A v o n 3

tRNAs amino acids

met arg met arg

Figure 4.7 Translation. Messenger RNA encodes a gene depicted at the top in triplet codons. Transfer RNAs (tRNAs), which translate the triplet codes into a sequence of amino acids, are depicted by the curlicue structure of their sugar-phosphate backbones. At one end of the tRNA are three letters corresponding to the triplet anticodons that are complementary to the codons in mRNA. The corresponding amino acids are attached to the opposite ends of the tRNAs. The process of making proteins involves breaking the bonds between amino acids and tRNA and making new bonds to hold the amino acids together in the new protein.

properly. This is achieved in cells through interactions with particles called ribosomes. Ribosomes are composed of many proteins and several RNA molecules. These are arranged in such a way that ribosomes can slide along an mRNA molecule, while cavities in the ribosome structure hold tRNA molecules loaded with an amino acid in the right orientation. This ensures that codons, anticodons, and amino acids are aligned properly in close proximity and in the right order. The full protein "assembly line" is depicted in figure 4.8. Note that protein synthesis start at the AUG RNA codon (with methionine). At a UGA codon, the ribosome cavity facing a UGA codon remains empty because no tRNA with an ACU anticodon exists, thus protein synthesis stops. The now fully formed protein then leaves the ribosome and becomes free to exercise its function where it is needed.

Transcription and translation basically work the same way in all living cells, from bacteria to humans.

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