FIGURE 5-1 Overview of four basic molecular genetic processes. In this chapter, we cover the three processes that lead to production of proteins 1 – 3 and the process for replicating DNA 4. Because viruses utilize host-cell machinery, they have been important models for studying these processes. During transcription of a protein-coding gene by RNA polymerase 1, the four-base DNA code specifying the amino acid sequence of a protein is copied, or transcribed, into a precursor messenger RNA (pre-mRNA) by the polymerization of ribonucleoside triphosphate monomers (rNTPs). Removal of noncoding sequences and other modifications to the pre-mRNA 2, collectively known as RNA processing, produce a functional mRNA, which is transported to the cytoplasm. During translation 3, the four-base code of the mRNA is decoded into the 20–amino acid language of proteins. Ribosomes, the macromolecular machines that translate the mRNA code, are composed of two subunits assembled in the nucleolus from ribosomal RNAs (rRNAs) and multiple proteins (left). After transport to the cytoplasm, ribosomal subunits associate with an mRNA and carry out protein synthesis with the help of transfer RNAs (tRNAs) and translation factor proteins. During DNA replication 4, which occurs only in cells preparing to divide, deoxyribonucleoside triphosphate monomers (dNTPs) are polymerized to yield two identical copies of each chromosomal DNA molecule. Each daughter cell receives one of the identical copies.