1 Gene Expression Every cell has the same DNA Yet, each cell is different, specialized Differences due to gene expression  Which genes are turned on  When the genes are turned on  How much product they make Different factors control all this

2 Gene Expression is Influenced by the Organization of DNA Prokaryotes have:  Several million base pairs in one chromosome  Related genes are grouped together  DNA that is mostly coding DNA Eukaryotes have:  Billions of base pairs in several chromosomes  Genes that are not grouped together  DNA that is mostly non-coding DNA

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 3 The Organization of DNA is Influenced by the Genome Size Genome: total component of DNA Prokaryotes  0.6 to 30 million base pairs  Approximately 2,000 genes Eukaryotes  12 million to 1 trillion base pairs  Humans have ~25,000 genes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 4 Gene Expression is Controlled by DNA Packaging Eukaryotic chromosomes are very large Must be packaged  Unavailable for transcription Unpacking must occur before transcription

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 5 Levels of DNA Packaging Fully condensed DNA, seen at metaphase Tightly packed loops 30 nm fibers Histone spool Double helix, even this has to be unpacked for transcription

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 6 Patterns of Gene Expression Bacteria directly exposed to environment Respond to changes in nutrient availability  Make enzymes for nutrients when they are present (turn on the gene)  Turn genes off when they are not

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 7 Patterns of Gene Expression Eukaryotic cells Tissue specific expression Housekeeping genes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 8 Gene Expression: Development Embryo development depends on gene expression Timing of expression is complex, yet vital Controlled by cascades of gene expression

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 9 Master switches Start gene cascades Coordinate development of specific structures Cell and timing specific expression Homeotic genes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 10 Homeotic genes Similar effects in different organisms

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 11 © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 11 Master Gene Switches: SRY During reproduction, males determine sex  If the sperm has an X, it’s a girl  If the sperm has a Y, boy Y chromosome has the SRY gene SRY is a master switch that controls the expression of other genes at other loci SRY changes the embryo to develop into a boy

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 12 Control of Gene Expression Based on two essential elements:  Regulatory DNA: switches gene on/off  Regulatory Protein: binds to regulatory DNA Control gene expression together Respond to environment or cell signals

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 13 Control of Gene Expression E. coli Tryptophan genes Repressor Operator

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 14 Eukaryotic Gene Expression More complex than prokaryotic Many different types of regulatory proteins Many DNA elements controlling each gene In summary, the organization of DNA controls gene expression in complex ways

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 15 Levels of Gene Control Tightly packed DNA is not expressed Transcription regulation Regulation of mRNA breakdown Inhibition of translation Regulation of proteins after translation Destruction of completed proteins

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 16 Levels of Gene Control 1. Packaging 2. Transcription 3. mRNA breakdown 4. Translation 5. Protein Regulation 6. Protein Degradation

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 17 Gene Control and Phenotype Can have powerful effects  Homeotic genes Can result from environmental conditions  Genes turn on and off  Phenotype changes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 18 Process Animation 14.2 The Tryptophan Operon  you tube link you tube link

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 19 Process Animation 14.3 Control of Gene Expression  You tube link You tube link