What two organelles are believed to be remnants of unicellular bacteria?

Mitochondria and chloroplast.

Explanation:

Endosymbiotic theory says that mitochondria and chloroplast are the two organelles which earlier existed as independently living unicellular bacteria. Mitochondria is believed to be remnant of a gram negative bacteria known as Rickettsia prowazekii while chloroplast is believed to be remnant of a cyanobacteria.

According to this theory, in past a eukaryotic cell engulfed these organelles and since then they become an integral part of these eukaryotic cells.

The reason behind this is that both these organelles have prokaryotic characteristics like "presence of 70S ribosome & circular DNA" and they also duplicate themselves by binary fission just like unicellular bacteria.

Apart from this, Rickettsia prowazekii is able to produce ATPs just like mitochondria does in eukaryotic cells. Also, Cyanobacteria and chloroplast are able to perform photosynthesis in a similar way.

In addition to this, the genome sequence similarity between mitochondria & Rickettsia prowazekii and Cyanobacteria & chloroplast further fortifies endosymbiotic theory.