What is the serial endosymbiotic theory? How does it explain the origin of mitochondria and chloroplast?

Hint: Organisms can be classified as prokaryotes (bacteria and archaea) or eukaryotes. Prokaryotes are unicellular organisms. Their genetic material is found in a region called nucleoid; they do not have a membrane-bound nucleus. Some of the cell organelles (mitochondria, chloroplasts) found in eukaryotes are absent in prokaryotes. Eukaryotes can be unicellular or multicellular.

Complete answer:
-In the beginning, there were prokaryotes, some of them could perform special functions such as preparing their food using light energy, some could use oxygen to produce energy, etc. Some bacteria were so large that they could ingest other bacteria. Some of these large cells ingested the other bacteria. The smaller bacteria remained intact inside the cell and began to live as symbionts.
-They performed the same functions that they performed while living outside the cell. These cells were believed to be the ancestors of the eukaryotes. Therefore, the endosymbiotic theory states that the eukaryotes evolved from prokaryotes.
-It is believed that mitochondria evolved from the bacterial cells that were able to utilize oxygen to produce energy. While chloroplast evolved from the bacterial cells that were able to perform photosynthesis.
-Both these organelles have their DNA. This is one of the reasons why it is believed that these organelles were once fully functional bacterial cells. The cell organelle DNA is arranged as prokaryotic DNA. This DNA is different from the nuclear DNA of the cell.
-The serial endosymbiotic theory explains the acquisition of more than one symbiont. The cell first acquires one symbiont (or endosymbiont as it is inside the cell). Then engulf another symbiont, probably another.

Note: It is believed that mitochondria evolved before plastids. This is because almost all eukaryotic cells have mitochondria. Even those with plastids have mitochondria. The bacterial cells with the symbiont that could perform photosynthesis were considered to be the ancestor autotroph eukaryote. The bacterial cells with the symbiont that could synthesize ATP were considered to be ancestor heterotrophic eukaryotes.