Molecular Information Flow and Protein Processing — DNA Replication, Transcription & Secretion Explained | Chapter 6 of Brock Biology of Microorganisms

Welcome back to Last Minute Lecture, your trusted source for in-depth, chapter-by-chapter textbook breakdowns. In this post, we explore Chapter 6 of Brock Biology of Microorganisms, which focuses on the central dogma of molecular biology: the flow of information from DNA to RNA to protein. This chapter also covers how proteins are folded, modified, and exported — all critical processes in microbial life.

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This guide is ideal for biology students, especially those preparing for exams or reviewing molecular genetics, microbiology, or biochemistry. Let’s dive into how genetic information drives microbial structure and function.

The Central Dogma and Genetic Elements

The central dogma of molecular biology describes the flow of genetic information:

• DNA → RNA → Protein

DNA exists as a double-stranded helix, supercoiled for compact storage. Key genetic elements include:

• Chromosomes: Essential genomic DNA
• Plasmids: Extrachromosomal DNA, often with antibiotic resistance genes
• Viruses and transposable elements: Mobile genetic material

Prokaryotic genomes are typically circular and compact.

DNA Replication

Replication is semiconservative—each new DNA strand pairs with one original strand. Key features:

• Origin of replication (oriC): Starting point for replication
• Leading strand: Synthesized continuously
• Lagging strand: Synthesized in Okazaki fragments

Key enzymes:

• Helicase: Unwinds DNA
• Primase: Lays down RNA primer
• DNA polymerase III: Synthesizes new DNA
• Ligase: Joins Okazaki fragments
• DNA polymerase I: Replaces RNA primers

The replisome is the full replication complex. Proofreading ensures high fidelity.

Transcription: Making RNA

RNA polymerase binds to promoter regions to initiate transcription. In Bacteria:

• Sigma factors: Help RNA polymerase recognize promoters
• Transcription stops at terminator sequences

RNA types:

• mRNA: Messenger RNA for translation
• tRNA: Transfers amino acids
• rRNA: Structural and catalytic parts of the ribosome

Prokaryotic mRNA is often polycistronic (encodes multiple proteins). In contrast, eukaryotic transcription occurs in the nucleus and includes processing: 5′ cap, splicing, and poly-A tail. Archaeal transcription is similar to eukaryotic transcription.

Translation: Protein Synthesis

Translation occurs on 70S ribosomes in prokaryotes (30S + 50S subunits). Key steps:

1. Initiation: Ribosome, mRNA, and initiator tRNA assemble
2. Elongation: Amino acids are added to the growing peptide chain
3. Termination: Release factors recognize stop codons (UAA, UAG, UGA)

Start codon: AUG (codes for fMet in Bacteria, Met in Archaea/Eukarya)

Polysomes: Multiple ribosomes translating the same mRNA simultaneously

Wobble pairing: Flexibility at the third codon base allows redundancy

tmRNA: Rescues stalled ribosomes and marks faulty mRNAs for degradation

Chaperones: Assist in folding newly made proteins

Protein Processing and Export

After translation, proteins often require folding and chemical modifications. Proteins may be:

• Directed to membranes or exported from the cell
• Modified with cofactors

Transport systems:

• Sec system: Translocates unfolded proteins across the membrane
• Tat system: Transports folded proteins
• SRP (Signal Recognition Particle): Targets membrane-bound proteins

Secretion Systems in Gram-Negative Bacteria

Gram-negative bacteria use multiple secretion systems to export proteins:

• Type I, III, IV, VI: One-step transport mechanisms that bypass the periplasm
• Type II, V: Two-step systems requiring Sec or Tat first

Notable types:

• Type III: Injectisome (syringe-like) for injecting proteins into host cells
• Type IV: Transfers DNA and proteins during conjugation
• Type VI: Contractile system for interbacterial competition

These systems are vital for virulence, communication, and environmental interaction.

Glossary Highlights

• Central Dogma: DNA → RNA → Protein
• Replisome: Multiprotein complex for DNA replication
• Plasmid: Small, circular DNA molecule with optional traits
• Okazaki Fragment: Short DNA segments on the lagging strand
• Sigma Factor: Promoter-recognizing unit of RNA polymerase
• Polycistronic mRNA: Single mRNA encoding multiple proteins
• Wobble: Flexible base pairing in codons
• tmRNA: Helps release stalled ribosomes
• Chaperone: Assists in protein folding
• Signal Sequence: Directs protein localization/export
• Injectisome: Type III secretion needle-like apparatus

Conclusion

Chapter 6 of Brock Biology of Microorganisms provides a foundational understanding of how genetic instructions are executed within cells. Mastering DNA replication, transcription, and translation — along with post-translational processes like folding and secretion — is key to understanding microbial physiology, evolution, and biotechnology applications.

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