Viral Genomics and Diversity | Chapter 11 of Brock Biology of Microorganisms

Welcome to Last Minute Lecture. This post summarizes Chapter 11 of Brock Biology of Microorganisms, which explores the astonishing variety of viral genomes and the mechanisms that drive virus replication and evolution. You'll also learn about subviral agents like viroids and prions, which challenge traditional definitions of life.

🎥 Watch the full podcast summary here:

This chapter is essential for understanding virus biology, classification systems like the Baltimore scheme, and the broader impact of noncellular infectious agents.

Viral Genomes and Classification

• Viral genomes: DNA or RNA; ss or ds; linear or circular
• Plus-strand RNA = same polarity as mRNA
• Baltimore classification sorts viruses by genome type and mRNA production (7 classes)
• RNA viruses need RNA replicase (carried or encoded)
• ICTV classification: host range, structure, replication, phylogeny
• No universal viral genes → phylogeny uses conserved proteins

DNA Viruses

Bacterial DNA Viruses

• ϕX174: ssDNA, overlapping genes, rolling circle replication
• M13: chronic infection, no lysis
• T4: linear dsDNA, circularly permuted, headful packaging
• T7: encodes its own RNA polymerase
• Lambda: temperate phage, cI and Cro repressors regulate lysis vs. lysogeny

Archaeal DNA Viruses

• Diverse morphologies: spindle, rod, pleomorphic
• Unique exit mechanisms (e.g., pyramid-based lysis)
• Use host RNAP; some encode their own TFs

Animal DNA Viruses

• Poxviruses: cytoplasmic replication
• Adenoviruses: protein primer replication
• Polyomaviruses: transformation potential (e.g., SV40)
• Herpesviruses: latent infection, rolling circle replication

RNA Viruses

• Belong to Baltimore Classes III–VI
• Require RNA replicase

Positive-Strand RNA Viruses

• MS2: small phage with RNA structure–based regulation
• Poliovirus: polyprotein strategy, VPg primer
• Coronaviruses: nested mRNAs, Golgi assembly

Negative-Strand RNA Viruses

• Rabies virus: bullet shape, monocistronic mRNAs
• Influenza virus: segmented genome, cap-snatching, antigenic shift

Double-Stranded RNA Viruses

• Reoviruses (e.g., rotavirus): segmented dsRNA, replicate in nucleocapsid

Subviral Agents

Viroids

• Small circular ssRNA infecting plants
• No coding capacity; replicate via rolling circle
• Use host RNAP; act via siRNA-like silencing

Prions

• Infectious proteins (no nucleic acid)
• PrPC → PrPSc conversion leads to aggregation
• Cause neurodegenerative diseases (e.g., mad cow, CJD)
• May have beneficial roles in some organisms

Glossary Highlights

• Concatemer – Long DNA strand with repeated units
• Rolling Circle Replication – Circular DNA replication method
• RNA Replicase – Enzyme copying RNA from RNA
• Polyprotein – Large protein cleaved into smaller functional proteins
• Positive/Negative Strand – RNA polarity in relation to mRNA
• Reverse Transcriptase – Synthesizes DNA from RNA
• Retrovirus – Uses reverse transcription (e.g., HIV)
• Viroid – Naked infectious RNA, plant pathogen
• Prion – Misfolded infectious protein

Conclusion

Chapter 11 of Brock Biology of Microorganisms expands your understanding of viral diversity, genome organization, and replication strategies — including unusual agents like viroids and prions. These topics form the foundation of virology and shed light on how life can exist and replicate in unconventional forms.

🎬 Watch the video above to review all key concepts and examples.

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