Prokaryotes vs. Eukaryotes

Prokaryotes, organisms in the domains Bacteria and Archaea, are much older than eukaryotes, organisms in the domain Eukarya, and prokaryotes appeared in the fossil record 3.5 billion years ago, while eukaryotes appeared around 2.7 billion years ago. Prokaryotic cells are typically unicellular, while eukaryotic cells are often multicellular and much more complex. Eukaryotic cells are also larger than prokaryotic cells, with an approximately 10,000 times greater cell volume. Eukaryotic cells range in size from roughly 10 µm (micrometers), such as red blood cells, to 100 µm or more, such as amoebas. Prokaryotic cells can be smaller than 1 µm and as large as 10 µm. However, prokaryotic cells have a larger surface area to volume ratio than do eukaryotic cells. This means that prokaryotic cells have a higher metabolic rate and growth rate and a shorter generation time (average time between two generations of an organism) compared to eukaryotic cells.

There are also many structural differences within the cells, the most important being that eukaryotes have a distinct nucleus and prokaryotes do not. The eukaryotic nucleus is enclosed by a double layer membrane and contains many chromosomes (~21,000 genes in human cells). In contrast, prokaryotes contain either a single circular chromosome, double circular chromosome, or linear chromosome in a compartment of the cytoplasm called the nucleoid. The genomes of prokaryotes are smaller than eukaryotes, for example Bacillus subtilis has just over 4,000 genes and Pseudomona aeruginosa has just under 6,000. Prokaryotic cells also lack membrane-bound organelles, such as the rough endoplasmic reticulum where proteins are synthesized, the Golgi apparatus, and mitochondria or chloroplasts (though they can have chlorophyll in their cytoplasm, such as cyanobacteria).

Both types of cells have a semipermeable cell membrane made of a double layer of phospholipids that contains the cytoplasm. However, eukaryotic cell membranes contain sterol molecules that stabilize the membrane, while prokaryotes do not have sterol in their cell membranes. Although animal cells do not have cell walls made of the polysaccharides peptidoglycan or pseudopeptidoglycan like prokaryotes, other eukaryotic cells do have cell walls. The polysaccharide components of eukaryotic cell walls are different, as fungi have cell walls made of chitin and plants have cell walls made of cellulose. Prokaryotes also have a number of external structures, including the glycocalyx (covering composed of polysaccharides) and pili (tubular appendage) for attachment and whip-like flagella for motility. Eukaryotic cells can also have flagella, but these differ from prokaryotic flagella by the number and arrangement of proteins involved in the structure.

Translation, or protein synthesis, is the assembly of amino acids into proteins by the ribosomes through the reading of messenger ribonucleic acid (mRNA) by transfer ribonucleic acid (tRNA) and the ribosome. Put more simply, it is the process by which cells build proteins from information that originated in the DNA. DNA is first transcribed into mRNA, which is then translated on ribosomes into chains of amino acids called polypeptides. The polypeptides are folded into three-dimensional proteins that have important functions in the cell. Both eukaryotes and prokaryotes carry out protein synthesis on ribosomes in the cytoplasm, but with several important differences.

In eukaryotes, the mRNA is first transcribed in the nucleus and then exported to the cytoplasm or the endoplasmic reticulum, where ribosomes facilitate translation. The mRNA is monocistronic, meaning it contains the sequence to make a single protein. Eukaryotic genes have noncoding sequences called introns in between protein coding regions called exons. Within the nucleus, the mRNA is processed and spliced to remove introns and join exons. It is then exported to the cytoplasm for protein translation. Eukaryotic mRNAs also have special structures on each end that play important roles in translation. At the five-prime (5´) end, which is the beginning of the mRNA, there is a guanine nucleotide cap connected to the mRNA by a triphosphate linkage. The cap aids in mRNA export from the nucleus and binds the mRNA to the ribosome. At the three-prime (3´) end of the mRNA is a long stretch of adenosine nucleotides called a poly(A) tail. The poly(A) tail also aids in nuclear export and translation and eventually marks the mRNA for degradation when it is old and no longer needed. Translation begins when the capped mRNA, along with over 10 other factors, binds the ribosome, and the first tRNA (bound to the amino acid methionine) is matched to the start codon in the mRNA. Prokaryotic mRNA is transcribed in the cytoplasm and translated on ribosomes that are free-floating in the cell using tRNA to link the individual amino acids. The mRNA is usually polycistronic, meaning it contains the sequences to make several, often functionally related, proteins. Translation of prokaryotic mRNA can either produce a large polypeptide that gets cleaved into distinct proteins after being released by the ribosome, or ribosomes can start at different internal sites on the mRNA to make different proteins. Typically transcription and translation in prokaryotic cells are coupled, and translation begins before transcription is complete. Bacterial genes do not have introns (but Archaea genes do), and therefore their mRNA does not undergo any processing prior to translation.