Chapter 9 Extranuclear Inheritance Genetics

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Chapter 9 Extranuclear Inheritance Genetics
2013-10-28 02:30:11
Genetics Chapter

notes chapter 9
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  1. Organelle heredity
    DNA contained in mitochondria or chloroplasts determines certain phenotypic characteristics of the offspring
  2. Infectious heredity
    results form symbiotic or parasitic association with a microorganism
  3. Maternal effect
    • nuclear gene products are stored in the egg and then transmitted through the ooplasm to the offspring
    • gene products in the ovule are distributed to the cells of the developing embryo and influence its phenotype
  4. Analysis of mutant alleles in chloroplast and mitochondria are difficult because:
    • the function of these organelles is dependent on gene products from the nucleus and the organelle DNA, making the origin of mutations affecting organelle function difficult
    • organelles are contributed to each progeny cell and exhibit great variation (heteroplasmy) making detection of the mutation hence analysis more complicated than Mendelian inheritance
  5. Chlamydomonas
    excellent model system for studying organelle heredity because it has a single large chloroplast that exhibits a uniparental inheritance pattern
  6. Ruth Sager (1954)
    showed that streptomycin-resistant Chlamydomonas show differences in reciprocal crosses with susceptible strains, and the trait is passed through the female parent
  7. Mitchell and Mitchell (1952)
    reveal that mitochondria likewise contain a diverse genetic system, and mutations are transmitted through the cytoplasm
  8. 3 types of mutations
    segregational, neutral, suppressive
  9. Segregational petites
    mitochondrial mutation studies with yeast Saccharomyces cerevisae have revealed that a small percent of these mutations are nuclear in nature
  10. Neutral petites
    cross with neutral petites show that mitochondria are inherited form both parental cells and are replicated in the offspring
  11. Suppressive mutation
    may not be observed initially in offspring but becomes apparent after a time
  12. Endosymbiotic theory
    • states that mitochondria and chloroplasts arose independently about 2 billion years ago form free-living bacteria
    • Progenitors posses the abilities now attributed to these organelles such as aerobic respiration and photosynthesis
    • theory proposes that these bacteria were engulfed by larger eukaryotic cells and a beneficial symbiotic relationship was developed
  13. Amount of DNA in today's mitochondrai
    is less than 10%, very miniscule when compared to DNA of free-living bacteria from which it was derived
  14. Chloropast DNA length
    ranges from 100-225 kb and genes carried on the DNA encode products involved in phtosynthesis and translation
  15. Comparison of Mitochondrial DNA (mtDNA) and DNA in chloroplasts (cDNA)
    • mitochondria DNA is smaller because the introns are absent and gene repititions are rare
    • most of protein encoding genes are located on single strand
    • replication in mitochondria is dependent on genes encoded by the nuclear DNA
  16. Human mitochondrial DNA
    • contains 16,569 base pairs
    • codes for 13 over 70 proteins required for aerobic cellular respiration
  17. Why is mtDNA susceptible to mutations?
    • doesn't have histones to protect from mutations
    • mitochondria have high concentrations of reactive oxygen species (ROS) generated by cell respiration [ROS damages organelle contents such as proteins, lipids, and mtDNA)
  18. Heterplasmy
    • condition in which adult cells have a variable mixture of normal and abnormal organelles
    • if a zygote receives a large number of organelles through the egg; a mutation in one or a few will be diluted out by many mitochondria that lack the mutation and function normally
  19. For human disorder to be attributed to mtDNA:
    • the inheritance must exhibit a maternal inheritance pattern
    • the disorder must reflect a deficiency in the bioenergetic function of the organelle
    • there must be a specific mutation in a mitochondrial gene
  20. Three disorders arising from mtDNA are:
    • myoclonic epilepsy and ragged red fiber disease (MERRF)
    • Leber's heredity optic neuropathy (LHON)
    • Kearns-Sayre syndrome (KSS)
  21. Maternal effect
    • implies that an offspring's phenotype is under the control of nuclear gene products present in the egg
    • nuclear genes of the female gamete are transcribed and the genetic products accumulate in the egg cytoplasm
    • these products are distributed among newly formed cells, influencing patterns/traits established early in development
    • Maternal effect genes, the genotype of hte female parent and not that of the embryo determines the phenotype of the offspring by overriding the genotype of the progeny
  22. Examples of maternal effect
    • Ephestia (larva)
    • small Limnaea peregra (shell coiling)
    • Drosophila