Viruses and Subviral Particles
High-Yield Summary
- Viruses: acellular obligate intracellular parasites — genetic material (DNA or RNA) in a protein capsid, sometimes with a lipid envelope.
- Genome varies: DNA/RNA, single/double-stranded; ssRNA further splits into positive-sense (directly translatable) vs. negative-sense (needs complementary strand first).
- Retroviruses (ssRNA) use reverse transcriptase to make DNA from their RNA, which integrates into the host genome.
- 5-step replication cycle: attachment → penetration → biosynthesis → assembly → release (lysis or budding/extrusion).
- Bacteriophages: lytic cycle (immediate replication, host lyses) vs. lysogenic cycle (dormant provirus/prophage, triggered exit to lytic).
- Prions (infectious misfolded protein, no genetic material) and viroids (naked circular ssRNA, no protein coat, plant-only) are subviral particles — simpler than viruses.
Key Terms
- Capsid
- Protein coat surrounding a virus's genetic material.
- Envelope
- Lipid layer around some viral capsids, typically derived from the host cell membrane.
- Reverse transcriptase
- Retrovirus enzyme that synthesizes complementary DNA from an RNA genome, integrated into host DNA.
- Provirus / prophage
- Viral genome integrated into host DNA during the lysogenic cycle; replicates passively with the host genome.
- Prion
- Infectious protein with no genetic material; causes disease by inducing misfolding of normal proteins (e.g. Creutzfeldt-Jakob, mad cow disease).
- Viroid
- Naked circular single-stranded RNA (no protein coat, no encoded proteins); infects plants by disrupting gene expression.
Viral Replication Cycle
- 1Attachment — virus binds a specific host-cell surface receptor.
- 2Penetration — virus enters via membrane fusion or endocytosis.
- 3Biosynthesis — viral genome released; hijacks host enzymes/ribosomes to replicate genome and make viral proteins.
- 4Assembly — viral components assembled into new virions.
- 5Release — virions exit via lysis (destroys host, common in non-enveloped viruses) or budding/extrusion (host survives, typical for enveloped viruses).
Lytic vs. Lysogenic Cycle
| Lytic | Lysogenic |
|---|---|
| Immediate replication, little regard for host survival | Genome integrates as provirus/prophage; stays dormant |
| Host cell lyses, releases new virions | Host survives; viral DNA replicates with host genome each division |
Prions vs. Viroids
| Prion | Viroid |
|---|---|
| No genetic material — infectious protein only | Circular ssRNA, no protein coat |
| Neural tissue; Creutzfeldt-Jakob, mad cow disease | Plants; disrupts gene expression, agricultural damage |
Common MCAT Trap
- Positive-sense RNA is directly translatable; negative-sense RNA is NOT — it needs a complementary strand made first.
- Prions lack genetic material entirely (protein only); viroids lack a protein coat (RNA only) — passages may test which false statement mismatches which agent.
- Retroviruses reverse the usual DNA→RNA flow: RNA→DNA via reverse transcriptase, then that DNA integrates into the host genome.
Quick Recall
What enzyme do retroviruses use, and what does it do?
What are the 5 steps of the viral replication cycle?
What distinguishes the lysogenic cycle from the lytic cycle?
What key feature do prions lack that all other infectious agents on this page have?