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SC.912.L.16.6 All cells contain a set of genes, which can be thought of as a set of instructions for making each of a very large number of proteins. The creation of a protein from its gene is called gene expression. However, for a given cell not all of these instructions are actually used, and among those that are, some are used more than others or only under certain circumstances. Controlling gene expression is critical to a cell because it allows it to avoid wasting energy and raw materials in the synthesis of proteins it does not need. Remember, transcription is the process of making a messenger ribonucleic acid (mRNA) copy of the deoxyribonucleic acid (DNA) gene. Transcription can only occur if RNA polymerase first attaches, or binds, to the DNA. Controlling this binding process is the major way that gene expression is controlled, and proteins are the major controllers of binding. A protein molecule that helps regulate binding can itself be regulated. This usually occurs when some other molecule binds to the protein, causing the protein to undergo a structural change, in other words, to change shape. In some cases this shape change will help RNA polymerase to bind to DNA, and in other cases it will prevent it from doing so. Regulation of gene expression is by necessity far more complex in eukaryotic cells (cells with a nucleus) than in prokaryotic cells. • eukaryotic cells are larger and more highly compartmentalized • multicellular eukaryotes pass through a number of developmental stages, each requiring different proteins, on the road to their final differentiated state. • multicellular organisms contain many different cell types, each of which expresses distinct sets of proteins. How does an organism know whether to turn a gene on or off? Operons-a group of genes that operate together Example: lac operon in E. coli, control genes that code for proteins that break down lactose for food • Lac operons are turned on by lactose and turned off by repressors (a protein that blocks the transcription of lac genes) Many genes are regulated by repressor proteins, while others can use proteins to speed up transcription Most eukaryotic genes are controlled individually and have regulatory sequences that are much more complex than prokaryotic operons. Genes are regulated by enhancer sequences located at a point where transcription beginsdifferent proteins can bind here causing transcription to speed up or not take place at all Example: TATA box-region of DNA containing sequence TATATA before the start of transcription • Helps position RNA polymerase