Inherited cytoplasmic factors, unlike nuclear genes, do not segregate or assort regularly in sexual reproduction. This category includes organelle genes, infectious agents, and mRNAs. Extranuclear genes reside in mitochondria, chloroplasts, and cytoplasmic intracellular parasites. Prions infect somatic cells of animals and thereafter are inherited cytoplasmically. This chapter focuses on the modes of cytoplasmic inheritance and methods of analysis.
Mitochondria and Chloroplasts
Characteristics of Mitochondria and Chloroplasts
The mitochondrion is the site of oxidative metabolism in eukarya. There are two membranes separated by a space; the innermost space of the mitochondrion is the matrix. Initial oxidation of fatty acids happens between the two membranes; the oxidation of acetyl-CoA, the citric acid cycle, and β-oxidation of fatty acids take place inside the matrix. The electron transport system and the enzymes of oxidative phosphorylation reside in the inner membrane. The number of mitochondria per cell may be 1 to ~105, depending on cell type and species.
Mitochondria reproduce clonally, by splitting. The mitochondrion has a small chromosome (14 to 2500 kb), present in multiple copies because its replication is not tightly coordinated with organelle division. Chromosomes segregate into daughter mitochondria. Mitochondria have their own machinery for protein synthesis, but nuclear genes encode most mitochondrial proteins.
Mitochondria are distributed to daughter cells during cell division in a haphazard way. Suppose a mutation arises in one of the many mitochondria in an actively cycling cell.