cytoplasmic inheritance / maternal effect

piRNA inheritance

maternal deposition of piRNAs into embryo cytoplasm provides resistance against P-elements in progeny

oocyte

• 100 μm

• largest cell in humans

• cytoplasm contains important cytoplasmic factors (proteins, RNAs, etc)

sperm

• head 5.1 μm by 3.1 μm

• tail 50 μm long

organelle genetics

• multiple organelles (mitochondria, chloroplasts) in each cell

• organelles contain circular genome (mtDNA, cpDNA)

• organelle DNA replication and division is stochastic, not coupled to cell cycle

mitochondrial genome in humans

• 16569 nucleotides

• circular DNA molecule

• coding 37 genes (13 polypeptides)

• dozens to hundreds of mitochondria per cell, multiple mtDNA copies per mitochondrion

• gene sequences most similar to α-proteobacteria

  • endosymbiotic theory

mitochondrial proteins

• 99% of proteins in mitochondria are encoded in nuclear genome

• the 13 mtDNA-encoded proteins all involved in electron transport chain

• nuclear mitochondrial genes similar to bacterial genes

  • moved to nucleus over time

cytoplasmic inheritance

• oocyte and sperm both contain mitochondria, oocyte contains far more

• upon fertilization, sperm mitochondria are destroyed or consumed by oocyte

• all mitochondria are inherited from the mother (maternal inheritance)

  • mtDNA can be used as a tool to trace maternal heritage

heteroplasmy

two distinct mitochondria populations in a single cell

mtDNA mutations

• ~1000-fold higher rate than nuclear genes

  • more frequent DNA replication

  • no DNA repair

• can lead to heteroplasmy

• random segregation of organelles at mitosis or meiosis can lead to homoplasmic cells for mutation

variegated four o’clock plants

• main shoot is variegated (made up of both mutant and wild-type cells, mixed)

• some branches can be homoplasmic

• white homoplasmic branches mainly contain mutant chloroplasts

• green homoplasmic branches mainly contain wild-type chloroplasts

pedigree of human mitochondrial disease caused by mitochondrial genes

• children of an affected mother all have the disease

• whether grandchildren have the disease or not is determined by sex of the child

• in some cases where a mother has a mixture of affected and unaffected mitochondria (heteroplasmic), her children may be normal

human diseases caused by dysfunction in mitochondria

• affect at least 1/5000 people

• progressive

• multi-system disease (high energy-demand tissues)

pronuclear transfer

• three-person in vitro fertilization

• parents’ nuclear material removed from oocyte with unhealthy mitochondria

• parents’ nuclear material placed inside donor egg with healthy mitochondria