Cells within cells

Were some organelles originally bacteria?

Some organelles, such as mitochondria and chloroplasts, found inside eukaryotic cells look rather like cells themselves. According to the endosymbiotic theory, eukaryotes originated when symbiotic bacteria (which exist alongside cells in a mutually beneficial relationship) began to live inside larger cells, giving them ready-made compartments. Over time, these bacteria became permanent additions – chloroplasts and mitochondria – to the cells we see today.

This would account for the loop of mitochondrial DNA (mtDNA) found within mitochondria that codes for ribosomal RNA (rRNA) and for components of the electron transport chain, which is involved in oxidative phosphorylation. mtDNA is maternally inherited through the egg and accounts for less than 1 per cent of the cell’s total DNA.

About this resource

This resource was first published in ‘The Cell’ in February 2011 and reviewed and updated in September 2015.

Cell biology
The Cell
Education levels:
16–19, Continuing professional development