Prokaryotic and Eukaryotic Cells
Written by tutor Anneliese A.
Prokaryotic cells evolved before eukaryotic cells (“pro” comes from the Greek word for “before”), and in general are much simpler in structure and function. Prokaryotes are found in the domains of Bacteria and Archaea, while eukaryotes make up the remaining domain. They tend to be much smaller in size than eukaryotic cells and contain no membrane-bound organelles, such as a nucleus, endoplasmic reticulum, or mitochondrion.
Prokaryotic DNA is in a circular format – called plasmids – rather than the linear format found in eukaryotes, and the genome tends to be much smaller/shorter. Importantly, some plasmids have the ability to donate and/or receive fragments of genetic code from other cells. Eukaryotic DNA is isolated within a nucleus, which has its own selectively permeable membrane, while prokaryotic DNA is found in the cytoplasm. Furthermore, genetic material is not packaged in prokaryotes, in contrast to the histones and chromosome condensation used in storing eukaryotic DNA. Despite these differences, the format of the genetic code itself is identical in both cell types.
While some eukaryotic cells have a cell wall (i.e. plant cells), others do not. Almost all prokaryotes, on the other hand, will have a cell wall formed of peptidoglycan layers. Particularly within the Archaea domain, this often means that prokaryotes can potentially withstand much more extreme conditions (temperature, pH) than eukaryotes.
Eukaryotes and prokaryotes do share some similarities. Both contain vacuoles and vesicles which are used in cell metabolism and cell clean-up. The lipid bilayer membrane is found in both, allowing cells to specifically select what may enter and exit. Both cell types are filled with cytosol; in eukaryotes, the cytosol contains organelles which perform the major functions of the cell, while in prokaryotes, it is the site of most metabolic pathways and DNA replication. While both prokaryotes and eukaryotes both contain ribosomes involved in protein synthesis, those in eukaryotes are larger and more complex.
It is generally believed that prokaryotes and eukaryotes share a common ancestry. Another widely held theory is that the mitochondria found in eukaryotes are in fact entire prokaryotic cells that were taken up by eukaryotic ancestors. Evidence in support of this idea includes structural similarities, and that mitochondria have their own DNA. Similarly, it is also believed that chloroplasts in plant cells were once enveloped prokaryotes.