6 Chapter 1 • Genetics and the Organism Protein synthesis takes place on cytoplasmic organelles called ribosomes. A ribosome attaches to one end of an mRNA molecule and moves along the mRNA, catalyzing the assembly of the string of amino acids that will consti-tute the primary polypeptide chain of the protein. Each kind of amino acid is brought to the assembly process by a small RNA molecule called transfer RNA (tRNA), which is complementary to the mRNA codon that is being read by the ribosome at that point in the assembly. Trains of ribosomes pass along an mRNA molecule, each member of a train making the same type of polypep-tide. At the end of the mRNA, a termination codon causes the ribosome to detach and recycle to another mRNA. The process of translation is shown in Figure 1-7. GENE REGULATION Let’s take a closer look at the structure of a gene, which determines the ±nal form of the RNA “working copy” as well as the timing of tran-scription in a particular tissue. Figure 1-8 shows the gen-
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