Gene Expression

Gene expression is the process where the information stored in a gene is used to create a functional product, typically a protein. This process is vital for growth, development, and everyday functioning of organisms and occurs in both prokaryotes and eukaryotes.

Basic Steps of Gene Expression:

Transcription:

• Process: The DNA sequence of a gene is transcribed (copied) into RNA.
• In Eukaryotes: The RNA is primarily messenger RNA (mRNA), and transcription occurs in the nucleus. The mRNA is often modified through splicing, capping, and the addition of a poly-A tail.
• In Prokaryotes: The mRNA is usually not extensively modified and is often ready for translation immediately after synthesis.

Translation:

• Process: The sequence of the mRNA is used to build a protein.
• Mechanism: Ribosomes read the mRNA sequence in sets of three nucleotides (codons) and translate each codon into a corresponding amino acid, forming a functional protein.

Regulation of Gene Expression:

Transcriptional Regulation:

• Involves controlling when and how much mRNA is synthesized from a gene. Transcription factors play a crucial role in this process.

Post-Transcriptional Regulation:

• After transcription, various mechanisms can modify the mRNA transcript, including splicing, editing, and controlling the stability and translation efficiency of the mRNA.

Translational and Post-Translational Regulation:

• Even after mRNA is made, its translation into a protein can be regulated. Proteins can also be modified after translation to control their activity, location, or lifespan.

Gene Expression in Bacteria:

• Overview: Gene expression in bacteria is controlled at the transcriptional level, allowing them to respond quickly to environmental changes.

Regulation of Gene Expression in Bacteria:

• Mechanisms: Includes the use of operons, which are clusters of genes regulated as a single unit. Regulatory proteins can either inhibit (repressors) or accelerate (activators) gene expression.

Operon Concept:

• Definition: An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter, transcribed together into an mRNA strand.

Types of Operon Systems in Bacteria:

1. Inducible Operons: Normally off, but can be turned on (induced) by a small molecule (inducer).
2. Repressible Operons: Typically on, but can be turned off (repressed) by a small molecule.

Inducible Operon System:

• In the Absence of Lactose (e.g., lac Operon): The repressor protein is bound to the operator, blocking RNA polymerase and preventing transcription.
• When Lactose is Present: Lactose binds to the repressor, detaching it from the operator and allowing RNA polymerase to transcribe genes for lactose metabolism.

Lac Operon: A Model of Regulation:

• Additional Control Mechanisms: The lac operon is subject to positive control mechanisms like the cAMP-CAP complex, enhancing transcription efficiency under certain conditions (e.g., low glucose levels).

Repressible Operon:

• In Absence of Tryptophan (e.g., trp Operon): The operon is active, synthesizing enzymes for tryptophan production.
• In Presence of Tryptophan: Tryptophan binds to the repressor protein, inhibiting transcription.

Trp Operon: Regulation by Attenuation:

• Concept: Premature termination of mRNA synthesis (transcription) when tryptophan levels are high, known as attenuation.
• Mechanism: Dependent on the interaction between the ribosome and the mRNA during the early stages of protein synthesis.