3 B 1 Gene Regulation Draw 8 boxes

3. B. 1 Gene Regulation Draw 8 boxes on your paper Gene regulation results in differential gene expression, leading to cell specialization.

Gene regulation accounts for some of the phenotypic differences between organisms with similar genes.

Gene regulation in bacteria • Control of gene expression enables individual bacteria to adjust their metabolism to environmental change • Cells vary amount of specific enzymes by regulating gene transcription – turn genes on or turn genes off • ex. if you have enough tryptophan in your cell then you don’t need to make enzymes used to build tryptophan – waste of energy – turn off genes which codes for enzymes 2005 -2006

Box #1 What is gene regulation?

Gene expression can involve: Regulatory Sequences Within Genes Regulatory Genes Small Regulatory RNAs (s. RNA)

So how can genes be turned off? • First step in protein production? – transcription – stop RNA polymerase! • Repressor protein – binds to DNA near promoter region blocking RNA polymerase • binds to operator site on DNA • blocks transcription 2005 -2006

Box # 2 How do you regulate genes?

Regulatory sequences are stretches of DNA that interact with regulatory proteins to control transcription.

Operons are a type of regulatory sequence consisting of clusters of genes under the control of a single regulatory region.

Genes grouped together • Operon – genes grouped together with related functions • ex. enzymes in a synthesis pathway – promoter = RNA polymerase binding site • single promoter controls transcription of all genes in operon • transcribed as 1 unit & a single m. RNA is made – operator = DNA binding site of regulator protein 2005 -2006

Operators are segments of DNA that a regulator molecule can bind to.

Promoters are nucleotide sequences that allow the genes of an operon to be transcribed. RNA polymerase binds to the promoter region to begin transcription.

Repressors are small regulatory proteins that halt transcription. They bind to the operator region of an operon and prevent RNA polymerase from binding.

Box # 3 Describe an operon (include the following terms in description, promoter, repressor and operator. )

Both positive and negative control mechanisms regulate gene expression in bacteria and viruses. Positive Control Negative Control • Stimulate Transcription • Inactivate Transcription

The expression of specific genes can be inhibited by the presence of a repressor. A repressor binds to the operator site of an operon, preventing RNA polymerase from binding and therefore preventing transcription of the operon (negative control).

Inducers are small proteins that stimulate transcription. They bind to repressors, inactivating them so that RNA polymerase can bind to the operator and begin transcription.

Positive Control Negative Control • Stimulate Transcription Inducer • Inactivate Transcription Repressor

Box # 4 Compare and contrast inducers and repressors

Terminators are nucleotide sequences that mark the end of a gene or operon.

Enhancers are short regions of DNA that can be bound with proteins to enhance transcription.

Box # 5 How do enhancers aid transcription?

A regulatory gene is a sequence of DNA encoding a regulatory protein (such as a repressor) or RNA.

Certain genes are continuously expressed; that is, they are always turned “on, ” regardless of environmental conditions.

Example of Prokaryotic Gene Regulation: The Trp Operon

The trp operon consists of a group of genes that code for the enzymes necessary to synthesize tryptophan, an amino acid.

The trp operon is an example of negative feedback. When there is too much tryptophan, tryptophan itself acts as a corepressor, which activates the repressor that shuts down this operon.

The trp operon is an example of a repressible operon; it is usually “on” but can be turned “off” when there is too much tryptophan.

Repressor protein model Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase RNA TATA repressor polymerase promoter operator gene 1 gene 2 gene 3 gene 4 DNA Repressor protein turns off gene by blocking RNA polymerase binding site. repressor 2005 -2006 repressor protein

Repressible operon: tryptophan Synthesis pathway model When excess tryptophan is present, binds to tryp repressor protein & triggers repressor to bind to DNA RNA polymerase – blocks (represses) transcription RNA TATA repressor polymerase gene 1 gene 2 repressor promoter operator gene 3 gene 4 DNA repressor protein tryptophan repressor 2005 -2006 tryptophan – repressor protein complex conformational change in repressor protein!

Tryptophan operon What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan binds allosterically 2005 -2006 to regulatory protein

Box # 6 Draw the trp operon and how its regulated by a repressor

Example of Prokaryotic Gene Regulation: The Lac Operon

The lac operon consists of a group of genes in e. coli that allow the bacteria to metabolize lactose when lactose is present in the gut of its host.

When there is no lactose present, e. coli does not need to produce the enzymes to break down lactose, instead using glucose as its primary nutrient.

In the absence of lactose, the lac repressor protein is made, which binds to the operator and halts the binding of RNA polymerase.

In the presence of lactose, an inducer binds to the repressor, altering its shape so that it is no longer able to bind to the operator. RNA polymerase can now bind and begin transcribing the operon.

The lac operon is an example of an inducible operon; it is usually “off” but can be turned “on” in the presence of lactose.

Inducible operon: lactose Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA RNA polymerase – induces transcription RNA TATA repressor polymerase gene 1 gene 2 repressor promoter operator gene 3 gene 4 DNA repressor protein lactose repressor 2005 -2006 lactose – repressor protein complex conformational change in repressor protein!

Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose binds allosterically to regulatory protein 2005 -2006

Box # 7 Describe the lac operon and the inducer

Operon summary • Repressible operon – usually functions in anabolic pathways • synthesizing end products – when end product is present in excess, cell allocates resources to other uses • Inducible operon – usually functions in catabolic pathways, • digesting nutrients to simpler molecules – produce enzymes only when nutrient is available • cell avoids making proteins that have nothing to do, cell allocates resources to other uses 2005 -2006

In eukaryotes, gene expression is complex and control involves regulatory genes, regulatory elements and transcription factors that act in concert.

Transcription factors are proteins that bind to specific DNA sequences and/or other regulatory proteins. They work alone or in complex.

Some of these transcription factors are activators (increase expression), while others are repressors (decrease expression).

The combination of transcription factors binding to the regulatory regions at any one time determines how much, if any, of the gene product will be produced.

Box # 8 What are transcription factors? How do they involved in eukaryotic gene expression?

Learning Objectives: LO 3. 18 The student is able to describe the connection between the regulation of gene expression and observed differences between different kinds of organisms. [See SP 7. 1] LO 3. 19 The student is able to describe the connection between the regulation of gene expression and observed differences between individuals in a population. [See SP 7. 1] LO 3. 20 The student is able to explain how the regulation of gene expression is essential for the processes and structures that support efficient cell function. [See SP 6. 2] LO 3. 21 The student can use representations to describe how gene regulation influences cell products and function. [See SP 1. 4]