Gene Expression and Regulation: Study Notes

Gene Expression

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA and then translated into protein.

• DNA → RNA → Protein

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of protein from an mRNA template.

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated to produce the hemoglobin protein.

Transcription in Prokaryotes vs. Eukaryotes

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm.

• Eukaryotes: Transcription occurs in the nucleus; mRNA is processed and exported to the cytoplasm for translation.

Key Point: Eukaryotic mRNA undergoes capping, polyadenylation, and splicing before translation.

Regulation of Gene Expression

Levels of Regulation

Gene expression can be regulated at multiple levels:

• Transcriptional control: Whether and how much a gene is transcribed.

• Post-transcriptional control: mRNA processing, stability, and transport.

• Translational control: Regulation of protein synthesis from mRNA.

• Post-translational control: Protein modification and degradation.

Operons in Prokaryotes

An operon is a cluster of genes under the control of a single promoter and regulatory elements. Common in prokaryotes, operons allow coordinated regulation of gene groups.

• Lac Operon: Controls metabolism of lactose in Escherichia coli.

• Trp Operon: Controls synthesis of tryptophan.

Example: The lac operon is induced in the presence of lactose and absence of glucose.

Regulatory Proteins

• Repressors: Proteins that bind to operators to block transcription.

• Activators: Proteins that enhance the binding of RNA polymerase to the promoter.

Gene Expression in Eukaryotes

Chromatin Structure and Epigenetics

• Chromatin: DNA wrapped around histone proteins; can be tightly packed (heterochromatin) or loosely packed (euchromatin).

• Epigenetic modifications: Chemical changes to DNA or histones (e.g., methylation, acetylation) that affect gene expression without altering DNA sequence.

Example: DNA methylation typically silences gene expression.

Transcription Factors

• General transcription factors: Required for transcription of all genes.

• Specific transcription factors: Bind to enhancers or silencers to regulate specific genes.

RNA Processing

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end of mRNA.

• Polyadenylation: Addition of a poly-A tail to the 3' end of mRNA.

• Splicing: Removal of introns and joining of exons.

Summary Table: Prokaryotic vs. Eukaryotic Gene Expression

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
mRNA Processing	None	Capping, polyadenylation, splicing
Operons	Common	Rare
Regulation	Mainly transcriptional	Multiple levels

Key Equations

• Transcription:

$\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

• Translation:

$\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein}$

Additional info:

• Some content inferred from context and standard biology curriculum due to partial or abbreviated notes.