The observation that chloroplasts and mitochondria each contain their own dna and synthesize some of the proteins that function in these organelles suggests that chloroplasts and mitochondria must divide each time the cell containing them divides are produced by the nucleus of the cell contain three sets of membranes are part of the endomembrane system

The answer to this question relates to the evolution of prokaryotic cells. The first simple prokaryotic cells likely evolved mitochondria from a cell that engulfed an aerobic prokaryotic cell, resulting in an endosymbiotic relationship, and gradual development into a mitochondrion. Something similar happened in regards to chloroplasts, except the original cell engulfed was probably a simple photosynthesising bacteria such as cyanobacteria.