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cysteinyl-tRNA

synthesis (.although DNA replication and transcription art! also often mediated by larger metltisubunit complexes). In contrast, the machinery for polymerizing amino acids is composed of at least three RNA molecules up to about three kilobases in size and more than 50 different proteins, with an overall molecular mass of greater than 2.5 megndaltons. Compared to the speed of DNA replication—200 to 1,000 nucleotides per second—translation takes place at a rate of only 2 to 20 amino acids per second.

In prokaryntes, the transcription machinery and the translation machinery are located in the same compartment. Thus, the ribosorne can commence translation of the mRNA as it emerges from the RNA polymerase. This situation allows the ribosome lo proceed in tandem with the RNA polymerase as it elongates the transcript (Figure 14-11). Recall that the 5' end of an RNA is syndiesized First, and thus translation, which also starts at the 5' end of the mRNA, nan commence on nascent transcripts as soon as they emerge from the RNA polymerase. Interestingly, there are several instances in which the coupling of transcription and translation is exploited during the regulation of gene expression, as we shall see in Chapter 10.

Although slow relative to DNA synthesis in prokaryotes, the ribosome is capable of keeping up with the transcription machinery. The typical prokaryotic rate of translation of 20 amino acids per second corresponds to the translation of 60 nucleotides (20 codons) of mRNA per second. This is similar to the rate of 50 to 100 nucleotides per second synthesized by RNA polymerase.

In contrast to the situation in prokaryotes, translation in eukary-otes is completely separate from transcription. Indeed, these events occur in separate compartments of the cell: transcription occurs in the nucleus, whereas translation occurs in the cytoplasm. Perhaps due to the lack of coupling to transcription, eukaryotic translation proceeds at the more leisurely speed of 2-4 amino acids per second.

FIGURE 14-11 Prokaryotic RNA polymerase and the ribosome at work or the same mRNA.

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