Different Sizes and Compositions
The sizes of ribosomes and other cell structures are described in terms of Svedberg units (S), which are actually a measure of sedimentation rate in a centrifuge. Prokaryotic ribosomes are 70S particle units with 30S and 50S subunits -- with the 30S subunit containing 16S RNA and 21 peptides. Eukaryotic ribosomes are 80S units with 40S and 60S subunits -- with the 40S subunit containing 18S RNA and 33 polypeptides and the 60S containing 28S RNA, 5.8S RNA, 5S RNA and 49 polypeptides.
Locations within the Cell
One of the major differences between prokaryotic and eukaryotic cells is that eukaryotic cells have cell organelles with distinct membranes and prokaryotic cells do not. This means that ribosomes in prokaryotic cells are free-floating in the cytoplasm. In eukaryotic cells, ribosomes can roam free in the cytoplasm, but they can also be bound to the exterior of the endoplasmic reticulum. Ribosomes can also be found in mitochondria and chloroplasts of eukaryotic cells.
Initiation of Protein Synthesis
Ribosomes synthesize proteins in both prokaryotic and eukaryotic cells, but the process is initiated differently in each cell type. In eukaryotic cells, the starter amino acid is methionine rather than the N-formylmethionine used by prokaryotic cells. In both cell types, a special transfer RNA (tRNA) participates in the initiation. Both cell types also use the RNA sequence AUG as an initiator. However, eukaryotes, unlike prokaryotes, do not use a specific sequence to differentiate initiator AUG sequences from internal ones not meant to kick off protein synthesis.
Elongation and Termination
Elongation, or the assembly-line production of proteins, is very similar in prokaryotes and eukaryotes, with the only notable difference being that the eukaryotic elongation factors EF1α and EF1βγ are EF-Tu and EF-Ts in prokaryotes. In prokaryotes, two proteins, RF-1 and RF-2, called “release factors” recognize the stop sequence on mRNA and terminate the protein-making process. Termination in eukaryotes is accomplished through a similar single release factor, called eRF1.