286 Chapter 9 • Proteins and Their Synthesis illustrated in Figure 9-15. The binding site for mRNA is completely within the small subunit. There are three bind-ing sites for tRNA molecules. Each bound tRNA bridges the 30S and 50S subunits, with its anticodon end in the former and its aminoacyl end (carrying the amino acid) in the latter. The A site (for aminoacyl) binds an incoming aminoacyl tRNA whose anticodon matches the codon in the A site of the 30S subunit. As we move in the 5 9 direc-tion on the mRNA, the next codon interacts with the anti-codon of the tRNA in the P site (for peptidyl) of the 30S subunit. The tRNA in the P site contains the growing pep-tide chain, part of which ±ts into a tunnel-like structure in the 50S subunit. The E site (for exit) contains a deacy-lated tRNA (it no longer carries an amino acid) that is ready to be released from the ribosome. Whether codon–anticodon interactions also take place between the mRNA and the tRNA in the E site is not clear. rRNA
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