Gene Expression and the Central Dogma in Prokaryotes and Eukaryotes

Gene Expression Overview

Introduction

Gene expression is the process by which genetic information encoded in DNA is converted into functional products, such as proteins. This process involves several key steps, including transcription, RNA processing (in eukaryotes), translation, and post-translational modifications. The "Central Dogma" of molecular biology describes the flow of genetic information from DNA to RNA to protein.

Bacterial Gene Expression

Translation Overview

Translation is the process by which messenger RNA (mRNA) is decoded to synthesize proteins. In bacteria, translation occurs in the cytoplasm and is closely coupled with transcription.

• Structure

  • Transfer RNA (tRNA): tRNA molecules carry specific amino acids to the ribosome and recognize codons in the mRNA via their anticodon region.

  • Ribosome: The ribosome is a complex molecular machine composed of ribosomal RNA (rRNA) and proteins. It facilitates the assembly of amino acids into polypeptide chains.

• Process

  • Initiation: The ribosome assembles at the start codon of the mRNA, and the first tRNA binds.

  • Elongation: tRNAs bring amino acids to the ribosome, where peptide bonds are formed, elongating the polypeptide chain.

  • Termination: When a stop codon is reached, the completed polypeptide is released.

Eukaryotic Gene Expression

Overview

Eukaryotic gene expression is more complex than in bacteria and involves additional steps and regulatory mechanisms.

• Transcription: The process by which RNA is synthesized from a DNA template by RNA polymerase.

• Transcription Processing: Includes modifications such as 5' capping, splicing to remove introns, and 3' polyadenylation.

• Translation: Similar to bacteria, but occurs in the cytoplasm and involves more initiation factors and regulatory steps.

• Post-translation Protein Modification: Proteins may undergo further modifications, such as phosphorylation or glycosylation, to become fully functional.

The Central Dogma of Molecular Biology

Explanation

The central dogma describes the flow of genetic information:

• DNA Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is transcribed into RNA by RNA polymerase.

• Translation: RNA is translated into protein by ribosomes.

Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Transcription in Detail

Mechanism

Transcription is the synthesis of RNA from a DNA template. RNA polymerase binds to the promoter region of DNA and synthesizes RNA in the 5' to 3' direction.

• Phosphodiester Linkage: RNA polymerase forms phosphodiester bonds between nucleotides as RNA is synthesized.

• Base Pairing: RNA nucleotides pair with complementary DNA bases (A-U, G-C).

Equation: $\text{(Nucleotide)}_{n} + \text{NTP} \rightarrow \text{(Nucleotide)}_{n+1} + \text{PP}_i$

Translation in Detail

Mechanism

Translation involves decoding the mRNA sequence into a polypeptide chain. The ribosome reads mRNA codons, and tRNAs bring the corresponding amino acids.

• Codon-Anticodon Interaction: Each tRNA has an anticodon that pairs with a specific mRNA codon.

• Peptide Bond Formation: The ribosome catalyzes the formation of peptide bonds between amino acids.

Comparison of Prokaryotic and Eukaryotic Gene Expression

Key Differences

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
RNA Processing	None	5' cap, splicing, poly-A tail
Coupling of Transcription and Translation	Yes	No
Initiation Factors	Fewer	More complex

Key Terms

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• RNA Polymerase: Enzyme that synthesizes RNA from a DNA template.

• tRNA (Transfer RNA): Adaptor molecule that brings amino acids to the ribosome during translation.

• Ribosome: Cellular machinery for protein synthesis.

• Codon: Three-nucleotide sequence in mRNA that specifies an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to the mRNA codon.

Example

In Escherichia coli (a bacterium), transcription and translation occur simultaneously in the cytoplasm, allowing rapid protein synthesis in response to environmental changes.

Additional info: The notes and images provided are consistent with introductory college-level biology topics on gene expression, transcription, and translation in prokaryotes and eukaryotes.