Molecular Biology: Gene Expression

Gene Expression

• Process where a gene activates in a cell to transcribe RNA and produce proteins
• Instructions in DNA convert into a functional product, usually a protein
• Information from a gene synthesizes a functional gene product like proteins, tRNA, or snRNA

Control of Gene Expression

• Gene expression is often controlled by transcription initiation
• Regulatory gene products are needed to initiate (turn on) or stop (turn off) expression
• Regulatory genes make regulatory proteins that bind to DNA, either blocking or stimulating transcription based on interaction with RNA polymerase

Types of Gene Expression

• Some genes are essential and continuously expressed in cells; they are known as housekeeping genes
• Gene expression levels for some genes change in response to external signals

Gene Expression in Multicellular Organisms

• Every cell contains all genes, but only certain proteins are needed
• Gene expression at the wrong time, in the wrong cell type, or in abnormal amounts can cause issues
• Induced expression occurs when some genes show higher expression once activated
• Repressed expression occurs when some genes are repressed and their expression levels are lower

Exons and Coding Sequences

• Exons are spliced after introns are removed from pre-mRNA
• Coding sequences are in exons, however not all exons contain coding sequences
• Untranslated regions (UTRs) are sequences in exons not translated into amino acids
• UTRs are found upstream and downstream of the protein-coding sequence

Gene Expression in Prokaryotes and Eukaryotes

• Eukaryotes have more complex gene expression controls than prokaryotes
• Eukaryotic complexity arises due to compartmentalization of cells
• Eukaryotes have different cell organelles performing specific functions, preventing functional interference
• Eukaryotic cells have more extensive transcript processing, regulation from a distance, cell and tissue specific gene expression, larger genome size, and genes scattered in the genome

Key Events in Gene Expression

• 4 major events include: transcription, RNA processing, translation, and protein processing

Gene Regulation

• Bacteria respond quickly to environmental changes
• Bacteria stop production if they have enough of a product
• Bacteria will utilize new food and energy sources quickly

Bacterial Metabolism

• Bacteria halt enzyme synthesis when enough product is available to conserve energy
• Bacteria initiate enzyme production for digestion when a new food source is found

Regulating Metabolism

• Feedback inhibition is where the product acts as an allosteric inhibitor of the first enzyme in a pathway
• Gene regulation blocks gene transcription for all enzymes in a pathway instead of blocking enzyme function
• Blocking transcription saves energy by avoiding unnecessary protein synthesis

Gene Regulation in Bacteria

• Cells regulate gene transcription to vary the amounts of specific enzymes produced
• Genes can be turned on or off
• Example is genes are turned off if the bacterium has enough tryptophan
• Genes are turned on to digest lactose if a bacterium encounters lactose

Jacob & Monod

• Francois Jacob and Jacques Monod: first to describe operon systems and coined the term "operon" in 1961

Bacterial Gene Grouping

• Genes with related functions are grouped together in the operon
• All enzymes in a metabolic pathway is an example of operons
• A single promoter controls transcription of all genes in an operon
• The genes are transcribed as one unit to produces a single mRNA
• The operator signifies the DNA binding site of the repressor protein

Operon Model (Gene Turned On)

• RNA polymerase binds to the promoter and the gene is transcribed

Operon Model (Gene Turned Off)

• The repressor binds to the operator site, which blocks RNA Polymerase

Repressor Protein

• Repressor protein binds to DNA at the operator site to block RNA polymerase and blocks transcription

The Tryp Operon Model

• Functions as a model for gene regulation due to the operator, promoter, and genes it controls

Repressible Operons

• Genes that are naturally on, and are turned off
• The trp operon synthesizes enzymes used in tryptophan synthesis

Inducible Operons

• Genes that are naturally off, and are turned on
• The lac operon produces enzymes used in lactose digestion

Tryptophan (Repressible) Operon

• Needed for use when the gene is naturally on
• Repressor protein initially exists as an inactive form
• Cell produces enzymes for tryptophan synthesis

Trytophan Synthesis

• When excess tryptophan is present, it binds to the trp repressor protein, triggering the repressor to bind to DNA and block transcription; it will then create a synthesis pathway model

Repressible Operon Example

• There is no need to make tryptophan-building enzymes when tryptophan is present
• Tryptophan is an allosteric regulator of repressor protein

Lactose Operon

• Active transport of lactose requires lactose-digesting enzymes
• The lactose operon is a digestive pathway model

Lactose Digestion

• Glucose is the preferred food source
• Lactose digesting enzymes are not a necessity
• Inducible operon that is naturally turned off

Inducible Operon

• When lactose is present, it binds to the lac repressor protein, triggering the repressor to release DNA, thereby inducing transcription

Repressible Operon Summary

• Functions in anabolic pathways in the body
• Synthesizes end products
• If the end product is present in excess it allocates resources to other uses

Inducible Operon Summary

• Functions in catabolic pathways in the body
• Digests nutrients to simpler molecules
• Enzymes produce only when the nutrient is available
• The cell avoids making proteins that are unneeded and allocates cell resources to other functions