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In eukaryotic cells, transcription occurs in the cell nucleus and translation occurs later in the extra-nuclear hyaloplasm.
In prokaryotes, which have no nucleus, not only do transcription and translation occur in the same region of the cell but they also occur at the same time.
In bacteria such as E. coli, the transcription of DNA is accompanied by the translation of the nascent messenger RNA. The mRNA is synthesized beginning with its 5′ end, and as soon as the mRNA strand is long enough, a ribosome attaches to the messenger and begins translation.
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As mRNA synthesis proceeds, more ribosomes attach to the elongating strand to form a polysome. Whereas in eukaryotes mRNA contains the codon sequence for a single polypeptide, prokaryotic mRNAs may be polycistronic (see earlier). Polycistronic mRNAs have multiple initiation codons and ribosomes may attach to any of these independently.
O. L. Miller and others have provided elegant proof of this in the form of electron micrographs such as the one shown in Figure 22-38. In this figure, a portion of the E. coli chromosomal DNA appears as a thin filament being actively transcribed by” a number of RNA polymerase molecules into mRNA strands. To each of these mRNA strands, ribosomes have attached to form polysomes.
In Figure 22-38, it appears that transcription proceeds from left to right along the DNA, as the mRNA length (and therefore polysome size) exhibits a general increase in that direction. Unfortunately, electron micrographs do not reveal the growing nascent polypeptide chains on the ribosomes, because the amino acids that make up the polypeptides are too small to be resolved.
