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QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Gene regulation in prokaryotes and eukaryotes • How do organisms respond correctly to their environment? • How do they control their growth and development? • How do they control which genes are expressed? Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The process by which genetic information flows from genes to proteins is called gene expression Prokaryotes Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes Figure 11.1A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings genes for related enzymes are often controlled in groups called operons lac operon regulator gene promotor operator i gene codes for repressor protein p o z gene y gene binding site codes for of RNA b-galactosidase, polymerase which clips lactose molecules a gene codes for permease enzyme that transports lactose into cells DNA lac operon regulator gene i gene RNA polymerase p operator o z gene promotor repressor protein blocks binding of RNA polymerase y gene a gene DNA no transcription repressor protein Regulatory proteins bind to control sequences in DNA and turn operons on or off 2 RNA polymerase binds to promoter p i gene o z gene y gene a gene DNA 3 transcription proceeds repressor mRNA transcript lactose b-galactosidase 1 lactose the (inducer) inactivates the repressor so that it cannot bind to the operator permease galactose glucose lactose • Two types of repressor-controlled operons Promoter Operator Genes DNA Active repressor Active repressor Tryptophan Inactive repressor Inactive repressor Lactose lac OPERON Figure 11.1C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings trp OPERON Eukaryotic gene control • Transcription is a major control point • Individual genes are regulated, rather than operons • Activation is more important than repression Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Activators bind to enhancer regions of DNA and to other transcription factors – These interactions turn the DNA transcription Transcription of eukaryotic factors genes on or off Enhancers Promoter Gene Activator proteins Other proteins RNA polymerase Bending of DNA Figure 11.8 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Transcription Control of transcription • Control access of enzymes to DNA: 1. Methylation of DNA - inhibits 2. Acetylation of histones - promotes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How do eukaryotes control genes of a single metabolic pathway? • genes for individual enzymes of one pathway are often located far apart in the genome • same enhancer region(s) common to all genes of a pathway. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Eukaryotic RNA may be spliced in more than one way • After transcription, alternative splicing may generate two or more types of mRNA from the same transcript Exons DNA RNA transcript RNA splicing mRNA Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings or Figure 11.9 Translation and later stages of gene expression are also subject to regulation • The lifetime of an mRNA molecule helps determine how much protein is made • Initiation of translation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The protein may need to be activated in some way • Rate of protein degradation Folding of polypeptide and formation of S–S linkages Initial polypeptide (inactive) Folded polypeptide (inactive) Cleavage Active form of insulin Figure 11.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings DNA double helix (2-nm diameter) multiple levels of DNA packing Histones “Beads on a string” Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Supercoil (200-nm diameter) •DNA packing tends to block gene expression 700 nm Metaphase chromosome Figure 11.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In female mammals, one X chromosome is inactive in each cell EARLY EMBRYO TWO CELL POPULATIONS IN ADULT Active X Allele for orange fur X chromosomes Orange fur Inactive X Cell division and X chromosome inactivation Inactive X Active X Black fur Allele for black fur Figure 11.7 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome DNA unpacking Other changes to DNA GENE GENE TRANSCRIPTION Exon RNA transcript Intron Addition of cap and tail Splicing Tail Cap mRNA in nucleus NUCLEUS Flow through nuclear envelope mRNA in cytoplasm CYTOPLASM Breakdown of mRNA Translation Brokendown mRNA Polypeptide Cleavage/modification/ activation ACTIVE PROTEIN Breakdown of protein Brokendown protein Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 11.11 Review: Multiple mechanisms regulate gene expression in eukaryotes • Each step of gene expression can be turned on or off, speeded up, or slowed down • most important control is usually the start of transcription • DNA can be mobile; position will affect transcription. Transposons Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. What is a gene? • First version: one gene, one polypeptide • Now: DNA segment that is transcribed to RNA • Proteins and RNA function in cells Nature vs nurture • Do environments or genes determine phenotype? • Himalayan rabbits and fur color • Twin studies - identical cp. fraternal Complex human traits • Language - FoxP2 • Depression • Social bonds Do 460 bp determine our life partner? Genome imprinting • Same gene is expressed differently, depending on whether it was inherited from the male or female parent • Ex. Corn kernel color • Ex. Xsome 15 deletion • Gene is “reset” during gamete formation