6.3 Gene Regulation

turn genes on/off

BPQ

Why regulate gene expression?

multicellular has cell specialization → altho every cell is genetically the same, diff genes expressed in diff cells

BPQ

How might regulation in be different in multicellular organism than unicellular organism?

turning genes on/off leads to evolution (changes over time)

BPQ

How might gene expression effect heredity and evolution?

Factors that can affect gene expression

• Transcriptional (Prokaryotes & Eukaryotes)

  • Transcription factors (Pro & Eu)

    • Activators

    • Inhibitors/Repressors

  • Histones (Eu & Archaea)

  • DNA Methylation (Pro & Eu)

• Post Transcriptional (Eu)

  • MicroRNA - miRNA

  • Alternate splicing

Transcription

Promoter – region where RNA polymerase binds

RNA polymerase creates mRNA from the template strand

Regulatory Sequence

• a noncoding region of DNA that can influence gene expression.

  • Promoter (Pro & Eu)

  • Operator (Pro)

  • Enhancer (Pro & Eu)

  • Silencers (Eu)

Regulatory Gene

a coding gene that produces a product that can influence gene expression

Transcription Factor

a protein that can influence gene expression by binding regulatory sequences.

Activator – turns on / promotes transcription

Repressor – turns off / decreases transcription

Prokaryotic Operon

• a series of related genes where transcription is initiated by one promoter

  • Creates 1 mRNA & multiple proteins

• often regulated by a repressor that attaches downstream of the promoter in a sequence called an operator (a regulatory sequence)

• Repressor physically blocks RNA polymerase

all the genes in an operon are LINKED

1 gene per promoter allows for complex expression in eukaryotes

BPQ

In Eukaryotes 1 promoter always transcribes 1 gene and only 1 gene.  What might be some reasons for this?  Why do Eukaryotes not use operons?

Lac Operon (Repressor: Inducible)

• Inducible Operator

• Lactose is an Inducer that triggers the transcription of genes responsible for the breakdown of lactose

Inducible Operator

transcription is “turned off” until an inducer binds to the repressor

• Inducer releases the repressor from the operator

Lac Operon (Activator)

• Activators bind to enhancers (regulatory sequence) and guide RNA polymerase onto the promoter.

• cAMP is a coactivator that is produced in the absence of glucose.  cAMP “turns on” the activator CAP and triggers transcription of genes responsible for the breakdown of lactose

1. off

2. off

3. off

4. on

both must be “on” for Lac Operon to turn on

BPQ

Determine if the Lac Operon is turned “on” or “off”

1. Glucose present, Lactose present

2. Glucose absent, Lactose absent

3. Glucose present, Lactose absent

4. Glucose absent, Lactose present

Trp Operon (Repressor: Repressible)

• Repressible Operator

• Tryptophan is a corepressor that stops the transcription of genes responsible for the synthesis of tryptophan

Repressible Operator

• transcription is “turned on” until a corepressor binds to the repressor.

  • Corepressor enables the repressor to bind to the operator

lactose comes from an inducible operator - off by default; lactose presence turns on the lactase gene

tryptophan comes from a repressible operator - on by default; trp presence turns off the gene bc there’s enough trp now

BPQ

Why does Lactose “turn on” gene expression while Tryptophan “turn off” gene expression?

Eukaryote Transcription Factors

• Transcription factors

  • Activators

  • Inhibitors

  • Basal/general transcription factors

• Regulatory sequences

  • Enhancer (upstream of the promoter)

  • Silencer(upstream of the promoter)

  • Promoter

• **unlike the Prokaryotes with their operators, Eukaryotes have no regulatory sequences downstream of the promotor

Basal/General Transcription Factors

• All Eukaryotic genes require basal transcription factors for the RNA polymerase to bind to the promoter

  • RNA Polymerase in prokaryotic cells can often bind on its own

Upstream Regulatory Sequences

• Eukaryotic regulator sequences can be very far upstream from the promotor

  • 1 regulator sequence can affect multiple promotors/genes

• Prokaryotic regulator sequences are always proximal to the promotor

  • Can only effect 1 promotor(but multiple gene/see operon)

DNA Methylation

• A methyl group can be added to the nitrogen bases C or A (usually C)

  • Affect promotors

    • “Turn off” gene expression

  • Affect Histones

    • “Turn off or on” gene expression

• often leads to more long-term patterns in gene expression

growth hormones for cell specialization

BPQ

Considering the long-term effects of DNA methylation, what is likely a common application of DNA methylation in multicellular organisms?

Post Transcriptions Gene Expression (Eu)

• MicroRNAs (miRNA)

  • Small noncoding RNAs that can bind to mRNA

    • Prevent translation

    • Accelerate mRNA breakdown

• Alternate splicing

Gene Expression and Cell Differentiation

Multicellular Eukaryotes need to express different genes in different cells

see which genes are turned on/off due to cell specialization (?)

BPQ

A Liver tumor is biopsied and found to be metastatic lung cancer

How can we tell that a tumor in the liver came from the lungs?

Gene Expression and Evolution

Changes in gene expression are often more consequential than changes to the genes themselves