Ribosomes in eukaryotic cells are larger than those in prokaryotic cells. They consist of a particle weighing 4,200 kilodaltons in eukaryotes, compared to a particle weighing 2,400 kilodaltons in prokaryotes. Biochemists use kilodaltons to express the mass of atoms and molecules. Several types of antibiotics utilize this ribosomal difference which disrupts translation in prokaryotic ribosomes only and does not affect translation.
In both types of organisms, transfer-RNA initiates protein synthesis at the ribosome by bringing in the first amino acid for the respective protein being synthesized. However, the type of transfer-RNA and the first amino acid differs. In prokaryotes, a special initiator transfer-RNA, called Met-tRNA, brings the amino acid N-formylmethionine to initiate protein synthesis. Eukaryotic translation, instead, is initiated with the amino acid methionine.
Prokaryotic and eukaryotic translation also differ in the initiation process. Translation commences when the start codon is found on the mRNA strand by the small ribosomal subunit. Eukaryotes have only one start codon, the nucleotide sequence AUG, and thus, only one protein is normally translated at a time. Prokaryotic genes, however, may have several different start codons meaning that several different proteins can be synthesized at one time.
Translation commences when the ribosome reaches a stop codon on the mRNA through a process called termination. These stop codons are recognized by proteins referred to as releasing factors. Eukaryotes utilize two releasing factors to carry out the termination step of protein synthesis, while prokaryotes have three different releasing factors.