Viruses

Core Concepts

Why viruses are not cellular organisms

Viruses lack most characteristics associated with life:

• They are not composed of cells.

• They have no metabolism — they do not produce or consume ATP.

• They cannot reproduce independently — they must infect a host cell.

• They do not maintain homeostasis.

• They do not grow or respond to their environment outside of a host.

However, viruses DO have genetic material (DNA or RNA), evolve by natural selection, and are exquisitely adapted to their hosts. They occupy a gray area between living and nonliving. Biologists generally classify them as infectious particles rather than organisms.

Viral structure: Genetic material (DNA or RNA, ss or ds) in a protein capsid (helical, icosahedral, or complex). Some have a lipid envelope derived from host membrane (e.g., influenza, HIV). Enveloped viruses are more sensitive to desiccation than non-enveloped.

Host specificity

Viruses are highly specific for their host — a given virus can infect only certain species and often only certain cell types within that species. Host specificity is determined by the interaction between viral surface proteins and specific receptor molecules on the host cell surface. This is why you cannot catch a plant virus, and why HIV infects specific immune cells but not skin cells.

Bacteriophages (phages)

Bacteriophages — viruses that infect bacteria — have been crucial models in molecular biology. The most studied are the T-even phages that infect E. coli.

Lytic cycle

1. Attachment: The phage binds to specific receptors on the bacterial surface.

2. Entry: The phage injects its DNA into the bacterium (the capsid remains outside).

3. Synthesis: The phage DNA directs the synthesis of phage proteins and replication of phage DNA, using the host cell's machinery. Host DNA is degraded.

4. Assembly: New phage particles are assembled from the replicated DNA and proteins.

5. Release: The host cell lyses (bursts), releasing hundreds of new phages that can infect nearby bacteria.

Lysogenic cycle

1. After entry, the phage DNA integrates into the bacterial chromosome, becoming a prophage.

2. The prophage is replicated along with the host DNA and passed to daughter cells. Most prophage genes are silenced.

3. The bacterium carrying the prophage is called a lysogen. The phage is dormant (latent).

4. An environmental trigger (e.g., DNA damage, stress) can cause the prophage to excise from the chromosome and enter the lytic cycle.

The lysogenic cycle allows the phage to persist and multiply without immediately killing the host.

Animal virus replication

Animal viruses follow a similar overall pattern but with key differences:

1. Attachment: Viral proteins bind to host cell receptors.

2. Entry: Enveloped viruses may fuse their envelope with the host plasma membrane, releasing the capsid into the cytoplasm. Other viruses enter by receptor-mediated endocytosis. Non-enveloped viruses may inject their genome or enter through endocytosis.

3. Uncoating: The capsid is removed, releasing the viral genome.

4. Replication and protein synthesis: The viral genome directs the production of viral mRNA, proteins, and new genomes. The details vary depending on the type of viral genome (DNA, RNA, retrovirus).

5. Assembly: New viral particles are assembled.

6. Release: Enveloped viruses typically bud from the host cell, acquiring their envelope from the host membrane. Non-enveloped viruses are usually released by cell lysis.

Retroviruses (e.g., HIV) are RNA viruses that use the enzyme reverse transcriptase to synthesize DNA from their RNA genome. The DNA is then integrated into the host chromosome, where it can direct the production of new viral RNA and proteins. Retroviruses violate the central dogma (RNA → DNA instead of DNA → RNA).

Latency: Some animal viruses can establish latent infections, in which the viral genome persists in the host cell without producing active infection. The virus may reactivate later. Examples: herpes simplex virus (cold sores), varicella-zoster (chickenpox and shingles).

Viral mutation and emerging viruses

Viruses — especially RNA viruses — have high mutation rates. This is because the enzymes that replicate viral RNA lack proofreading activity. High mutation rates, combined with rapid replication and large population sizes, enable viruses to evolve quickly, evading immune responses and crossing species barriers. Emerging viruses are viruses that have recently appeared in a population or are rapidly expanding their range. Examples: SARS-CoV-2, Ebola, Zika, new influenza strains.

Vaccination (biological basis)

Vaccination works by exposing the immune system to a harmless version of a pathogen (or a piece of it), allowing the body to develop a memory response. Upon subsequent exposure to the actual pathogen, the immune system can mount a rapid, effective defense before the infection takes hold.

Vaccines may contain:

• Inactivated (killed) virus

• Attenuated (weakened) live virus

• Viral protein subunits

• mRNA encoding a viral protein (newer technology, as used in some COVID-19 vaccines)

Vaccination does not guarantee that a vaccinated person will never become infected, but it dramatically reduces the likelihood of severe illness by preparing the immune system in advance.

Viruses versus bacteria

ELI Example

A virus is a USB drive with malware. Alone, it does nothing — no processor, no power. Plug it into a computer (host cell), and the malware takes over, making copies and disrupting normal function. Vaccination is installing antivirus before the malware arrives.

Do Not Confuse

High-Yield Memory Anchors

• Virus = genetic material (DNA or RNA) + protein capsid (+ optional envelope). Not cellular.

• Obligate intracellular parasites: must infect a host cell to reproduce.

• Lytic cycle = infect → replicate → lyse. Lysogenic = integrate → dormant → may reactivate.

• Vaccines train the immune system to recognize a pathogen before infection.

• Antibiotics do NOT work on viruses.

Quick Check

Q1 (Foundational): List three reasons why viruses are generally not considered living organisms.

Q2 (Application): A bacteriophage infects a bacterial cell. Instead of immediately destroying the cell, the phage DNA integrates into the bacterial chromosome. The bacterium survives and divides normally for many generations. Later, the phage DNA excises, and the cell begins producing new phage particles. Name and describe the two cycles involved in this scenario.

Q3 (Comparison/Reasoning): Compare the structure and replication of viruses and bacteria. Why are antibiotics effective against bacteria but not viruses?

Quick Check Answers

A1: (1) Viruses are not composed of cells. (2) Viruses have no metabolism — they do not produce or use ATP independently. (3) Viruses cannot reproduce independently — they must infect a host cell and hijack its machinery. (Also acceptable: no homeostasis, no independent growth, no response to environment outside a host.)

A2: The first phase is the lysogenic cycle: the phage DNA integrates into the bacterial chromosome as a prophage, replicating passively with the host DNA without harming the cell. The second phase — when the prophage excises and begins producing new phage particles — is the switch to the lytic cycle, which ends with bacterial cell lysis and release of progeny phages.

A3: Structure: Bacteria are cellular (prokaryotic), with plasma membranes, ribosomes, and metabolic machinery. Viruses are noncellular — protein capsid with genetic material, optional envelope. Replication: Bacteria reproduce independently by binary fission. Viruses require a host cell to replicate — they inject their genome and commandeer the host's machinery. Antibiotics: Antibiotics target bacterial-specific structures and processes — peptidoglycan cell wall synthesis (penicillins), 70S ribosomes (tetracyclines), DNA replication enzymes unique to bacteria. Viruses use the host cell's machinery for replication; targeting viral processes requires antiviral drugs that interfere with viral-specific enzymes (reverse transcriptase, proteases) or viral entry. Broad-spectrum antibiotics have no targets in viruses because viruses lack ribosomes, cell walls, and independent metabolism.

Chapter Summary

Viruses are noncellular infectious particles — genetic material (DNA or RNA) in a protein capsid, sometimes with a lipid envelope. They are obligate intracellular parasites. Bacteriophages follow either a lytic cycle (immediate replication, cell lysis) or a lysogenic cycle (integration, dormancy, potential reactivation). Animal viruses enter by endocytosis or membrane fusion and exit by budding or lysis. Viral mutation rates are high, facilitating rapid evolution. Vaccination exposes the immune system to a harmless form of the virus, building protective memory. Viruses and bacteria are fundamentally different — antibiotics target bacterial structures absent from viruses.

Common Mistakes

Mistake: "Viruses are living organisms."

Reality: Viruses lack cells, metabolism, and independent reproduction. They occupy a gray area between living and nonliving. Most biologists classify them as infectious particles, not organisms.

Mistake: "Antibiotics can treat viral infections."

Reality: Antibiotics target bacteria, not viruses. Taking antibiotics for a cold, flu, or COVID-19 is ineffective and contributes to antibiotic resistance. Antiviral medications (a different class of drugs) target specific viral processes.

Mistake: "All viruses kill their host cells quickly."

Reality: Some viruses establish latent or persistent infections that do not kill the host cell. The host may carry the virus for life, with periodic reactivation (e.g., herpes viruses).

Mistake: "Viruses are primitive precursors to cellular life."

Reality: Viruses likely evolved after cells, as escaped genetic elements. They require cells for replication, so they could not have predated the first cells.