© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Gene Expression

Every cell has the same genes 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 this

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Organization of DNA

ProkaryotesSeveral million base pairs - one chromosomeRelated genes groupedDNA is mostly coding DNA

EukaryotesBillions of base pairs – several chromosomesGenes not grouped togetherDNA is mostly non-coding DNA

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Genome Size

Genome: total component of DNA Prokaryotes

0.6 to 30 million base pairsApproximately 2,000 genes

Eukaryotes12 million to 1 trillion base pairsHumans have ~25,000 genes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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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

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Levels of DNAPackaging Fully condensed,

seen at metaphase Tightly packed loops 30 nm fibers Histone spool Double helix

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Patterns of Gene Expression

Bacteria directly exposed to environment Respond to changes in nutrient availability

Make enzymes for nutrients when they are present

Turn genes off when they are not

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Patterns of Gene Expression

Eukaryotic cells Tissue specific

expression Housekeeping

genes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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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

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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

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Homeotic genes

Similar effects in different organisms

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Control of Gene Expression

Based on two essential elements:Regulatory DNA: switches gene on/offRegulatory 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

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Control of Gene Expression

E. coli Tryptophan

genes Repressor Operator

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Eukaryotic Gene Expression

More complex than prokaryotic Many different types of regulatory proteins Many DNA elements controlling each gene

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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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

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Levels of Gene Control1. Packaging

2. Transcription

3. mRNA breakdown

4. Translation

5. Protein Regulation

6. Protein Degradation

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Gene Control and Phenotype

Can have powerful effectsHomeotic genes

Can result from environmental conditionsGenes turn on and off Phenotype changes

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Process Animation 14.2The Tryptophan Operon

MacintoshWindows

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

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Process Animation 14.3Control of Gene Expression in Eukaryotes

MacintoshWindows