In both prokaryotic and eukaryotic cells, mRNA molecules are translated simultaneously by multiple ribosomes; see page 000 (Figure 15.25). The resulting structure — an mRNA with several ribosomes attached—is called a polyri-bosome. Each ribosome successively attaches to the ribo-some-binding site at the 5' end of the mRNA and moves toward the 3' end; the polypeptide associated with each ribosome becomes progressively longer as the ribosome moves along the mRNA.

In prokaryotic cells, transcription and translation are simultaneous; so multiple ribosomes may be attached to the 5' end of the mRNA while transcription is still taking place at the 3' end, as shown in Figure 15.26; see page 000. Until recently, transcription and translation were thought not to be simultaneous in eukaryotes, because transcription takes place in the nucleus and all translation was assumed to take place in the cytoplasm. However, research findings have now demonstrated that some translation takes place within the eukaryotic nucleus, and evidence suggests that, when the nucleus is the site of translation, transcription and translation may be simultaneous, much as in prokaryotes.

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