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Gene Expression and Cell Differentiation There are hundreds of different types of cells in your body, and each type has a unique function. We’re going to compare a few different types of cells to see how much they have in common. Your Best Guess For each pair of cells in your body, you are going to predict what percentage of DNA is the same in the two types of cells. What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your blood cells and nerve cells? What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your rods (eye) and lung cells? What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your liver and bone cells? What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your blood cells and nerve cells? Answer: 100% What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your rods (eye) and lung cells? Answer: 100% What percentage of the DNA is the Images Courtesy of Wikimedia Commons same in your liver and bone cells? Answer: 100% Hmmm… How can these cells that have very different functions have the exact same DNA in the nucleus? Hmmm… How does the body use the exact same set of instructions to make such different structures? • http://www.dnatube.com/video/2933/The-HumanGenome-Project-Video--3D-Animation-Introduction • http://www.dnalc.org/resources/3d/10-tripletcode.html • http://www.dnalc.org/resources/3d/09-how-much-dnacodes-for-protein.html Gene Expression • The explanation for how many different cell types are made from the exact same DNA lies in how the genes are expressed in each cell type. • The gene to make a certain protein is turned on in some cells and turned off in others. • All cells have all the instructions in their DNA to make all proteins, but they only use the sections of DNA or genes that they need. • Gene expression is a regulated process. Cell Differentiation In a fetus, cells with the exact same DNA are directed to differentiate by chemical signals that cause certain genes to be switched on or off in each one. Promoter: The Switch A promoter is a sequence of DNA needed to turn a gene on or off. The process of transcription is initiated at the promoter. Usually found near the beginning of a gene, the promoter has a binding site for the enzyme used to make a messenger RNA (mRNA) molecule. Repressor A repressor is a protein that turns off the expression of one or more genes. The repressor protein works by binding to the gene's promoter region, preventing the production of messenger RNA (mRNA). Introns and Exons • Introns – Regions of DNA that are not: • Expressed • Translated – Regions that are removed Introns and Exons • Exons – Regions that code for proteins – Regions that are expressed – Regions that remain after RNA splicing Watch for a Demonstration!!! Epigenome • The term epigenome is derived from the Greek word epi, which literally means "above" the genome. • The epigenome consists of chemical compounds that modify, or mark, the genome in a way that tells it what to do, where to do it, and when to do it. • Different cells have different epigenetic marks. • The environment causes changes in our epigenetics. Epigenetics Use the link below to watch the NOVA video “Epigenetics”. http://video.pbs.org/video/1525107473# DNA Microarrays Images Courtesy of Wikimedia Commons This technology helps scientists understand the differences in different types of cells, despite the fact that they have the exact same DNA. DNA microarrays allow scientists to observe which genes are expressed in different types of cells.