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 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 cells genes is known as a GENE EXPRESSION PROFILE. Each cell type has a unique gene expression profile. Insulin Muscle Cell DNA? Protein?
X Pancreatic Cell Slide adapted from Genetic Science Learning Center, University of Utah 201 Gene Expression in Bacteria Bacteria are singlecelled 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 hostyou! 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. Fig. 18-2
Feedback inhibition Precurs or trpE gene Enzyme 1 Enzyme 2 trpD gene trpC gene Regulati on of gene
expressi on trpB gene Enzyme 3 trpA gene Tryptopha n (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 Fig. 18-3a
Promot er DNA trp R Regulator y gene mRNA 5 Protein Promot er trp operon Genes of operon trp
trp trp D C Operat E orStart Stop 3 codon codon mRNA RNA 5 polymeras E D C e Inactive represso r
trp B trp A B A Polypeptide subunits that make up enzymes for tryptophan (a) Tryptophan absent, repressor inactive, synthesis operon on Fig. 18-3b-1 DNA No RNA
made mRNA Protein Tryptophan (corepressor) Active represso r (b) Tryptophan present, repressor active, operon off Fig. 18-3b-2 DNA No RNA made
mRNA Protein Tryptophan (corepressor) Active represso r (b) Tryptophan present, repressor active, operon off Video for Gene Expression http://education-portal.com/academy/les son/regulation-of-gene-expression-trans criptional-repression-and-induction.html Different Types of Operons A
repressible operon is one that is usually ONbinding a repressor to the operator turns off transcription. (The trp operon is like this) An inducible operon is one that is usually OFFa 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.
Fig. 18-4 Regulato ry gene Promot er lacI DN A Operat or lacZ No RNA made
3 mRN A RNA polymerase 5 Active Protei represso n r (a) Lactose absent, repressor active, operon off lac operon DN A
mRN A Protei n lacZ lacI 3 5 lacY lacA RNA polymeras e mRNA 5
Galactosidase Inactive Allolactos repressor e (inducer) (b) Lactose present, repressor inactive, operon on Permeas e Transacetyla se Fig. 18-4a DN A
Regulato ry gene lac I Promot er Operat or lacZ No RNA made 3 mRN A 5
RNA polymerase Active Protei represso n r (a) Lactose absent, repressor active, operon off Fig. 18-4b lac operon DNA mRN A Protei n
lacI lacZ 3 5 lacY lac A RNA polymeras e mRNA 5 Galactosidas
e Inactive Allolacto repressor se (inducer) (b) Lactose present, repressor inactive, operon on Permea se Transacetyla se 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.
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