The genetic code (page 75) dictates the sequence of amino acids in a protein molecule. The synthesis of proteins is quite complex, requiring three types of RNA. Messenger RNA (mRNA) contains the code and is the template for protein synthesis. Transfer RNAs (tRNAs) are adapter molecules that carry amino acids to the mRNA. Ribosomal RNAs (rRNAs) form part of the ribosome that brings together all the components necessary for protein synthesis. Several enzymes also help in the construction of new protein molecules. This chapter describes how the nucleotide sequence of an mRNA molecule is translated into the amino acid sequence of a protein.
Figure 8.1 shows the basic mechanism of protein synthesis, also called translation. In the first step, free amino acids are attached to tRNA molecules. In the second step, a ribosome assembles on the mRNA strand to initiate synthesis. In the third step, the ribosome travels along the mRNA. At each codon on the RNA a tRNA binds, bringing the amino acid defined by that codon to be added to the growing polypeptide chain. In the last, fourth, step the ribosome encounters a stop codon and protein synthesis is terminated.
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