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Extranuclear Inheritance

Extranuclear Inheritance. Dr.Shivani Gupta, PGGCG-11, Chandigarh. Commonly defined as transmission through the cytoplasm (or things in the cytoplasm, including organelles) rather than the nucleus Generally only one parent contributes. Extranuclear Inheritance. Organelle heredity

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Extranuclear Inheritance

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  1. Extranuclear Inheritance Dr.Shivani Gupta, PGGCG-11, Chandigarh

  2. Commonly defined as transmission through the cytoplasm (or things in the cytoplasm, including organelles) rather than the nucleusGenerally only one parent contributes

  3. Extranuclear Inheritance • Organelle heredity • Organelles that contain chromosomes • Chloroplasts and mitochondria • Infectious heredity • Involves a symbiotic or parasitic association with a microorganism • Maternal effect • Nuclear gene products are stored in ooplasm and distributed to cells as the fertilized egg divides to form developing embryo

  4. Chloroplasts and Mitochondria • These organelles contain DNA • First explanation for [some] maternal inheritance patterns • Endosymbiont theory • Analysis of mutant alleles in organelles can be complex because many genes for organelle components are nuclear-encoded • And even subunits of a multicomponent enzyme may be partially encoded in both locations • Heteroplasmy makes things even worse…

  5. Chloroplasts • Carl Correns • A codiscoverer of Mendel’s work • Worked with four o’clock plants (Mirabilis jalapa) • Had branches with either white, green or variegated leaves • Type of offspring dependent only upon the phenotype of the branch from which the ovule was derived—not the pollen F(figure 9-1) • Concluded that leave color was dependent upon the chloroplasts and that these or other factors were contributed through the ovule cytoplasm

  6. Four O‘Clocks

  7. Saccharomyces petite Mutations • petite mutations give rise to small colonies • Aerobic respiration blocked • Live anaerobically • S. cerevisiae is a facultative anaerobe • Two types • Segregational petites encoded by nuclear genes showing Mendelian inheritance • cytoplasmic transmission pattern petites • Neutral petites demonstrate (give all wt offspring when crossed to wt) • Suppressive petites (behave like poky in Neurospora)

  8. petite Mutant Crosses

  9. Mitochondrial/Chloroplast Evolution • Endosymbiont theory – Lynn Margulis • Mitochondria and chloroplasts arose independently about 2 billion years ago as free-living prokaryotes • Primitive eukaryotes without these abilities engulfed the prokaryotes as endosymbionts • Relationship ultimately changed to that of an organelle • Organelles have circular DNA • Most genes moved to “nucleus” (<10% remain) • Targeting peptides added • Organelle genes/expression still “prokaryotic”

  10. Chloroplast Genes/Expression • Chloroplasts have circular DNA and a complete gene expression system • Components derived from cpDNA and nuclear DNA encoded genes • cpDNA commonly 100-225 kbp in size • No nucleosomes, but has introns and large intergenic regions • Multiple copies/organelle (75 in Chlamydomonas) and recombination can occur • Encode rRNAs, tRNAs, rproteins (~70S ribosome) and other proteins/enzymes (92 encode thylakoid proteins in the liverwort)

  11. Mitochondrial Genes/Expression • mtDNA is circular, generally relatively small • 16-18 kbp in mammals, 75 kbp in yeast, but 367 kbp in Arabidopsis (a mustard plant) • 5-10 copies/organelle in vertebrates, 20-40 in plants • Introns generally absent, small intergenic spaces in small mtDNAs, reverse in larger ones such as yeast • Genetic code similar but modified • Encodes rRNAs, tRNAs and 13 polypeptides in humans (portions of electron transport chain)

  12. Mitochondrial Genes/Expression • Protein synthetic apparatus combination of mtDNA and nuclear-encoded • But nuclear-encoded proteins distinct from their cytoplasmic or nuclear counterparts • RNAP is single polypeptide and is inhibited by rifampicin/rifamycin • But sensitive to antibiotics targeted normally against prokaryotes • Ribosomes range from 55-80S

  13. Many proteins encoded by nuclear genes have products transported to mitochondria and RNAs ….

  14. mtDNA Mutations and Human Genetic Disorders • Human mtDNA is 16,569 bp • Encodes 13 proteins, 22 tRNAs and 2 rRNAs • Heteroplasmy • Variable mixture of genetically distinct mitochondria/mtDNAs • Properties of mtDNA-encoded traits • Maternal inheritance pattern • Deficiency in bioenergetic function of organelle • Specific mutation in an mtDNA gene

  15. Human mtDNA Disorders • Myoclonic epilepsy and ragged red fiber disease (MERRF) • Fibers from proliferation of aberrant mitochondria • Mutation in mtDNA tRNA gene

  16. Human mtDNA Disorders • Leber’s hereditary optic neuropathy (LHON) • Sudden bilateral blindness 9average age 27 yrs) • Most mutations in NADH dehydrogenase gene • Maternal transmission to all offspring • Many cases appear to be “new” mutations • No family history

  17. Infectious Heredity • Cytoplasmic transmitted phenotypes in eukaryotes due to an invading microorganism or particle (e.g. virus)

  18. Kappa in Paramecium • Certain strains of P. aurelia are called killer strains because they release paramecin, a substance toxic to sensitive strains • Paramecin produced by kappa particles (100-200 per cell) that replicate in cytoplasm • Kappa particles contain DNA and protein and require a nuclear gene (K, “little k” strains are sensitive) for maintenance • Kappa particles are bacterialike and may contain temperate phage

  19. Infective Particles in Drosophila • CO2 sensitivity • Flies fail to recover from CO2 anesthetization (permanently paralyzed) • Sensitivity due to presence of virus called sigma • Transfer to other insect species unsuccessful, suggesting Drosophila genes essential for its continued propagation/function • Sex ratio in D. bifasciata and D. willistoni • Some flies produce offspring at an altered sex ratio • Mostly female at below 21 degrees Celsius • Trait transmitted only to daughters • Agent shown to be a protozoan that is lethal only to males • And protozoan may have a virus that is actually responsible…

  20. Maternal Effect/Maternal Influence • Offspring phenotype under control of nuclear gene product present in the egg • Genetic information of mother used to produce products present in the egg cytoplasm • Snail Limnaea peregra shell coiling is an example

  21. Snail Limnaea peregra Shell Coiling • Hermaphroditic snails • Some shells have right-handed (DD or Dd) coiling while others have left-handed (dd)coiling • Reciprocal crosses (reverse mail and female genotypes) of true-breeding snails • Offspring phenotype depends upon maternal genotype—not maternal phenotype

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