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Gene Expression

Transcription

  1. Eukaryotes: Transcription in the nucleus; mRNA modified with splicing, capping, poly-A tail.
  2. Prokaryotes: mRNA usually unmodified, ready for translation.

Translation

  1. Process: Ribosomes read mRNA, translate codons into amino acids.
  2. Result: Amino acid chain folds into functional protein.

Gene Expression Regulation

  1. Transcriptional Regulation: Transcription factors control mRNA synthesis.
  2. Post-Transcriptional Regulation: mRNA modified by splicing, editing, stability control.
  3. Translational and Post-Translational Regulation: Control over protein synthesis and modification.

Gene Expression in Bacteria

General

  1. Control Level: Primarily at transcriptional level.

Operon Concept

  1. Definition: Cluster of genes under a single promoter.
  2. Types: Inducible (normally off) and Repressible (normally on) operons.

Inducible Operon (e.g., lac Operon)

  1. Without Inducer (no lactose): Repressor binds operator, blocks transcription.
  2. With Inducer (lactose present): Inducer inactivates repressor, allows transcription.

Repressible Operon (e.g., trp Operon)

  1. Without Co-repressor (no tryptophan): Operon active, synthesizes enzymes.
  2. With Co-repressor (tryptophan present): Co-repressor activates repressor, blocks transcription.

Special Regulation Mechanisms

  1. lac Operon: Also controlled by cAMP-CAP complex for efficient transcription.
  • The genes in the operon encode proteins that allow the bacteria to use lactose as an energy source.
  • i gene - Repressor
  • z gene - β- galactosidase
  • y gene - Permease
  • a gene - Transacetylase
  1. trp Operon: Regulated by attenuation - transcription stops early when tryptophan is high.


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