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.)
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