Acellular Life: MDCAT Notes and MCQs
Viruses — the basics
- Viruses are acellular, obligate intracellular parasites: no metabolism, no ribosomes, reproduce only inside a living host cell.
- A complete virus particle is a virion: nucleic acid core (either DNA or RNA, never both) + protein coat (capsid, made of capsomeres). Some have a lipid envelope taken from the host membrane (e.g. HIV, influenza).
- Size ≈ 20–250 nm; seen only with an electron microscope. Tobacco Mosaic Virus (TMV) was the first virus studied (Ivanowsky, 1892).
Classification
| Basis | Types / examples |
|---|---|
| Capsid shape | Helical (TMV), polyhedral / icosahedral (adenovirus, polio), complex (bacteriophage T4), enveloped (HIV, influenza) |
| Nucleic acid | dsDNA (pox, herpes, adeno), ssDNA (parvo), dsRNA (reovirus), ssRNA (polio, TMV, HIV, influenza) |
| Host | Plant viruses (TMV), animal viruses, bacterial viruses = bacteriophages |
| Disease | Polio, measles, mumps, hepatitis, rabies, herpes, influenza, AIDS, COVID-19 |
Bacteriophage life cycles
- Lytic: attach → inject DNA → replicate → assemble → lyse host (virulent phage, e.g. T4).
- Lysogenic: phage DNA joins host chromosome as a prophage and is copied with it (temperate phage, e.g. lambda).
HIV & AIDS
- Cause: Human Immunodeficiency Virus — an enveloped retrovirus with 2 copies of ssRNA and the enzyme reverse transcriptase (RNA → DNA).
- Target: helper T-lymphocytes (CD4+ cells) and macrophages → collapse of cell-mediated immunity.
- Transmission: unprotected sexual contact, infected blood/transfusion, shared needles/syringes, mother to child (placenta, birth, breast-feeding). Not by touching, sharing food, mosquitoes.
- Symptoms: early flu-like illness, then long latent period; AIDS stage → weight loss, chronic diarrhoea, fever, swollen lymph nodes, opportunistic infections (TB, pneumonia, Kaposi's sarcoma).
- Diagnosis: ELISA (screening) confirmed by Western blot / PCR. No cure; antiretroviral therapy controls it.
PMDC learning outcomes for this unit
- Viruses: 1.1 Classify viruses on basis of their structure / number of strands / diseases / hosts etc.
- AIDS and HIV Infection: 1.2 Identify symptoms, mode of transmission and cause of viral disease (AIDS)