Origin of Cytoplasmic Organelles
Extranuclear inheritance associated with cytoplasmic organelles meets the preceding requirements and deserves an explanation, although organelles represent only a very small proportion of genetic material, perhaps a few hundred genes, based on the amount of functional DNA that is involved.

The fascinating possibility suggested by several earlier investigators and recently elaborated by Margolis is that mitochondria chloroplasts were once free living bacteria. Over long periods of time, they established a hereditary symbiosis with their eukaryotic host cells and ultimately evolved into organelles with animal and plant cells.

At the same time the host lost some of its genes so it could no longer produce energy efficiently; it became dependent on its guest for energy. Several lines of evidence point to this symbiotic association.Transcription in organelles is inhibited by the antibiotic rifampicin, which inhibits prokaryotic, but not eukaryotic RNA Pol.

Even the subunits of RNA polymerase are similar to prokaryotes. Promoters for gene expression and protein synthesis in organelles also shows 100% similarity to that of prokaryotes. The size and sequence of rRNA genes and ribosomes are more closely related to prokaryotic genes rather than eukaryotic genes.