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Gene Regulation • An expressed gene is one that is transcribed into RNA • Not all genes are expressed by every cell • How does an organism know when to “turn on” or “turn off” a gene? Typical Gene Regulation • See page 309 in your textbook Figure 12-22 stop start gene promoter Regulatory sites Promoters- DNA sequence where RNA polymerase will bind Operator (regulatory sites)- stretches of DNA where proteins can bind to regulate transcription Start and stop signals- for transcription of gene Prokaryotic Gene Expression usually uses OPERONS. OPERONS- are a group of genes that operate together the Lac operon is a great example! It’s a set of genes needed for E.coli to make proteins so the bacteria can use the sugar lactose as food! The Lac operon has 1. a promoter (the nucleotide sequence where RNA Polymerase binds- green) 2. an operator (segment of DNA used to turn gene expression on or off, repressor protein binds here- dotted/shaded region) 3. Three genes for making proteins involved in breaking down lactose See page 310 Fig.12-23 How the Lac Operon works or operates…. Lac genes turned OFF by the repressor protein and turned ON by the presence of Lactose WITHOUT Lactose- DNA binding protein is bound to the OPERATOR (where protein binds) so RNA polymerase CAN’T WORK (can’t make the mRNA) WITH Lactose- lactose binds to repressor DNA binding protein, this causes the protein to CHANGE SHAPE and FALL OFF the DNA Operator so RNA polymerase CAN WORK (transcribes the gene) Without Lactose: With Lactose: Lac operon animation • http://www.sumanasinc.com/webcontent/a nimations/content/lacoperon.html Eukaryotic Gene Regulation: • Most eukaryotic genes are controlled individually NOT with operons • Only a few of the available genes will be expressed in the cells of different tissues (cell specialization/differentiation) • Complex! Eukaryotic Gene Regulation See page 311 in your textbook Figure 12-24 TATA Box exon enhancer intron exon intron exon Promoter sequences Enhancer sequence- lots of proteins can bind here on different sequences (that’s why eukaryotic gene regulation is more complex!!) Promoters- usually just before the TATA Box (where RNA polymerase binds) TATA Box- helps position RNA polymerase in front of gene Exons- expressed Introns- cut out before translation Proteins that bind to enhancer sequences of a gene can work to: a. Open up tightly packed chromatin. b. Attract RNA polymerase. c. Block access to genes. RNA Editing unit of transcription in a DNA strand exon intron exon intron exon 3’ 5’ transcription into pre-mRNA poly-A tail cap 5’ 3’ (snipped out) (snipped out) 5’ 3’ mature mRNA transcript Fig. 14.9, p. 229 Prokaryotic vs. Eukaryotic Gene Expression • http://highered.mcgrawhill.com/sites/0072437316/student_view0/ chapter15/animations.html# Hox genes Development and Differentiation • Hox genes are master control genes that control the differentiation of cells and tissues in the embryo. • A mutation in hox gene in fruit flies had a leg growing where antenna should be.