Genetics Lecture 12

• Malignant cells:

     - immortal

     - divide indefinitely

     - cells become a lump

     - cancerous cells can even grow on

       smooth surfaces

     - normal cells when crowded, cease dividing

       but cancerous cells continue dividing

     - do not need growth factors

     - lose of characteristics

     - become aneuploids
• A mutation is an alteration in DNA sequence
• Mutations may be single-base pair substitutions deletion or insertion of one or more base pairs major alteration in chromosomal structure
• Mutations may occur in somatic or germ cells
• Spontaneous mutations happen naturally and randomly and are usually linked to normal biological or chemical processes in the organism
• Rates of spontaneous mutations vary among loci in different organisms

  • Exceedingly low
  • Rates vary between different organisms
  • Within species, rates vary from gene to gene
• Induced mutations result from the influence of an extraneous factor, either natural or artificial

  • Radiation
  • UV light
  • Natural and synthetic chemical
• Somatic mutations occur in any cell except germ cells and are not heritable
• Germ-line mutations occur in gametes and are inherited
• Autosomal mutations occur within genes located on the autosomes
• X-linked and Y-linked mutations occur within genes located on the X and Y chromosome, respectively
• When a recessive autosomal mutation occurs in a somatic cell of a diploid organism, it is unlikely to result in a detectable phenotype.
• Inherited autosomal mutations will be expressed phenotypically in the first generation
• X-linked recessive mutations arising in the gametes of a homogametic female may be expressed in hemizygous male offspring
• Mutations can be classified based on type of nucleotide change
• Point mutation or base substitution is a change of one base pair to another

  • May result in a new triplet code for a different amino acid: missense mutation
  • May result in a triplet code for a stop codon: nonsense mutation (translation terminated prematurely)
  • New triplet code still codes for the same amino acid: silent mutation
• Base substitutions can be classified as

  • transitions: a pyrimidine replaces a pyrimidine, or a purine replaces a purine
  • transversions: a purine and a pyrimidine are interchanged
• Frameshift mutations result from insertions or deletions of a base pair

  • Can be very severe, especially if they occur early in the coding sequence
  • Except multiples of three, which would reestablish the initial frame of reading
• Mutations can be classified according to their phenotypic effects as

  •loss-of-function mutations

  •gain-of-function mutations

  •visible (morphological) mutations

  •nutritional (biochemical) mutations

  •behavioral mutations

  •regulatory mutations
• Lethal mutations interrupt an essential process and result in death

  • Various inherited biochemical disorders

  ▪Tay-Sachs
• The expression of conditional mutations depends on the environment in which the organism finds itself
• Temperature-sensitive mutation

  ▪Gene product functions at one temperature but not another (temperature sensitive coat color variations in Siamese cat and Himalayan rabbits)