Viruses and Subviral Particles
Learn viral structure, genome types, the replication cycle, lytic vs. lysogenic cycles, and subviral particles (prions and viroids) for the MCAT.
Viruses are a unique class of biological entities: they contain genetic material — either DNA or RNA — but unlike living cells, they're acellular and cannot survive or replicate on their own. Because they require a host cell to reproduce, viruses are considered obligate intracellular parasites. This page covers viral structure, genome types, the replication cycle, and two even simpler infectious agents — prions and viroids.
Key Takeaways
Viruses are acellular obligate intracellular parasites: genetic material (DNA or RNA) in a protein capsid, sometimes with a lipid envelope.
Viral genomes vary by type (DNA/RNA), strandedness, and — for single-stranded RNA — sense (positive vs. negative); retroviruses use reverse transcriptase to convert RNA into DNA.
The viral replication cycle has five steps: attachment, penetration, biosynthesis, assembly, release.
Bacteriophages can follow the lytic cycle (immediate replication, host cell lysis) or the lysogenic cycle (dormant integration as a provirus/prophage, triggered exit into the lytic pathway).
Prions are infectious proteins with no genetic material, causing neurodegenerative disease by inducing protein misfolding.
Viroids are protein-free, circular single-stranded RNA that disrupt gene expression in plants.
What Is a Virus?
Capsid and Envelope Structure
A virus's genetic material is surrounded by a protein coat called a capsid. In many cases, the capsid is also enclosed by a lipid-containing envelope, typically derived from the host cell's membrane.
Bacteriophage Structure
One specific type of virus is the bacteriophage, which infects bacteria. Bacteriophages have a specialized structure that includes a tail sheath, which acts like a syringe to inject genetic material, and tail fibers, which help the virus recognize and bind to specific bacterial hosts.
Viral Genome Types
DNA vs. RNA, Single- vs. Double-Stranded
A viral genome can take several forms: it may be made of DNA or RNA, and it may be single-stranded or double-stranded.
Positive-Sense vs. Negative-Sense RNA
Single-stranded RNA viruses are further classified by sense:
Positive-sense RNA can be directly translated by the host's ribosomes.
Negative-sense RNA must first be converted into a complementary strand before it can be translated.
Retroviruses and Reverse Transcriptase
Retroviruses contain a single-stranded RNA genome, but unlike typical RNA viruses, they use an enzyme called reverse transcriptase to synthesize a complementary DNA strand from their RNA. This DNA is then integrated into the host genome, allowing the virus to replicate along with the host cell's DNA.
The Viral Replication Cycle
The viral life cycle typically follows a sequence of five steps:
Attachment — the virus attaches to a specific receptor on the surface of a host cell. This receptor recognition is highly specific and determines which hosts and cell types a virus can infect.
Penetration — the virus enters the host cell, either by membrane fusion or endocytosis, depending on the type of virus and whether it has an envelope.
Biosynthesis — the viral genetic material is released into the cytoplasm, and the host cell's own enzymes and ribosomes are hijacked to replicate the viral genome and synthesize viral proteins.
Assembly — the viral components are assembled into new virions.
Release — mature virions are released from the host cell. This can occur through lysis, which destroys the host cell (more common in non-enveloped viruses), or through extrusion/budding, which allows the virus to exit while keeping the host cell alive (typical for enveloped viruses).
Lytic vs. Lysogenic Cycle
Some viruses, especially bacteriophages, can enter an alternative cycle known as the lysogenic cycle. Whether a bacteriophage enters the lytic or lysogenic cycle often depends on environmental conditions and host cell status.
Cycle | What Happens | Outcome |
|---|---|---|
Lytic | Virus immediately replicates, with little regard for host survival | Host cell lyses, releasing new virions |
Lysogenic | Viral genome integrates into host DNA as a provirus/prophage; stays dormant | Host cell survives; viral DNA replicates with host genome each division |
In the lytic cycle, the virus immediately begins replicating upon infection, producing large numbers of virions and ultimately causing the host cell to lyse, releasing the new viral particles.
In the lysogenic cycle, the viral genome integrates into the host's DNA, becoming a provirus (or prophage). In this dormant state, the viral genetic material is replicated along with the host's genome each time the cell divides. The virus may remain in this latent form for extended periods until a trigger — such as stress or UV radiation — causes it to exit the lysogenic cycle and enter the lytic pathway.
Subviral Particles
Two unique infectious agents fall outside the standard definition of a virus: prions and viroids. They're classified as subviral particles because they're even simpler than viruses, but they can still cause disease under specific circumstances.
Genetic Material | Protein Coat | Typical Host / Disease | |
|---|---|---|---|
Virus | DNA or RNA | Yes (capsid, often with envelope) | Varies widely |
Prion | None | No (infectious protein only) | Neural tissue — Creutzfeldt-Jakob disease, mad cow disease |
Viroid | Circular, single-stranded RNA | No | Plants — agricultural damage |
Prions
Prions are infectious proteins that do not contain any genetic material. What makes them dangerous is their ability to induce the misfolding of normal proteins in the host, particularly in neural tissue. This misfolding disrupts normal protein structure and function, leading to progressive neurodegenerative diseases like Creutzfeldt-Jakob disease and mad cow disease. Since prions are made entirely of protein and lack RNA or DNA, they challenge the traditional understanding of what constitutes an infectious agent.
MCAT Callout — No Genetic Material Required: Every other infectious agent on this page — viruses and viroids — relies on genetic material to cause disease. Prions don't. They spread disease purely by causing normal proteins to misfold into the same abnormal shape.
Viroids
Viroids are composed solely of a short strand of circular, single-stranded RNA. Unlike viruses, they don't encode proteins and don't have a protein coat. Instead, viroids exert their effects by interfering with gene expression in the host. They're known to infect plants and can disrupt normal growth and development, causing significant agricultural damage.
Why Viruses and Subviral Particles Matter for the MCAT
Genome-type reasoning — positive-sense vs. negative-sense RNA, and the retrovirus/reverse transcriptase exception — connects directly to central dogma reasoning tested elsewhere on the exam.
Lytic vs. lysogenic cycle questions often test why a virus would "choose" dormancy over immediate replication, which requires understanding the trigger-based switch, not just the vocabulary.
Prions and viroids are a classic MCAT trap category: passages may ask which statement about a given infectious agent is false, testing whether you know prions lack genetic material entirely and viroids lack a protein coat.
Do not assume specific AAMC blueprint weighting — these are simply well-established, frequently tested structural distinctions.
Common MCAT Mistakes
Assuming all viruses have an envelope. Only some do — the envelope is a lipid layer derived from the host membrane, present in enveloped viruses but absent in non-enveloped ones. The capsid, not the envelope, is the universal feature.
Mixing up positive-sense and negative-sense RNA. Positive-sense RNA can be translated directly by host ribosomes. Negative-sense RNA must first be converted into a complementary strand before translation can occur.
Treating the lysogenic cycle as a dead end. It isn't — the viral genome integrates as a dormant provirus/prophage, but a trigger (such as stress or UV radiation) can still push the virus into the lytic cycle later.
Confusing prions and viroids. Prions are infectious proteins with no genetic material at all. Viroids are the opposite in one key way — they're pure genetic material (circular ssRNA) with no protein coat. Neither fits the standard virus definition, but for different reasons.
MCAT-Style Concept Check
Question: A newly discovered infectious agent is found to contain single-stranded RNA that cannot be directly translated by host ribosomes upon entry into the cell. Which of the following best describes this agent?
A) A positive-sense RNA virus
B) A negative-sense RNA virus
C) A prion
D) A viroid
Answer: B
Explanation: The defining clue is that the RNA "cannot be directly translated" upon entry — that's the definition of negative-sense RNA, which must first be converted into a complementary strand before translation. Positive-sense RNA (A) can be translated immediately, ruling it out. Prions (C) contain no genetic material at all, and viroids (D) are non-coding circular RNA that isn't translated into viral proteins in the first place — neither matches an agent whose RNA is being evaluated for translatability.
FAQ
What's the difference between a capsid and an envelope?
The capsid is the protein coat that directly surrounds a virus's genetic material — every virus has one. The envelope is an additional lipid layer, typically derived from the host cell's membrane, that surrounds the capsid in some (but not all) viruses.
Why do retroviruses need reverse transcriptase?
Retroviruses carry a single-stranded RNA genome, but they replicate by integrating into the host's DNA genome. Reverse transcriptase converts their RNA into complementary DNA, which can then be inserted into the host chromosome.
What triggers a lysogenic virus to enter the lytic cycle?
A dormant provirus/prophage can be triggered to exit the lysogenic cycle by environmental stressors, such as cellular stress or UV radiation, at which point it switches to active, lytic replication.
Are prions and viroids technically viruses?
No. Both are classified as subviral particles because they lack features required to be classified as a virus — prions have no genetic material, and viroids have no protein coat.
Part of: