Endosymbiotic Origins

Eukaryotic Cell

Prokaryotic Cell

      There are two basic types of cells found in organisms, prokaryotic cells and eukaryotic cells. Prokaryotic cells are those found in bacteria and archaea, while protists, fungi, animals, and plants are composed of eukaryotic cells. One of the main differences between these two types of cells is that chromosomes are located in the membrane-bound nucleus in eukaryotes and in a non-enclosed area known as the nucleoid in prokaryotes. Eukaryotes also contain other membrane-bound organelles while prokaryotes do not. Endosymbiotic theory posits that these membrane-bound organelles were originally small prokaryotes that were engulfed by larger predatorial cells.

     There is a large amount of evidence in support of this theory. For example, both mitochondria and chloroplasts have their own DNA which is found in one circular molecule not associated with histones. These organelles also have their own transfer RNAs, ribosomes, and other molecules needed for transcription and translate. Also, mitochondria and chloroplasts both divide in a process more similar to that of binary fission which occurs in prokaryotes. Mitochondria and chloroplasts have ribosomes which are more similar to prokaryotic ribosomes in terms of nucleotide sequence, size, and antibiotic sensitivities.
     You might be wondering how and why this would happen. As far as how, the smaller prokaryotic cells most likely entered the eukaryotic cells as internal parasites or undigested prey. From there a mutual relationship developed, benefiting both cells. The smaller cells gained protection in an increasingly oxygen enriched environment from the eukaryotic host, while the host was able to use nutrients from the photosynthetic prokaryote. Both cells became increasingly dependent on the benefits gained from the relationship until they were no longer separate organisms.