DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC TRANSLATION
Translation in eukaryotes differs from prokaryotes in many ways (Fig. 2.18). First of all, mRNA is made in the nucleus, but translation occurs on the ribosomes in the cytoplasm. Therefore, there is no coupled transcription and translation in eukaryotes. Eukaryotic ribosomes have 60S and 40S subunits that combine to form an 80S ribosome, which is a little larger than bacterial ribosomes. Additionally, eukaryotes have more initiation factors than prokaryotes, and they assemble the initiation complex in a different order. Overall, more proteins are involved in eukaryotic translation to deal with the greater complexity of regulation.
Despite this, the binding of the mRNA is simpler in eukaryotes. Eukaryotic mRNA does not have a Shine-Dalgarno sequence. Instead, eukaryotic ribosomes recognize the 5′ cap structure and begin protein synthesis at the first AUG. Only one gene per mRNA is found (unlike bacteria, which often have polycistronic messages and whose ribosomes recognize separate Shine-Dalgarno sequences for each coding sequence). The first amino acid in each new polypeptide is methionine, as in bacteria. However, unlike in bacteria, this methionine is not modified with a formyl group. Finally, many eukaryotic proteins are modified after translation by addition of chemical groups. (Although bacteria do modify some proteins, this is much rarer and the variety of additions is much more limited.)