Chapter 14

  • Organelle genomes lead to non-Mendelian inheritance
    • four-o’clocks have green or variegated leaves
    • this trait is inherited from the mother (maternal inheritance), due to genes found on chloroplast genome.
    • mitochondria and chloroplasts are nonnuclear organelles with their own small genomes
  • Structure and function of mitochondria
    • mitochondria: membrane bound cytoplasmic organelle
    • many mitochondria in each eukaryotic cell
    • outer membrane of mitochondria surrounds wrinkled inner membrane
    • mitochondria produce energy packets (ATP) through the Krebs cycle and oxidative phosphorylation
  • Human mitochondrial genome
    • compact gene arrangement
    • 16.5 kb genome
    • 37 genes encoding tRNAs, rRNAs, and proteins for oxidative phosphorylation
    • no introns
  • Variation in mitochondrial genomes
    • vary in size from 6-2400 kb
    • some have introns and space between genes
    • some have circular mtDNA (people, animals) and some have linear mtDNA (plants, fungi)
    • some protozoan have a single mitochondrion (kinetoplast) with interlocking circular mtDNA
  • Mitochondrial exceptions to the universal code
    • mitochondrial genetic code varies in different organisms
    • in humans, there are 5 differences between the universal and mitochondrial genetic codes
    • UGA universal: stop; UGA mtDNA: Trp
    • AGG universal: Arg; AGG mtDNA: Stop
    • AGA universal: Arg; AGA mtDNA: Stop
    • AUA universal: Ile; AUA mtDNA: Met

    AUU universal: Ile; AUU mtDNA: Ile-elongation Met-initiation

  • Structure and function of chloroplasts
    • chloroplasts: membrane bound cytoplasmic organelles in plants
    • in corn, each cell has 40-50 chloroplasts
    • outer membrane surrounds wrinkled inner membrane
    • capture solar energy and store in chemical bonds of carbohydrates
    • Chloroplast genomes
    • most Chloroplast genomes between 120-160 kb long
    • more than one copy of Chloroplast genome in each chloroplast
    • compact gene arrangements (no introns) in Chloroplast genomes
    • circular, linear, and branched forms in Chloroplast genomes
    • Chloroplast genomes have more genes than mitochondria
  • Making transgenic chloroplast by biolistic transformation
    • use gene gun: coat small metal particles (bullets) with DNA, shoot DNA bullets at cells, DNA rarely enters chloroplasts and recombines into genome
    • transgenic cells identified using selectable marker
    • transgenic cell cultured to produce a transplastomic plant
  • Mitochondria and chloroplasts have characteristics of prokaryotic cells
    • Endosymbiont theory: mitochondria and chroloplasts descended from bacteria and fused with nucleated cells
    • have their own DNA, like bacteria, mtDNA and cpDNA not arranged into nucleosomes
    • inhibitors of bacterial translation block mitochondrial and chloroplast translation, but not eukaryotic translation
    • comparisons of rRNA gene sequences suggest mitochondrial and chloroplast genomes derive from a common ancestor of non-sulfur and cyanobacteria, respectively
  • Cooperation between nuclear and organellular genomes
    • mitochondria and chloroplasts require nuclear gene products to assemble and function
    • cytochrome oxidase c: functions in mitochondrial electron transport; 7 subunits (3 encoded by mitochondrial genome genes, 4 by nuclear genes)
    • nuclear genes encode majority of protein required for gene expression in mitochondria and chloroplasts
  • Implications of gene transfer between organelles and the nucleus
    • organelles must function inside the cell
    • organelle genes incorporated into nuclear genome
    • organelle copy of the genes becomes redundant
    • changes may make it non-functional
    • different organelle genes transferred to nucleus in different lineages leading to organelle diversity
  • Mutations in organelle genes often produce whole-organism phenotypes
    • mtDNA mutations may result in slow cell growth leading to small cell colonies or weak tissues
    • cpDNA mutations may decrease chlorophyll production leading to a change in leaf color
    • DNA polymorphisms can be followed by DNA sequencing
  • Mechanisms leading to maternal inheritance
    • Gamete size:
    • female gamete may be much larger
    • zygote receives many female organelles and few paternal organelles
    • paternal organelles may be actively excluded or destroyed
    • paternal organelles may segregate into non-embryonic cells
    • fertilization may prevent organelles from entering the egg
  • Inheritance of organellular genomes explains variegation in four-o’clocks
    • variegated plants have green, white, and variegated branches
  • the plant is variegated because it has 2 types of chloroplasts
    • wild-type cpDNA genes make chlorophyll
    • mutant cpDNAs have a mutation that prevents chlorophyll production
    • heteroplasmic cells: mix of organelle genomes
    • homoplasmic cells: one type of organelle genome
  • Segregation of organelles during mitosis
    • mitotic progeny of homoplasmic cells are also homoplasmic
    • mitotic progeny of heteroplasmic cells can either be heteroplasmic, homoplasmic wild-type, or homoplasmic mutant
    • uneven distribution of organellular genomes has distinct phenotypic consequences
  • Relationship between cytoplasmic segregation and variegation
    • threshold effect: a certain fraction of wild-type organelles is sufficient for the normal phenotype
    • in four-o’clocks, heteroplasmic cells make enough chlorophyll to be green
  • Characteristic pedigree for mitochondrial disease
    • several diseases of the human nervous system are caused by mutations in the mitochondrial genome
    • mutations are passed from mothers to children
    • symptoms vary due to heteroplasmy
  • Mitochondrial mutations may have an impact on aging
    • oxidative phosphorylation system in the mitochondria generates free radicals, which can damage DNA
    • accumulation of mtDNA mutations over time may result in age-related decline in oxidative phosphorylation
    • evidence in support of role of mtDNA and aging:
    • percentage of heart tissue with a mitochondrial deletion increases with age
    • brain cells of people with Alzheimer’s diseases have abnormally low energy metabolism
    • 20%-35% of mitochondria in brain cells of most AD patients have mutations in cytochrome c oxidase genes, which may explain the low energy metabolism
  • Oocyte nuclear transplantation can sidestep transmission of mitochondrial disease
  • Oocyte nuclear transfer is possible in primates