Introduction to Gene Expression

Gene Expression Gene expression is the process by which information from a gene is used in the synthesis of a functional protein. Several steps in the gene expression process may be modulated, including: Transcription RNA Splicing (taking out Introns) Translation Post translational modification of a protein

Gene Expression

Constitutive Genes that are ALWAYS ON Genes that specify cellular components that perform housekeeping functions Structure is simpler Example: The ribosomal RNAs Proteins involved in protein synthesis Regulatory proteins

Non-Constitutive Genes: Inducible and Repressible Genes Are expressed only as needed: Their amount may vary depends on the condition Their structure is relatively complicated with some response elements. Why? Would anabolic/ catabolic pathway have genes that are inducible or repressible?

Anabolic Pathway Repressible Anabolism: Synthesis of molecules from simpler subunits Example: If an essential nutrient is not present in the environment, bacteria must be able to synthesize the nutrient on its own However, bacteria do not need to produce the essential nutrient if it is already present in the environment

Catabolic Pathway Inducible Catabolism: Breakdown of complex molecules into simpler units Example: It would be a waste of energy for bacteria to make enzymes for the breakdown of a substance if that substance does not exist

Control Mechanisms There are 42,000 genes that code for proteins in humans Not all proteins are required at all times E.g. Insulin is only required in a cell when glucose levels are high Regulation is therefore vital to an organism’s survival

Why turn genes on and off? Cell specialization Each cell of a multicellular eukaryote expresses only a small fraction of its genes Development Different genes needed at different points in life cycle of an organism Responding to organism’s needs Cells must be able to turn genes on and off in response to signals from their external and internal environment

Gene regulation Regulation of gene expression is of two types: Positive regulation: When the expression of a gene is increased by the presence of specific regulatory element (aka Activator or positive regulator) Negative regulation: When the expression of a gene is decreased by the presence of specific regulatory element (aka repressor )

Control of Gene Expression Controlling gene expression is often accomplished by controlling transcription initiation. Regulatory proteins bind to DNA to either block or stimulate transcription, depending on how they interact with RNA polymerase.

Control of Gene Expression Prokaryotic organisms regulate gene expression in response to their environment. Eukaryotic cells regulate gene expression to maintain homeostasis in the organism.

Regulatory Proteins Gene expression is often controlled by regulatory proteins binding to specific DNA sequences. regulatory proteins gain access to the bases of DNA at the major groove regulatory proteins possess DNA-binding motifs

Regulatory Proteins DNA-binding motifs are regions of regulatory proteins which bind to DNA helix-turn-helix motif homeodomain motif zinc finger motif leucine zipper motif

Helix-Turn-Helix Motif

Homeodomain Motif

Zinc Finger Motif

Leucine Zipper Motif

Principles of Gene Regulation Most prokaryotic genes are regulated in units called Operons Operon: A group of genes that share a single promoter Genes code for proteins that have related functions

Operon Components Promoter Region on DNA where RNA Polymerase binds to start transcription Acts as the on/off switch for genes Operator Region on DNA that controls the access of RNA Polymerase to the genes Can activate or repress transcription Structural genes -Genes to be transcribed by RNA Polymerase

Operon Regulation Regulatory gene Region on DNA that codes for the production of the regulatory protein Upstream of the operon Constitutive expression: transcribed continuously Regulatory protein Binds to perator and blocks attachment of RNA Polymerase Allosteric: Alternates bewteen active and inacive forms

Example: Trp Operon Regulatory Gene: TrpR Allosteric regulatory protein: - Has 2 forms: Active or inactive

Operon Regulation Effector molecule: Any molecule that can regulate the activity of a protein Inducer: effector molecule that binds repressor protein to cause it to fall off operator Corepressor: effector molecule that binds repressor protein to cause it to bind to the operator