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 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
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
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
Four O Clocks
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)
petite Mutant Crosses
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
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)
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)
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
Many proteins encoded by nuclear genes have products transported to mitochondria and RNAs.
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
Human mtdna Disorders Myoclonic epilepsy and ragged red fiber disease (MERRF) Fibers from proliferation of aberrant mitochondria Mutation in mtdna trna gene
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
Infectious Heredity Cytoplasmic transmitted phenotypes in eukaryotes due to an invading microorganism or particle (e.g. virus)
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
Infective Particles in Drosophila CO 2 sensitivity Flies fail to recover from CO 2 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
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
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