Classification of Genes Based on Expression Control
📌 Genes are categorized into three main groups based on expression: constitutive, inducible, and repressible.
⚙️ Constitutive genes are continuously expressed, encoding essential proteins like tRNA, rRNA, and ribosome components needed constantly.
📊 Inducible and repressible genes vary their expression levels, sometimes highly expressed and sometimes suppressed almost entirely.

Inducible vs. Repressible Genes (Metabolic Context)
🌟 Inducible genes are typically involved in catabolism (breaking down molecules); for example, genes for processing lactose in *E. coli* are induced only when lactose is present.
🧪 Repressible genes are generally involved in anabolism (biosynthesis); for example, genes for tryptophan synthesis are repressed when external tryptophan is available.
📉 Inducible systems show increased enzyme activity upon addition of the substrate (e.g., lactose $\rightarrow$ high enzyme activity).
⬆️ Repressible systems show decreased enzyme activity upon addition of the product (e.g., tryptophan $\rightarrow$ low enzyme activity).

Mechanisms of Genetic Regulation (Control Types)
🧠 Regulation involves regulator genes producing either activators (positive control) or repressors (negative control).
🛑 Repressors bind to the Regulator Protein Binding Site (RPBS), often near the promoter, to block RNA polymerase attachment.
➕ Activators must bind to the RPBS to facilitate RNA polymerase attachment, crucial for positive control systems.
🔄 Effector molecules (inducers or corepressors) determine whether the regulator protein (activator or repressor) can bind to the DNA site.

Operon Structure and Function in Prokaryotes
🔗 An operon is a segment of DNA comprising several structural genes, an operator, and a promoter.
💡 The key advantage of an operon is that a single regulatory mechanism can simultaneously activate or inactivate multiple genes involved in the same metabolic pathway, leading to energy savings and efficient regulation.
🧬 Within the operon, the operator functions as the RPBS (binding site for regulator proteins), while the promoter is the site for RNA polymerase binding.
🌐 Operons can be regulated as either inducible (e.g., *lac* operon) or repressible systems, controlled by one set of regulatory elements affecting all structural genes simultaneously.

Key Points & Insights
➡️ Focus on the distinction: Inducible genes promote catabolic enzymes when the substrate is present; repressible genes halt anabolic enzymes when the product is present.
➡️ Understand the roles: Inducers increase expression (e.g., bind repressor to release DNA); Corepressors decrease expression (e.g., bind repressor to enable DNA binding).
➡️ In negative control, the regulator produces a repressor; in positive control, the regulator produces an activator.
➡️ The operon structure is highly efficient as it links the expression of multiple functional genes under the control of a single promoter/operator system.

📸 Video summarized with SummaryTube.com on Nov 18, 2025, 14:55 UTC