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Gene Expression and Regulation Chapter 12-5 • All of your cells contain the same DNA. • Your nerve cells do not look like or act like skin cells, so how do they know what to do and what to produce? – Genes are to be turned on or turned off based on whether they will be used or not – When a gene is actually transcribed and translated, it is said to be expressed. I. Basic Structure of a gene • When looking at a gene sequence, there are several important regions that enzymes and other proteins recognize. Regulatory Promoter sites Start transcription DNA strand Stop transcription 1. Promoter- region of gene where RNA Polymerase binds 2. Start- transcription begins (TAC) 3. Stop- Transcription ends (stop codon) 4. Regulatory sites- near the promoter where regulatory proteins can bind (turn genes on and off) II. Prokaryotic Gene Regulation • Prokaryotes have a single chromosome and are unicellular • Turn genes on and off when needed • Operon- group of genes that operate together for a function, can be – Inducible – Repressible • A repressor molecule can block, or repress, transcription by binding to a region called the operator. – Inducible- in presence of a substance, the substance causes the repressor to let go – Repressible- in presence of a substance, the substance causes the repressor to grab onto the operator Example: lac operon • The lac operon is a series of genes in E. coli that operates together to metabolize (use as food) lactose, the sugar found in milk – Lactose is a disaccharide made of glucose and galactose – In the presence of lactose, certain enzymes must be produced to break them down Example: lac operon • P (promoter)- region that RNAP binds • O (operator)- region that a repressor can bind, blocking RNAP from transcribing • Gene Z- codes for β-galactosidase – which breaks down lactose into glucose and galactose • Gene Y- codes for Permease – Which causes the cell membrane to be more permeable to lactose (let it in) • Gene A- codes for a protein whose function is unknown Steps for lac operon • In absence of lactose, a repressor binds to the operator, blocking RNAP • 1. In the presence of lactose, lactose binds to the repressor causing it to let go of the operator – 2. RNAP transcribes gene Z (makes βgalactosidase) and gene Y (makes permease) – 3. Ribosomes translates β-galactosidase and permease – 4. β-galactosidase breaks down lactose into glucose and galactose – 5. Permease allows lactose to flood into the bacterial cell – 6. Once lactose is all broken down, lactose lets go of the repressor so it can rebind to the operator – 7. This stops transcription and translation of β-galactosidase and permease III. Eukaryotic Gene Regulation • No operons, genes are regulated individually • Similar process, but is much more complex than prokaryotic gene regulation – Prokaryotes have no cell specialization • There is a promoter region, several enhancer sequences and a “TATA box”. – TATA- helps position RNA Polymerase – enhancers can act like operators and block transcription, and others can signal to unpack chromatin • Exons actually get “EXPRESSED” • Introns are cut out of the mRNA copy and not transcribed and translated by ribosomes. • After fertilization and mitosis occurs, cells specialize into their life-long functions through a process called differentiation • Differentiation is controlled by hox genes. – Some genes get turned off permanently (your liver cells do not express genes that make proteins in the skin) – Like master controls of what cells become what part of the body – Manipulation of these genes can alter what parts grows where (a leg out of your head?!)