C17 Gene Expression in Prokaryotes and Eukaryotes: Central Dogma, Transcription, and Translation

Bacterial Gene Expression

Translation Overview

Gene expression in bacteria involves the conversion of genetic information from DNA into functional proteins. This process is governed by the central dogma of molecular biology, which describes the flow of genetic information from DNA to RNA to protein.

• Structure:

  • Transfer RNA (tRNA): Small RNA molecules that carry amino acids to the ribosome during protein synthesis. Each tRNA has an anticodon region that pairs with the corresponding codon on mRNA.

  • Ribosome: A complex molecular machine composed of rRNA and proteins, responsible for synthesizing proteins by translating mRNA sequences.

• Process:

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

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

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

The Central Dogma in Prokaryotic Cells

Overview of Genetic Information Flow

The central dogma describes the process by which genetic information is transferred from DNA to RNA and then to protein. In prokaryotes, these processes occur in the cytoplasm and can be coupled.

• Replication: DNA is copied to produce identical genetic material for cell division.

• Transcription: DNA is used as a template to synthesize RNA by RNA polymerase.

• Translation: RNA is decoded by ribosomes to produce proteins.

Equation:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

Transcription in Prokaryotes

Mechanism and Molecular Details

Transcription is the process by which RNA is synthesized from a DNA template. RNA polymerase binds to the promoter region and synthesizes RNA in the 5' to 3' direction.

• Phosphodiester Linkage: RNA polymerase forms phosphodiester bonds between nucleotides as it synthesizes RNA.

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

• Directionality: RNA is synthesized in the 5' to 3' direction, using the 3' to 5' DNA template strand.

Equation:

$\text{NTP} + (RNA)_{n} \xrightarrow{\text{RNA polymerase}} (RNA)_{n+1} + PP_i$

RNA Polymerase and Transcription Initiation

Role of RNA Polymerase

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template. It binds to specific DNA sequences called promoters to initiate transcription.

• Promoter Recognition: RNA polymerase recognizes and binds to the promoter region upstream of the gene.

• Initiation Complex: The enzyme unwinds the DNA and begins RNA synthesis.

• Downstream Elongation: RNA polymerase moves along the DNA, elongating the RNA transcript.

Example: In Escherichia coli, the sigma factor helps RNA polymerase recognize the promoter.

Eukaryotic Gene Expression

Overview and Key Steps

Gene expression in eukaryotes is more complex than in prokaryotes, involving additional steps and regulatory mechanisms. The processes occur in distinct cellular compartments.

• Transcription: Occurs in the nucleus, where DNA is transcribed into pre-mRNA by RNA polymerase II.

• Transcription Processing: Pre-mRNA undergoes modifications such as 5' capping, splicing, and 3' polyadenylation to become mature mRNA.

• Translation: Mature mRNA is exported to the cytoplasm, where ribosomes synthesize proteins.

• Post-translation Protein Modification: Proteins may be further modified (e.g., phosphorylation, glycosylation) to become fully functional.

Additional info: Eukaryotic transcription involves multiple RNA polymerases (I, II, III) and complex regulation by transcription factors.