Acellular Life
Acellular infectious agents — viruses, and the simpler prions and viroids — lack a true cellular organization yet can replicate or propagate inside a host. The AAMC MCAT content outline expects you to describe viral structure and replication (lytic vs lysogenic cycles), understand retroviruses and reverse transcriptase, explain the biology of HIV / AIDS in detail, and recognize prions and viroids. This is a high-yield area for the Bio/Biochem section.
Viruses
A virus is an obligate intracellular parasite that consists of a nucleic acid genome (DNA or RNA) enclosed in a protein coat called the capsid. Viruses lack ribosomes, mitochondria, and metabolic machinery; outside a host they exist as inert particles called virions.
Why viruses are "acellular"
Viruses do not satisfy the cell theory: they have no plasma membrane, no cytoplasm, and they cannot synthesise proteins or generate ATP on their own. They are therefore considered at the boundary of living and non-living. Inside a host cell they hijack ribosomes and enzymes to replicate; outside they behave like chemical crystals and can even be crystallised (Stanley, 1935 — tobacco mosaic virus).
General structure
- Capsid
- Protein shell built from repeating subunits called capsomeres. Determines viral shape (helical, polyhedral / icosahedral, or complex).
- Nucleic acid core
- Either DNA or RNA, single- or double-stranded, linear or circular — but never both DNA and RNA in the same virion.
- Envelope
- An outer lipid-bilayer membrane derived from the host cell, present in some viruses (HIV, influenza, herpes). Studded with viral glycoprotein spikes used for host attachment.
- Bacteriophage
- A virus that infects bacteria. The classic T4 phage has a polyhedral head, helical tail, base plate, and tail fibres — a "complex" morphology.
Helical — rod-shaped, capsomeres arranged in a spiral around the nucleic acid (e.g., tobacco mosaic virus, TMV). Polyhedral / icosahedral — 20 triangular faces (e.g., adenovirus, poliovirus). Enveloped — helical or polyhedral capsid surrounded by a lipid membrane (e.g., HIV, influenza). Complex — mixed geometry such as bacteriophages with head-and-tail design.
Lytic vs Lysogenic cycle (bacteriophages)
| Property | Lytic cycle | Lysogenic cycle |
|---|---|---|
| Outcome | Host cell lysed (destroyed) | Host cell survives and reproduces with phage DNA |
| Phage DNA fate | Replicates independently | Integrates into host chromosome as prophage |
| Steps | Attachment → penetration → biosynthesis → assembly → release | Attachment → penetration → integration → cell division (prophage replicates with host) |
| New virions produced? | Yes (~100–200 per cell) | Not until induced |
| Trigger to switch | — | UV light, X-rays, mutagens → prophage exits and lytic cycle begins |
| Time scale | Minutes to hours | Indefinite (many host generations) |
| Example | T-even phages (T2, T4) of E. coli | Lambda (λ) phage of E. coli |
Viruses are grouped by genome type: dsDNA (herpes, adenovirus, smallpox), ssDNA (parvovirus), dsRNA (rotavirus), ssRNA (+ sense) (polio, hepatitis A, SARS-CoV-2), ssRNA (- sense) (influenza, rabies), and retroviruses (HIV) which carry reverse transcriptase to copy RNA → DNA.
Common human viral diseases include influenza, measles, mumps, polio, rabies, chickenpox, smallpox, hepatitis A/B/C, dengue, COVID-19, and AIDS. Plant viruses include TMV; phages infect bacteria.
Prions are infectious proteins that contain no nucleic acid. A misfolded prion (PrPSc) forces the normal cellular form (PrPC) to refold into the abnormal shape, triggering a chain reaction that accumulates as aggregates. They cause fatal transmissible spongiform encephalopathies — Creutzfeldt-Jakob disease and kuru in humans, BSE (“mad cow”) and scrapie in animals. They resist heat and proteases, and because they carry no genome they are the classic exception to the central dogma.
Viroids are the smallest known pathogens: short, circular, single-stranded RNA molecules with no protein coat. They infect plants (e.g., potato spindle tuber disease) and are not translated — the naked RNA itself is infectious.
AIDS and HIV Infection
AIDS (Acquired Immunodeficiency Syndrome) is the late-stage clinical condition resulting from infection with the human immunodeficiency virus (HIV). HIV is a retrovirus (family Retroviridae, genus Lentivirus) that progressively destroys the immune system, leaving the patient vulnerable to opportunistic infections and certain cancers.
The pathogen
- HIV
- An enveloped, single-stranded (+) RNA retrovirus, ~120 nm in diameter, with two copies of its RNA genome and the enzyme reverse transcriptase.
- Glycoprotein spikes
- gp120 binds the CD4 receptor on helper T-cells; gp41 mediates fusion of viral envelope with the host membrane.
- Target cell
- Primarily CD4+ helper T-lymphocytes; also macrophages and dendritic cells. CD4 count falling below 200 cells/µL defines AIDS.
- Reverse transcriptase
- RNA-dependent DNA polymerase that copies viral RNA into a complementary DNA strand — the hallmark enzyme of retroviruses.
1. Attachment of gp120 to CD4 + co-receptor (CCR5 / CXCR4). 2. Fusion of envelope with host membrane via gp41. 3. Reverse transcription of viral RNA into dsDNA. 4. Integration into host chromosome as a provirus (catalysed by integrase). 5. Transcription & translation of viral genes. 6. Assembly of new virions at the membrane. 7. Budding and maturation (catalysed by protease).
HIV spreads through unprotected sexual contact, contaminated blood or blood products (transfusion, shared needles, unsterilised dental/surgical tools), and vertical transmission from infected mother to child during pregnancy, delivery, or breastfeeding. HIV is not spread by handshakes, coughing, sharing utensils, mosquitoes, or toilet seats.
Acute (window) phase — flu-like illness 2-4 weeks after exposure; antibodies not yet detectable. Clinical latency — asymptomatic, may last years while CD4 count gradually declines. AIDS — CD4 below 200/µL; opportunistic infections (Pneumocystis pneumonia, TB, candidiasis), Kaposi sarcoma, severe weight loss.
ELISA screens for anti-HIV antibodies; Western blot confirms; PCR detects viral RNA directly (used during the window period and in newborns). Treatment is HAART — Highly Active Antiretroviral Therapy — a cocktail of reverse-transcriptase inhibitors, protease inhibitors, and integrase inhibitors. There is currently no cure or licensed vaccine.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — it's where the deeper concept lives.
Q1. Viruses are considered acellular because they lack:
Viruses have nucleic acid (DNA or RNA) and a protein capsid, but no cytoplasm, no ribosomes, and no metabolic machinery — so they fail the cell theory and are classed as acellular. They mutate readily, which is why flu vaccines are reformulated yearly.
Q2. Which enzyme is unique to retroviruses such as HIV?
Reverse transcriptase is an RNA-dependent DNA polymerase that synthesises a DNA copy from a viral RNA template — the defining enzyme of retroviruses. It is the target of zidovudine (AZT) and other NRTIs in HAART.
Q3. HIV principally infects which type of human cell?
The viral glycoprotein gp120 binds the CD4 receptor present on helper T-cells (and to a lesser extent on macrophages and dendritic cells). Loss of helper T-cells cripples both cellular and humoral immunity, defining AIDS.
Q4. Which of the following is NOT a recognized mode of HIV transmission?
HIV is not transmitted by mosquitoes; the virus does not replicate in arthropods and the mechanical transfer of blood is far below the infectious dose. Sexual, blood-borne and vertical routes are the recognized transmission pathways.
Q5. A bacteriophage that integrates its DNA into the host chromosome and remains dormant is said to be in the:
In the lysogenic cycle, phage DNA is incorporated into the bacterial chromosome as a prophage and is replicated passively with the host. Environmental triggers (UV, chemicals) can switch it to the lytic cycle, leading to lysis.
Quick Recap
- Virus = nucleic acid (DNA or RNA, never both) + capsid ± envelope; obligate intracellular parasite.
- Shapes: helical, polyhedral, enveloped, complex (bacteriophages).
- Lytic cycle → host lysis; lysogenic cycle → prophage in host DNA.
- HIV is an enveloped ssRNA retrovirus that targets CD4+ T-cells via gp120.
- Reverse transcriptase converts viral RNA → dsDNA → integrated provirus.
- HIV transmission: sexual, blood, mother-to-child — not casual contact.
- Diagnosis: ELISA → Western blot / PCR. Treatment: HAART. No cure or licensed vaccine.
- Prions = infectious misfolded proteins, no nucleic acid (CJD, mad cow); viroids = naked circular RNA infecting plants.