6.5/6.6 - Gene Expression

Gene Expression: The process by which instructions in the DNA are transcribed and translated into functional proteins. This process must be tightly regulated to ensure proper cell function and response to environmental changes.

Importance of Regulation

1. Cell Differentiation & Specialization: Different cell types express different genes to perform specialized functions.

2. Timing: Genes need to be turned on or off at specific times during development or in response to environmental signals.

3. Environmental Influence: External factors can influence gene expression, such as temperature affecting the fur color of rabbits or the sex of reptiles based on incubation conditions.

Mechanisms of Regulation

Regulatory Sequences: DNA regions that can promote (+) or inhibit (-) protein synthesis.

• Examples:

  • Promoter: Region where RNA polymerase binds to initiate transcription.

  • Operator: Region where repressor proteins bind to turn genes on or off.

  • Repressor: Proteins that inhibit gene expression.

  • Enhancer: DNA sequences that increase the rate of transcription when bound by regulatory proteins.

Regulatory Proteins: Proteins that can promote (+) or inhibit (-) protein synthesis.

• Examples:

  • Transcription Factors: Proteins that help initiate or suppress transcription.

  • Repressor Proteins: Bind to operator regions to block transcription.

  • Activator Proteins: Increase the rate of transcription by binding to enhancer regions.

Operons in Prokaryotes

Operons: Groups of genes controlled together, primarily found in prokaryotes.

• Components:

  • Regulatory Gene: Produces repressor proteins.

  • Promoter Region: Where RNA polymerase binds to initiate transcription.

  • Operator: The on/off switch where repressor proteins bind.

  • Structural Genes (A, B, C): Genes that are transcribed together.

  • Terminator Region: Signals the end of transcription.

Inducible Operon (Lac Operon):

• Default State: Off position.

• Activation: An inducer molecule binds to the repressor protein, inactivating it and allowing transcription.

Epigenetics

Epigenetics: Changes that affect gene expression without altering the DNA sequence. These changes are often reversible and involve modifications to DNA or histones.

• Functions: Control whether a section of DNA is accessible for transcription.

Key Epigenetic Mechanisms:

1. Histone Acetylation: Addition of acetyl groups to histones, which generally makes DNA more accessible by loosening the chromatin structure.

2. DNA Methylation: Addition of methyl groups to DNA, typically silencing gene expression by making the DNA less accessible.