Splicing removes introns to form mRNA that is transported to the cytoplasm, where it will be translated.

The mRNA in eukaryotic cells must be transported out of the nucleus before it can be translated in the cytoplasm. Thus, the same mRNA molecule cannot be transcribed and translated at the same time or even in the same cellular location. Unlike in prokaryotes, the mRNA of eukaryotes is generally mono-cistronic. Translation of the message generally begins at the first occurrence of AUG in the molecule.

The ribosomes of eukaryotes are different from those of prokaryotes. Whereas the prokaryotic ribosome is 70S, made up of 30S and 50S subunits, the eukaryotic ribosome is 80S, made up of 40S and 60S subunits. The differences in ribosome structure account for the ability of certain types of antibiotics to kill bacteria without causing significant harm to mammalian cells.

Some of the proteins that play essential roles in translation differ between eukaryotic and prokaryotic cells. Diphtheria toxin, which selectively kills eukaryotic but not prokaryotic cells, illustrates this difference. This toxin is produced by Corynebacterium diphtheriae; it binds to and inactivates one of the elongation factors of eukaryotes. Since this protein is required for translocation of the ribosome, translation ceases and the eukaryotic cell dies, resulting in the typical symptoms of diphtheria. ■ diphtheria toxin, p. 570

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