<P> The division between prokaryotes and eukaryotes is usually considered the most important distinction or difference among organisms . The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus . Both eukaryotes and prokaryotes contain large RNA / protein structures called ribosomes, which produce protein . </P> <P> Another difference is that ribosomes in prokaryotes are smaller than in eukaryotes . However, two organelles found in many eukaryotic cells, mitochondria and chloroplasts, contain ribosomes similar in size and makeup to those found in prokaryotes . This is one of many pieces of evidence that mitochondria and chloroplasts are themselves descended from free - living bacteria . This theory holds that early eukaryotic cells took in primitive prokaryotic cells by phagocytosis and adapted themselves to incorporate their structures, leading to the mitochondria we see today . </P> <P> The genome in a prokaryote is held within a DNA / protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope . The complex contains a single, cyclic, double - stranded molecule of stable chromosomal DNA, in contrast to the multiple linear, compact, highly organized chromosomes found in eukaryotic cells . In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids . </P> <P> Prokaryotes lack mitochondria and chloroplasts . Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane . However, prokaryotes do possess some internal structures, such as prokaryotic cytoskeletons . It has been suggested that the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane - bound cellular structures . However, further investigation revealed that Planctomycetes cells are not compartmentalized or nucleated and like the other bacterial membrane systems are all interconnected . </P>

Where did the first prokaryotic cell come from