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

2. trp Operon: Regulated by attenuation - transcription stops early when tryptophan is high.