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Gene Regulation in Bacteria: Lac and Trp Operons
- Overview of Gene Regulation in Bacteria
- Bacterial genetics often involves operons, which are clusters of genes regulated together.
- Key examples include the lac operon and trp operon.
- Lactose (Lac) Operon
- The lac operon is a model for understanding gene regulation in bacteria.
- Components of the lac operon:
- Regulatory region: Contains the lacI gene coding for the lac repressor which binds to the operator.
- Structural genes: Includes lacZ (beta-galactosidase), lacY (lactose permease), and lacA (transacetylase).
- Functionality:
- Lac operon is responsible for lactose metabolism.
- It operates as a polycistronic unit, producing a single mRNA that codes for multiple proteins.
- Regulation Mechanisms
- Default state: Lac operon is usually off due to the repressor binding to the operator.
- Presence of lactose: When lactose is present, it binds to the repressor, changing its shape and allowing RNA polymerase to transcribe the operon, leading to the production of enzymes for lactose metabolism.
- Absence of lactose: In the absence of lactose, the repressor remains bound, preventing transcription.
- Constitutive mutations:
- I- (mutant repressor) leads to constitutive expression of the operon regardless of lactose presence.
- Glucose Impact on Lac Operon
- Effect of glucose:
- When glucose is present, the lac operon is inhibited due to catabolite repression.
- High glucose levels lead to low cAMP, preventing CAP (Catabolite Activator Protein) from facilitating RNA polymerase binding.
- Maximum transcription: Achievable with high lactose and low glucose conditions.
- Scavenging mechanism: In the presence of glucose, low levels of transcription may still occur with lactose present.
- Human Lactose Metabolism
- Genetic variations lead to differences in lactose tolerance (lactase persistence) among populations.
- Lactase persistence refers to continued production of lactase in adulthood, a result of mutations primarily in the MCM6 gene, influencing lactase gene expression.
- Genetic adaptations: Mutations that favor lactose digestion were historically selected for in pastoralist societies.
- Tryptophan (Trp) Operon
- The trp operon functions differently from the lac operon.
- Default state: The trp operon is typically on, allowing tryptophan biosynthesis.
- Repression: Activated when intracellular tryptophan levels are high, reducing further production.
- Attenuation: Involves a leader sequence and a regulator that can form different secondary structures in RNA, depending on charged tRNA availability, determining whether transcription continues or terminates.
- Important Terms and Concepts
- Cis vs. Trans:
- Cis elements are those that are physically connected (like promoters).
- Trans elements can act at a distance (such as diffusible proteins like repressors or activators).
- Polycistronic mRNA: Allows for coordinated expression of genes with related functions.
- Scavenging: Refers to low-level expression in absence of a direct signal, ensuring readiness for sudden availability of the substrate.
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Explore Top NotesNoteStudied by 250 peopleNoteStudied by 18 peopleNoteStudied by 66 peopleNoteStudied by 7 peopleNoteStudied by 17 peopleNoteStudied by 7 people
Study Guide: Pyongyang by Guy Delisle
NoteStudied by 250 peopleCh. Spanish Allophones
NoteStudied by 18 peopleChapter 4: The Laws of Motion
NoteStudied by 66 peoplelearning and motivation unit three (week 11)
NoteStudied by 7 peopleAccounting ALL chaps
NoteStudied by 17 peopleUnit 11: The Industrial Revolution and Imperialism. The division of the world - Point 5
NoteStudied by 7 people