AP Biology 01/27 Chapter 15

Definition of Repressor: A protein that inhibits gene expression by binding to an operator and blocking transcription.
Function of Repressor: It turns genes off, effectively preventing the synthesis of mRNA and subsequent protein production.
Metaphor: Like repressing emotions, a repressor keeps genes from functioning by 'hiding' them.
Inactive Repressor: The default state of a repressor is inactive, making it unable to bind to the operator due to its inappropriate shape.

Definition of Operon: A cluster of genes under the control of a single promoter and operator.
Components of an Operon:
- Promoter: The sequence where RNA polymerase attaches to start transcription.
- Operator: A segment of the operon where the repressor can bind.
- Genes: The coding sequences (e.g., genes A, B, C, D) that will be transcribed into one continuous mRNA strand, translating into multiple proteins.

Efficiency: Bacterial operons allow for simultaneous transcription of related genes that are all necessary for a specific function, such as synthesizing tryptophan.
Evolutionary Advantage: Having clustered genes in a single operon means bacteria can respond quickly to their metabolic needs and survive better in varied environments.

Default State: The system is usually 'on' because tryptophan is essential for survival.
Repressor Activation:
- Tryptophan acts as a corepressor, binding to the inactive repressor.
- The active repressor can now bind to the operator to prevent transcription of tryptophan synthesis genes.
Negative Control: The use of the repressor to stop gene expression is classified as negative control.

Characteristics: Typically active but can be turned off in response to the presence of a specific metabolic product.
Example: The tryptophan operon, which is repressed when tryptophan levels are sufficient.

Characteristics: Usually inactive (off) but can be activated when necessary.
Example: The lac operon, which is turned on when lactose is present.
Induction Mechanism: An inducer (lactose) binds to the active repressor, causing it to detach from the operator, thus allowing transcription to proceed.

Negative Control: Repressors inhibit transcription, effectively acting as switches that turn genes off.
Positive Regulation: The presence of certain conditions can also enhance gene expression, often involving activators that facilitate RNA polymerase binding.
Glucose Influence: When glucose levels are low, an activator binds the operon to enhance transcription, ensuring adequate enzyme levels for lactose breakdown.

Default State: The lac repressor is actively bound to the operator, keeping the operon off.
Role of Lactose: Lactose (through its isomer allolactose) serves as an inducer that removes the repressor, turning the operon on for lactose metabolism.
Transcription Activation: Once the repressor is inactive, transcription of the structural genes occurs, allowing the bacterium to digest lactose.

Repressible Operon:
- Default is on.
- Can be turned off by a corepressor (e.g., tryptophan).
Inducible Operon:
- Default is off.
- Can be turned on by an inducer (e.g., lactose).

Understanding both operons (tryptophan and lac operons) is crucial as they demonstrate different regulatory mechanisms bacteria employ to manage their metabolic processes efficiently.