Molecular Biology Chapter on Gene Regulation

Regulation of Gene Expression

• Gene expression is controlled at many steps, from initial transcription to protein processing and degradation.
• Prokaryotic gene regulation controls the production of enzymes needed for specific metabolic functions in response to environmental changes.
• Cells alter enzyme production by activating or deactivating operons.
• Operons are groups of genes that are transcribed together.
• Two common types are repressible and inducible operons,.
• The trp operon is a repressible operon.
• The lac operon is an inducible operon.
• Inducers turn the operon ON.
• In the trp operon, tryptophan is a corepressor that activates a repressor protein, which turns the operon OFF.
• In the lac operon, lactose acts as an inducer and binds to a repressor to turn the genes involved in lactose digestion ON.
• Eukaryotic gene regulation is more complex than prokaryotic gene regulation, and it involves multiple steps.

Eukaryotic Gene Regulation

• Eukaryotic gene regulation occurs at multiple steps: chromatin modification, transcription, RNA processing, RNA transport, translation, and protein processing.

• Chromatin modification affects DNA accessibility.

• DNA methylation generally reduces gene expression.

• Histone acetylation loosens DNA structure making transcription easier.

• Eukaryotes use transcription factors to control gene expression.

• Enhancers and silencers are DNA control elements involved in coordinating gene regulation.

• Enhancers and silencers are DNA control sequences that are far from the gene.

• They are bound by activator or repressor proteins, which control the rate of transcription.

• Activators promote transcription while silencers repress it.

• Alternative RNA splicing creates a family of proteins from a single gene.

• Different mRNAs are made from one primary RNA transcript.

• mRNA degradation controls protein synthesis by controlling mRNA longevity..

• The length of time an mRNA molecule is present in the cytoplasm determines the amount of protein made.

• mRNA degradation is regulated, with different mRNAs having different lifespans.

• Small RNAs (miRNAs and siRNAs) interfere with translation.

Control of Translation

• Regulatory proteins can attach to the 5' end of mRNA and block ribosomal subunits and initiator tRNA from attaching inhibiting translation.

Protein Processing and Degradation

• Proteins are often modified after translation.
• Modifications can include folding, cleavage, and addition of other components like sugars.
• Protein targeting directs proteins to their final locations.
• Proteins are degraded by a process called ubiquitin tagging and proteasome degradation.
• This process can help the cell recycle components.
• The proteasome is a complex that breaks down targeted proteins into smaller peptides.

Summary of Eukaryotic Gene Regulation

• Eukaryotic gene regulation involves various steps including chromatin modification, transcription, RNA processing, mRNA degradation, translation, and protein processing and degradation.
  • All are important in coordinated gene expression.

Embryonic Development

• Embryonic development occurs through cellular differentiation and morphogenesis.
• Key mechanisms in development include determination, cytoplasmic determinants, and induction.
• Cell division(Mitosis) creates a large number of identical cells in the developing embryo.
• Cells are determined if they are irreversibly committed to a particular specialization.
• Cytoplasmic determinants are maternal substances in the egg distributed unevenly that create gradients to determine the "head," "tail" and other body axes.
• Induction is the process by which a tissue or cell causes another to change in fate and take on new structures.
• Signaling molecules influence neighboring cells and help coordinate changes in structure and function.
• Hox genes are a class of homeotic genes that are essential for specifying body plan during embryonic development.
• Mutations in these genes have dramatic effects on the positions of structures and can create body parts in wrong positions, or cause extra body parts to form.
• Apoptosis (programmed cell death) is vital during development.

Cancer Results from Genetic Changes That Affect Cell Cycle Control

• Cancer arises from cumulative mutations in genes that control cell division.
• Proto-oncogenes stimulate cell division while tumor suppressor genes inhibit cell division.
• Mutations in either can lead to uncontrolled cell growth and cancerous development.