L10: extranuclear/ cytoplasmic/ organelle inheritance

what is uniparental inheritance usually

maternal inheritance but can be paternal too

cytoplasmic organelles

mitrochondion and chloroplast

what do mitochondrion and chloroplasts have

their own genomes

organelle heredity

an organism’s phenotype is affected by the expression of genes contained in the DNA of mitochondria or chloroplast, rather than the nucleus

endosymbiotic theory of origin

• chloroplast and mitochondria are supposedly the early prokaryotic endosymbionts

• cyanobacteria and aerobic free-living bacteria

• ingested by eukaryotes

• transitioned into semi-autonomous organelles

basis of endosymbiotic theory

• mitochondria and chloroplasts have their own DNA in circular form with its size and structure like prokaryotes

• have ribosomes that more closely resemble those in prokaryotes (70S)

• codon usage similar to prokaryotes

• reproduce by fission like prokaryotes

• rRNA sequences similar to prokaryotes

what percent of proteins required for chloroplast function are synthesized by the nuclear genome

95

what percent of proteins required for mitochondrial function are synthesized by the nuclear genome

99

why is the analysis of inheritance patterns resulting from mutant alleles in chloroplasts and mito more complex than mendelian patterns

• function of these organelles is dependent on gene products from both nuclear and organelle DNA, making the discovery of genetic origin of mutations unclear

• many mito and chloroplasts are present in each cell - if only one of the few organelles contain a mutant gene in a cell among a pop of mostly normal mito, the mutant phenotype may not be revealed

who passes cytoplasmic organelles usually

mother

why is the passing of mito and chloroplasts uniparental and maternal

• in heterogamous species, female gamete is larger - has more cytoplasmic content and organelles compared to male

• after fertilization, female gamete provides most of the cytoplasm to the resulting zygote while the male gamete provides only a little more than a nucleus

• fertilzation triggered autophagy event causes the selective degradation of paternal mitochondria post fertilization

paternal leakage

in a species with maternal inheritance, occasionally a male parent may provide mitochondria to zygote

heteroplasmy

the occurence of 2 different varieties of DNA within the cytoplasm of a single cell

how is heteroplasmy caused

• a mutation arising within one DNA molecule within one organelle generates a mixture of organelles within the cell

• some mutant, some wildtype

homoplasmy

cells with only mutant or only wildtype DNA

how is homoplasmy caused

• when a heteroplasmy cell dvides, the organelles split randomly into the progeny cells

• most progeny is heteroplasmic but sometimes by chance may only have mutant/ wt seqeunces

what does heteroplasmy generally lead t

homoplasmy over several gens

variegation in four o’clock plants

• branches have either while, green or variegated white and green leaves

• white areas in variegated and completely white leaves lack chlorophyll

• inheritance in all possible combos of crosses is strictly determined by the phenotype of the ovule source

what did Correns conclude

inheritance was transmitted through the cytoplasm of the maternal parent because the pollen, which contributes little or no cytoplasm to the zygote, had no apparent influence of the progeny phenotypes

chlorophyll defect

genetic defect that eliminates chlorophyll in the white patches on leaves is a mutation in the DNA housed in the chloroplast

cytoplasmic male sterility

• condition under which a plant is unable to produce functional pollen

• sterility results from mitochondrial gene mutations causing cytoplasmic dysfunction

how is CMS inherited

maternally, often associated with mitochondrial mutations

how is CMS fixed

• male fertility can be restored by nuclear encoded fertility restorer (Rf) gene

• fertility restoration relies on nuclear genes that suppress cytoplasmic dysfunction

• fertility is maintained by to a male-fertile line

what are CMS systems used for

• production of hybrids in species that could otherwise undergo self fertilization

• ex. maize, rice, cotton

heterosis

• aka hybrid vigor

• hybrids often exhibit heterozygote advantage

• hybrid progeny exhibit superior growth characteristics relative to either of the parental lines

• parental lines are homozygotes

what do CMS cells pass on to progeny

• cytoplasm

• mitochondria

what gender is a cell with a toxic pollen

female

saccharomyces cerevisiae mitochondrial mutations

2 types of colonies: petite and grande

petite mutants

• unable to carry out aerobic resp

• obtained all their energy from anaerobic resp which is less efficient

2 types of petite mutations

• segregational petite mutations

• vegetative petite mutations

segregational petite mutations

• mutation in nuclear gene resulting in defective mitochondria

• segregate in mendelian fashion

• 2:2 ratio in unordered spores - 2 WT, 2 petite

• both parents contribute nuclear DNA equally

vegetative petite mutations

• mutations or deletions in mitochondrial DNA resulting in defective mitochondria

• unequal contributions from parents

• show uniparental inheritance

2 categories of vegetative petite mutations

• neutral petite mutations

• suppressive petite mutations

neutral vegetative petite

• all normal colonies (4:0)

• lack most of their mitochondrial DNA, therefore no petite gene

explanation for for neutral vegetative

• both normal and defective mitochondria are inherited (heteroplasmic)

• normal mitochondria outcompete defective mitochondria (lack most of mtDNA)

• DNA cannot recombine cuz there’s not enough defective DNA

suppressive petite strain

• lack small segments of mtDNA

• petite suppresses WT

• all petite progeny

• both normal and defective mitochondria inherited

• defective mitochondria outcompete normal mitochondria

2 explanations for petite supression

• defective mitochondria replicate more rapidly

• crossovers happen between normal and defective strains

human diseases with maternal inheritance

• mitochondrial myopathy - mutation in tRNA-Leu

• cardiomyopathy - mutation in tRNA-Gly