Gene Expression AP Biology

Questions to Ponder….. How do your cells “know” what kind of cell they are? How do your cells “know” when to make a particular protein? When to stop making it? How does the environment affect your cells? ANSWER: Gene Expression

What makes cells from the same individual look different? Stem Cells Liver Cells Red Blood Cells Cartilage Cells DNA sequence in each cell is the same, but different cell types have different “GENE EXPRESSION PATTERNS”

Each cell type has a unique gene expression profile. Insulin DNA? When a gene is “on” and its protein or RNA product is being made, scientists say that the gene is being EXPRESSED. The on and off states of all of a cell’s genes is known as a GENE EXPRESSION PROFILE. Each cell type has a unique gene expression profile. Insulin DNA? Protein? Muscle Cell X Pancreatic Cell Slide adapted from Genetic Science Learning Center, University of Utah  2013

Gene Expression in Bacteria Bacteria are single-celled organisms who are surrounded on all sides by their environment. They must be able to regulate expression of their genes in response to environmental changes.

Bacteria Respond by Regulating Transcription Bacteria cells that can conserve resources and energy have a selective advantage over cells that cannot do so. Natural selection has favored bacteria that express only the genes they need.

E. Coli Regulation of Tryptophan An individual E. coli cell living in the erratic environment of the human colon, is dependent for its nutrients on the whimsical eating habits of its host—you! If the environment is lacking in the amino acid tryptophan, which the bacterium needs to survive, it responds by activating a metabolic pathway that makes tryptophan from another compound. If tryptophan becomes available, it shuts down this pathway.

Regulation of a Metabolic Pathway In the pathway for tryptophan synthesis, an abundance of tryptophan can both inhibit the activity of the first enzyme (a rapid response) OR repress expression of the genes encoding the enzymes in the pathway (a longer response). This is an example of feedback inhibition. It allows for a cell to adapt to short-term fluctuation in the supply of a substance it needs.

(a) Regulation of enzyme activity (b) Regulation of enzyme production Fig. 18-2 Precursor Feedback inhibition trpE gene Enzyme 1 trpD gene Regulation of gene expression Enzyme 2 trpC gene trpB gene Figure 18.2 Regulation of a metabolic pathway Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production

Gene Expression Controls Which Enzymes are Made and When In many cases, this occurs in the process of transcription. Many genes may be switched on or off by changes in the metabolic status of the cell. One example was discovered in 1961 by Francois Jacob and Jacques Monod at the Pasteur Institute in Paris. This method is called the Operon Model.

Operons: The Basic Concept A cluster of functionally related genes that can be under coordinated control by a single on-off “switch”. The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter. An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control.

The operon can be switched off by a protein repressor The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase The repressor is the product of a separate regulatory gene

The repressor can be in an active or inactive form, depending on the presence of other molecules. A corepressor is a molecule that cooperates with a repressor protein to switch an operon off.

Bacteria can synthesize tryptophan by utilizing the trp operon. By default, the trp operon is on and the genes for tryptophan synthesis are transcribed When tryptophan is present, it binds to the trp repressor protein, which turns the operon off The repressor is active only in the presence of its co-repressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high

Polypeptide subunits that make up enzymes for tryptophan synthesis Fig. 18-3a trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Regulatory gene Operator Start codon Stop codon 3 mRNA 5 mRNA RNA polymerase 5 E D C B A Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes (a) Tryptophan absent, repressor inactive, operon on

(b) Tryptophan present, repressor active, operon off Fig. 18-3b-1 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes (b) Tryptophan present, repressor active, operon off

(b) Tryptophan present, repressor active, operon off Fig. 18-3b-2 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes (b) Tryptophan present, repressor active, operon off

Video for Gene Expression http://education-portal.com/academy/lesson/regulation-of-gene-expression-transcriptional-repression-and-induction.html

Different Types of Operons A repressible operon is one that is usually ON—binding a repressor to the operator turns off transcription. (The trp operon is like this) An inducible operon is one that is usually OFF—a molecule called an inducer inactivates the repressor and starts transcription. (The lac operon is this type)

The lac Operon The lac operon is an inducible operon (usually off) and contains genes that code for enzymes that break down the sugar lactose (found in dairy products) By itself, the lac repressor is active and therefore shuts the lac operon off most of the time. A molecule called an inducer inactivates this repressor which turns the lac operon on.

(a) Lactose absent, repressor active, operon off Fig. 18-4 Regulatory gene Promoter Operator DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacI lacZ lacY lacA RNA polymerase Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes For the Cell Biology Video Cartoon Rendering of the lac Repressor from E. coli, go to Animation and Video Files. 3 mRNA mRNA 5 5 -Galactosidase Permease Protein Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off Fig. 18-4a Regulatory gene Promoter Operator DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes Active repressor Protein (a) Lactose absent, repressor active, operon off

(b) Lactose present, repressor inactive, operon on Fig. 18-4b lac operon DNA lacI lacZ lacY lacA RNA polymerase 3 mRNA mRNA 5 5 -Galactosidase Permease Transacetylase Protein Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

Inducible Enzymes Inducible enzymes (such as those found in the lac operon) are usually catabolic enzymes, which means they break things apart. Their synthesis is usually induced by some kind of signal. In the lac operon, the signal is the presence of the lactose sugar molecule.

Repressible Enzymes Repressible enzymes (such as those in the trp operon) usually function in anabolic pathways which build things or put things together. Since these are almost always ON, they are repressed (shut down) when there are high levels of the end-product present.