Chapter Review

Variety of Life

Viruses and AIDS/HIV

Viruses: Nature and Characteristics

Viruses are non-cellular, obligate intracellular parasites containing either RNA or DNA enclosed in a protein coat. They lack independent metabolic machinery and replicate only inside living host cells.

Key Points

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    Size ranges from 20 nm (parvoviruses) to 250 nm (poxviruses) — 10 to 1000x smaller than bacteria
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    Filterable through porcelain filters that trap bacteria — historically called 'filterable viruses'
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    Cannot be grown on artificial media; reproduce only in living cells via replication
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    Resistant to broad-spectrum antibiotics (penicillin, streptomycin) — these target bacterial processes
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    Study of viruses is called virology; word derived from Latin 'venome' meaning poisonous fluid
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    Prions are protein-only infectious agents discovered in 1983 — contain no nucleic acid

Virus Structure: Virion Components

A virion (complete infectious particle) consists of a nucleic acid genome surrounded by a protein capsid made of capsomeres. Some viruses also have a host-derived envelope.

Key Points

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    Genome = nucleic acid core (DNA or RNA) carrying viral genetic information
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    Capsid = protein coat giving the virion its definite shape, built from capsomeres
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    Capsomere number is fixed per virus type (Herpes: 162, Adenovirus: 252)
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    Envelope is an additional host-derived membrane covering the nucleocapsid (not present in all viruses)
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    Naked virions = nucleocapsid only; enveloped virions = nucleocapsid + host-derived envelope
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    Nucleocapsid = combined structure of genome + capsid
Formula

Bacteriophages: T-Phage Structure

Bacteriophages are viruses that infect bacteria. The T4 phage has a complex head-and-tail structure resembling a tadpole, with volume about 1/1000 of its E. coli host cell.

Key Points

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    Discovered by Twort (1915) and D'Herelle (1917); named 'bacteria-eaters'
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    Head: elongated pyramidal, hexagonal prism containing double-stranded DNA
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    Tail consists of inner core tube enclosed in a sheath, with collar and end plate
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    Six tail fibers attach to bacterial receptor sites for infection
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    Tail sheath contracts during infection, forcing the core through the cell wall
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    Only viral DNA is injected into the host — the protein coat remains outside

Lytic vs Lysogenic Cycle

The lytic cycle destroys the host cell and releases ~200 new phages in ~25 minutes. The lysogenic cycle integrates viral DNA (prophage) into the host chromosome for silent replication.

Key Points

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    Lytic cycle: Attachment → Penetration → Synthesis → Maturation → Release
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    Lytic: virulent phage; Lysogenic: temperate phage (can follow either pathway)
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    Penetration uses lysozyme to dissolve bacterial cell wall; sheath contracts to inject DNA
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    Prophage is replicated passively during bacterial cell division in lysogeny
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    Lysogenic bacteria are immune to infection by the same or related phages
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    Induction = prophage detaches from chromosome and enters the lytic cycle
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    Prophage (bacteria) vs Provirus (animal cells) — same concept, different host

Virus Classification

Viruses are classified by genetic material (DNA vs RNA), envelope presence, morphology, and host range. These criteria predict viral stability, transmission route, and disease pattern.

Key Points

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    DNA viruses: herpes, adenovirus, poxvirus (smallpox), hepatitis B
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    RNA viruses: influenza, poliovirus, mumps, measles, HIV, hepatitis A and C
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    Most respiratory and STI viruses are enveloped; many enteric viruses are naked
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    Naked viruses survive better outside the body (no fragile envelope)
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    Shapes: rod (TMV), spherical (polio, HIV), icosahedral (adenovirus), complex (poxvirus), tadpole (T4 phage)
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    Host range: animal viruses, plant viruses, bacterial viruses (bacteriophages)

Hepatitis Types Comparison

Hepatitis is liver inflammation caused by different viruses with distinct genetic material, transmission routes, and severity profiles.

Key Points

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    HAV — RNA, Naked, Faecal-oral transmission, Mild/short-term, Vaccine available
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    HBV — DNA, Enveloped, Body fluids transmission, Can be chronic, Vaccine available
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    HCV — RNA, Enveloped, Blood transmission, Often chronic liver disease, NO vaccine
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    HDV — requires co-infection with HBV (delta hepatitis)
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    HEV — faecal-oral transmission; pig may be a source
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    HBV is common in Asia, China, Philippines, Africa, and Middle East

Retroviruses and Reverse Transcriptase

Retroviruses are single-stranded enveloped RNA viruses that reverse the central dogma using reverse transcriptase to convert their RNA genome into DNA.

Key Points

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    Also called oncoviruses; spherical, ~100 nm, enveloped by host plasma membrane
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    Reverse transcriptase converts ssRNA → dsDNA — reverses normal flow (DNA → RNA → protein)
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    Viral DNA integrates into host genome as a provirus, replicated during cell division
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    Some retroviruses are oncogenic (cause cancer) by disrupting normal gene regulation
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    Most are host-specific, requiring specific receptor molecules on the target cell
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    Not all retroviruses cause cancer — HIV causes immune deficiency, not cancer
Formula

HIV/AIDS: Infection and Transmission

HIV is a retrovirus that selectively destroys helper T-lymphocytes (CD4+ cells), leading to progressive immune failure. AIDS is the resulting syndrome of severe immune deficiency.

Key Points

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    HIV primarily targets helper T-lymphocytes via CD4 receptor sites on their surface
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    HIV cycle: Attachment → Reverse Transcription → Integration → Transcription/Translation → Assembly → Budding
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    Enters as RNA — must use reverse transcriptase to make DNA inside the host cell
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    Provirus integrates into host chromosomal DNA; can also infect CNS cells
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    Transmission: sexual contact, infected blood, breastfeeding, mother-to-child during birth
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    NOT transmitted by casual contact — hugging, handshakes, food sharing, mosquito bites
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    Key symptoms: severe pneumonia, Kaposi's sarcoma, weight loss, swollen lymph nodes

Historical Milestones in Virology

Key discoveries shaped understanding of viruses from the first vaccination to viral crystallization.

Key Points

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    Jenner (1796): first vaccination using cowpox material against smallpox (Latin 'vacca' = cow)
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    Chamberland (1884): rabies agent passes through porcelain filters — filterable agents exist
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    Ivanowski (1892): tobacco mosaic disease caused by a filterable agent
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    Stanley (1935): first to crystallize TMV — proved viruses are simple (nucleic acid + protein only)
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    Smallpox declared eradicated by WHO in 1980 — only human disease ever fully eradicated
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    HIV identified in 1984, named HIV in 1986

Formulas

Lytic Cycle Output

~200 new phages produced in ~25 minutes from one infected bacterium

Capsomere Counts

Fixed number of capsomeres per virus — diagnostic for identification

Retroviral Reverse Transcription

RNA → DNA via reverse transcriptase, reversing the central dogma

T4 Phage Volume Ratio

Phage volume relative to its E. coli host cell