1 Gene Expression Every cell has the same DNA Yet, each cell is different, specialized Differences...

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

Gene Expression is Influenced by the Organization of DNA Prokaryotes have:

Several million base pairs in one chromosomeRelated genes are grouped togetherDNA that is mostly coding DNA

Eukaryotes have:Billions of base pairs in several chromosomesGenes that are not grouped togetherDNA that is mostly non-coding DNA

The Organization of DNA is Influenced by the 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

Gene Expression is Controlled by DNA Packaging Eukaryotic chromosomes are very large Must be packaged

Unavailable for transcription Unpacking must occur before transcription

Levels of DNAPackaging Fully condensed DNA,

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

has to be unpacked for transcription

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

Patterns of Gene Expression

Eukaryotic cells Tissue specific

expression Housekeeping

Gene Expression: Development

Embryo development depends on gene expression

Timing of expression is complex, yet vital Controlled by cascades of gene

expression

Master switches Start gene cascades Coordinate development of specific

structures Cell and timing specific expression

Homeotic genes

Similar effects in different organisms

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

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

Control of Gene Expression

E. coli Tryptophan

genes Repressor Operator

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

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

Levels of Gene Control1. Packaging

2. Transcription

3. mRNA breakdown

4. Translation

5. Protein Regulation

6. Protein Degradation

Gene Control and Phenotype

Can have powerful effectsHomeotic genes

Can result from environmental conditionsGenes turn on and off Phenotype changes

Process Animation 14.2The Tryptophan Operon

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Process Animation 14.3Control of Gene Expression

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1 Gene Expression Every cell has the same DNA Yet, each cell is different, specialized Differences...

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