⚡ Core Concept: Obligate Intracellular Parasitism & Capsid Architectures
Viruses are submicroscopic, acellular infectious entities possessing either a DNA or RNA genome enclosed within a protective proteinaceous capsid, sometimes surrounded by a host-derived lipid envelope. Lacking metabolic machinery and ribosomes, they replicate exclusively by hijacking host cellular biosynthetic pathways.
1. Viral Capsid Symmetries & Baltimore Classification
Viral genomes display unmatched architectural diversity categorized under the seven Baltimore classes based on their mRNA synthesis pathway:
Figure 3: Retroviral Architecture & The Bacterial Cell Envelope Matrix
Microbial Cytology
Figure 3: Left: Enveloped retroviral ultrastructure (HIV-1: dimeric RNA genome, reverse transcriptase, capsid shell, and surface gp120/gp41 spikes). Right: Gram-positive vs. Gram-negative bacterial envelope architecture.
2. The Seven Baltimore Replication Classes
| Class | Genome Configuration | mRNA Synthesis Mechanism | Representative Viruses |
|---|---|---|---|
| Class I | dsDNA | Direct transcription by host or viral RNA Pol | Adenovirus, Herpes simplex (HSV), Poxvirus |
| Class II | ssDNA (+) | Synthesizes complementary (-) strand → dsDNA intermediate | Parvovirus B19, Circoviruses |
| Class III | dsRNA | Viral RNA-dependent RNA polymerase (RdRp) copies (-) strand | Reovirus, Rotavirus |
| Class IV | (+)ssRNA | Genome serves directly as mRNA; RdRp translated immediately | Poliovirus, SARS-CoV-2, Hepatitis C (HCV) |
| Class V | (-)ssRNA | Must package structural viral RdRp inside virion to transcribe (+) mRNA | Influenza, Rabies, Ebola, Measles |
| Class VI | (+)ssRNA with DNA intermediate | Reverse Transcriptase copies RNA into dsDNA → proviral integration | HIV-1, HTLV-1 (Retroviruses) |
| Class VII | dsDNA with RNA intermediate | Transcribes pregenomic RNA → reverse transcribed inside capsid | Hepatitis B Virus (Hepadnavirus) |
3. Retroviral Replication Cycle: From gp120 Binding to Budding
- Adsorption & Entry: Viral envelope glycoprotein gp120 binds primary CD4 receptor and chemokine co-receptors (CCR5 or CXCR4), inducing conformational change in gp41 that drives membrane fusion.
- Reverse Transcription: Viral Reverse Transcriptase (RT) converts (+)ssRNA into linear double-stranded proviral DNA in the cytoplasm, exhibiting high error rates due to lack of proofreading.
- Nuclear Integration: Viral Integrase (IN) inserts proviral DNA covalently into host chromosomal euchromatin, establishing lifelong persistent infection.
- Assembly & Maturation: Gag and Gag-Pol polyproteins assemble at the plasma membrane, bud outward, and are cleaved into mature infectious proteins by the viral Protease (PR).
Lytic vs. Lysogenic Bacteriophage Cycles
Temperate phages (e.g., Bacteriophage λ) toggle between two cycles governed by the genetic switch between cI repressor (favors lysogeny: stable prophage integration into bacterial genome) and Cro protein (favors lytic cycle: rapid viral assembly and host cell burst via holin-endolysin peptidoglycan lysis).
