The genes encoded by the DNA nucleotide sequences control cellular functions by directing the synthesis of structural proteins and enzymes; enzymes, in turn, facilitate all cellular reactions. Typically, one gene controls the synthesis of one protein. Because DNA does not move out of the nucleus and because proteins, including enzymes, are synthesized in ribosomes in the cytoplasm, the translation of a gene's nucleotide sequence into a protein's amino acid sequence must be accomplished by an intermediary messenger, messenger RNA (mRNA). All three types of RNA are produced via a process called transcription, which is very similar to the process of DNA replication. Transcription begins when the two DNA strands uncouple. Unlike in DNA replication, in transcription, only one of the two DNA strands is ever transcribed. An enzyme, RNA polymerase, brings ribonucleotides to the DNA template, thus the deoxyribonucleotides of the DNA determine the order of the ribonucleotides of the RNA. From the nucleus, mRNA moves to the ribosomes where it acts as the template for the synthesis of polypeptide chains. The ribosomes are partly composed of ribosomal RNA (rRNA). Ribosomal RNA is synthesized based on the DNA that makes up the nucleoli of the nucleus and serves to bind the mRNA to the ribosome during protein synthesis. The ribosomes are nonspecific devices, capable of helping with the synthesis of any protein the mRNA is coded to produce. Commonly, a single strand of mRNA will become associated with up to half a dozen ribosomes, forming a complex called a polyribosome. Each ribosome in the polyribosome reads the information passing
