter 20 Gene Expression

• What are gene mutations?

  Changes in the DNA base pair sequence
• How do gene mutations occur?

  They occur spontaneously during DNA replication
• Why do most gene mutations not significantly change polypeptides?

  Because the genetic code is degenerate
• What can cause mutations in the DNA base sequence?

  Insertion, deletion, or substitution of nucleotides
• What is substitution of bases in gene mutations?

  Replacing one nucleotide with another different base
• What are the three possible consequences of base substitution mutations?

  Silent, missense, and nonsense mutations
• What is a silent mutation?

  A mutation that does not alter amino acid sequence
• How does a missense mutation affect a polypeptide?

  It alters a single amino acid in the chain
• What is a nonsense mutation?

  A mutation that creates a premature stop codon
• How does a nonsense mutation affect protein structure?

  It produces an incomplete polypeptide chain
• What is deletion of bases in gene mutations?

  The loss of a nucleotide base from DNA sequence
• What is a frameshift mutation?

  A mutation that changes the reading frame of codons
• How does deletion near the beginning of a gene affect the phenotype?

  It has a much greater impact than later deletions
• What is insertion of bases in gene mutations?

  The addition of a nucleotide base into DNA sequence
• How does insertion affect the DNA sequence?

  It creates a new triplet of bases
• What is duplication of bases in gene mutations?

  Repeating one or more bases in the sequence
• How can duplication of bases contribute to evolution?

  It allows the second copy to develop new functions
• What is inversion of bases in gene mutations?

  A group of bases is reversed in the sequence
• How does inversion affect protein function?

  It usually results in a non-functional protein
• What is translocation of bases in gene mutations?

  A group of bases moves to a different chromosome
• What are the potential effects of translocation on gene expression?

  It can lead to abnormal phenotypes and cancer
• What are the types of gene mutations?

  • Substitution
  • Deletion
  • Insertion
  • Duplication
  • Inversion
  • Translocation
• What are the consequences of deletion and insertion mutations?

  • Deletion: Causes frameshift, altering amino acids
  • Insertion: Causes frameshift, altering amino acids
• What are the effects of gene mutations on proteins?

  • Silent: No effect on protein
  • Missense: Alters one amino acid
  • Nonsense: Creates premature stop codon
• How do mutations contribute to evolution?

  • Duplication allows new functions
  • Mutations can lead to new traits
  • Natural selection acts on mutations
• Causes of gene mutations

  The rate that mutations occur can be estimated as around one mutation per 100 000 genes per generation. Exposure to mutagenic agents can increase the rate of mutation, they include:

  High energy ionising radiation, such as alpha, beta or gamma radiation which disrupt the structure of DNA

  Chemicals, such as nitrogen dioxide, may directly alter the structure of DNA or interfere with transcription, or benzopyrene from tobacco smoke which is a powerful mutagen that inactivates a tumour-suppressor gene leading to cancer.
• What is cell differentiation?

  Process where cells become specialized
• How do cells in complex organisms differ from unicellular organisms?

  Complex organisms have specialized cells
• What process do all cells in complex organisms derive from?

  Mitosis
• What happens to cells once they differentiate?

  They lose ability to divide and totipotency
• What changes occur during the differentiation process?

  Changes in size, shape, and metabolism
• Is differentiation reversible in mammals?

  No, it is generally not reversible
• What are cells that can differentiate called in animals?

  Stem cells
• What are cells that can differentiate called in plants?

  Meristematic cells
• What is a totipotent cell?

  A stem cell that can produce any body cell
• What is the role of stem cells?

  To divide and replace damaged cells
• Where do embryonic stem cells come from?

  Embryos in early stages of development
• What are umbilical cord blood stem cells similar to?

  Adult stem cells
• What type of stem cells are found in the placenta?

  Placental stem cells
• What is the function of adult stem cells?

  To maintain and repair tissue