Acellular Life: Viruses, Bacteriophages & HIV | EverExams.com

Course Overview

What You’ll Learn

This comprehensive guide covers all aspects of acellular life, focusing on viruses and related entities. The course is structured to help you understand the fundamental concepts of virology through detailed explanations, visual aids, and interactive quizzes.

Course Content:

  • Discovery of Viruses – Historical milestones in virology
  • Characteristics of Viruses – Living vs non-living features
  • Classification of Viruses – Based on host, structure, and genome
  • Structure of Viruses – Core, capsid, and envelope
  • Bacteriophages – Structure and life cycles (lytic & lysogenic)
  • Viral Diseases – Common viral infections in humans
  • HIV and AIDS – Structure, transmission, and impact

Each section includes memory tips and tricks to help you retain important information effectively.

How to Use This Guide

  • Start with the “Overview” section to understand the course structure
  • Proceed through topics in order for logical learning flow
  • Pay attention to the “Tips & Tricks” boxes in each section
  • Test your knowledge with the 50-question quiz at the end
  • Use day/night mode and font controls for comfortable studying
  • Refer to the study guidelines for effective learning strategies

Discovery of Viruses

Historical Milestones

The word virus is derived from the Latin word ‘venom’ meaning ‘poison’. Historically, the term virus was associated with infectious diseases with unknown causes.

Key Discoveries:

  • 1884 – C. Chamberland (working with L. Pasteur) discovered that rabies causative agents could pass through porcelain filters (pore size: 100-1000 nm)
  • 1892 – D. Ivanowski demonstrated the filterable nature of Tobacco Mosaic Virus (TMV)
  • 1915, 1917 – Twort and D’Herelle independently discovered bacteriophages
  • 1935 – W. M. Stanley isolated, purified, and crystallized TMV
Scientist Year Contribution
E. Jenner 1796 Developed 1st vaccine against smallpox
C. Chamberland 1884 Filterable nature of rabies viruses
D. Ivanowski 1892 Filterable nature of Tobacco Mosaic Virus (TMV)
Twort and D’Herelle 1915, 1917 Discovered bacteriophages
W. M. Stanley 1935 Isolation, purification and crystallization of TMV

Modern Definition: Viruses are non-cellular infectious entities containing either RNA or DNA, normally encased in a proteinaceous coat, and reproduce only inside living cells.

Memory Tips

  • Virus = Venom: Remember the Latin origin to understand why viruses were originally associated with poison
  • Filterable Agents: Chamberland (1884) and Ivanowski (1892) both used filters to discover viruses
  • Chronological Order: Jenner (1796) → Chamberland (1884) → Ivanowski (1892) → Twort/D’Herelle (1915-17) → Stanley (1935)
  • TMV Milestones: Ivanowski discovered it, Stanley crystallized it – remember “I to S”

Characteristics of Viruses

Key Features

  • Extremely small (20-250 nm), visible only under electron microscope
  • Cannot grow on artificial culture media; require living cells
  • Reproduce by replication within host cells
  • Resistant to broad-range antibiotics (penicillin, streptomycin, etc.)
  • Size ranges from 20nm (parvoviruses) to 250nm (poxviruses)
  • 10 to 1000 times smaller than most bacteria
  • Exhibit both living and non-living characteristics

Living Characteristics:

  1. Exist in different varieties or strains
  2. Contain genetic material (DNA or RNA) that can mutate
  3. Reproduce using host cell’s metabolic machinery
  4. Obligate intracellular parasites (cause diseases)
  5. Destroyed by UV rays

Non-Living Characteristics:

  1. Lack cellular structure and metabolic activities
  2. No enzyme synthesis system
  3. Can be crystallized and stored
  4. Do not respire; behave as inert particles outside host

Related Infectious Particles:

Prions: Infectious particles made of proteins only; cause brain infections (mad cow disease, Creutzfeldt-Jakob disease).

Viroids: Minute RNA particles without protein coats; cause diseases in plants and animals.

Memory Tips

  • Size Range: “Parvo (20) is small, Pox (250) is big” – remember parvoviruses are smallest (20nm), poxviruses are largest (250nm)
  • Living vs Non-living: Viruses are like “zombies” – have some life traits (genetics, reproduction) but lack others (metabolism, respiration)
  • Prions & Viroids: Prions = PROtein INfectious particleS; Viroids = VIral-like without proteiN coat
  • Obligate Parasites: Remember they MUST be inside cells to reproduce – “No cell, no life”

Classification of Viruses

Based on Host Organisms

Host Virus Type Description
Bacteria Bacteriophages Attack bacteria; DNA genome (some RNA); polyhedral head with tail
Plants Plant Viruses 2000+ types; mostly RNA genome; rod-shaped capsid (e.g., TMV)
Animals Animal Viruses Parasites in animals; cause various diseases:
  • Aphthovirus → Foot and mouth disease
  • Rous sarcoma virus → Cancer
  • Poxvirus → Smallpox
  • Picornavirus → Polio, Hepatitis A
  • Paramyxovirus → Measles, Mumps

Based on Structure & Genome

By Capsid Structure:

Capsid Type Example
Helical capsid Tobacco mosaic virus
Polyhedral capsid Adenoviruses
Enveloped Influenza viruses
Complex capsid Bacteriophages

By Genome Type:

Genome Type Example
ds-DNA Poxvirus, Adenovirus, Herpes virus
ss-DNA Parvovirus (causes mild rashes)
ds-RNA Reovirus (causes diarrhea)
ss-RNA (serves as mRNA) Rubella virus
ss-RNA (template for mRNA) Orthomyxovirus (influenza)
ss-RNA (template for DNA) Retrovirus (HIV)

Classification Mnemonics

  • Bacterial viruses: Bacteriophages = “Bacteria eaters”
  • Plant viruses: Most have RNA – “Plants are R-N-Awesome”
  • DNA vs RNA viruses: Animal viruses can have both; plant viruses mostly RNA
  • Retrovirus trick: RNA → DNA (reverse of normal) = RETROgrade
  • Enveloped viruses: Influenza, HIV, Herpes – “IEH” = I Enveloped Highly

Structure of Viruses

Virion Components

A complete, mature, infectious viral particle is called a virion. It consists of two main parts:

1. Central Core

  • Contains viral genome (DNA or RNA, single or double-stranded)
  • Includes various proteins and enzymes
  • Core proteins: Enzymes that facilitate viral action in host cells
  • Example: Reverse transcriptase in retroviruses (converts RNA to DNA)

2. Outer Coat

  • Consists of capsid and sometimes an envelope
  • Capsid: Protective protein coat made of repeating subunits called capsomeres
  • Number of capsomeres is virus-specific (e.g., 162 in herpes, 252 in adenovirus)
  • Arrangements:
    • Icosahedral (20 triangles) – polyhedral/spherical
    • Helical (hollow coil) – rod-shaped
  • Envelope: Additional lipoprotein layer derived from host cell membrane
  • Non-enveloped viruses = “naked viruses”

Structure Memory Aids

  • Virion = Virus + Ion: Think of it as the complete, active “ion” of virus
  • Capsid = Capsule: Like a protective capsule around the genetic material
  • Capsomeres: “Meres” = parts/units (like polymers have monomers)
  • Envelope origin: Stolen from host cell membrane – viruses are “thieves”
  • Herpes vs Adenovirus capsomeres: Herpes (162) < Adenovirus (252) - "H < A" alphabetically and numerically

Bacteriophages

Structure & Life Cycles

Bacteriophage = “Bacteria eater” – viruses that infect bacteria.

Structure:

  • Tadpole-shaped with head, neck, and tail
  • Head contains DNA genome (some have RNA)
  • Neck/collar separates head and tail
  • Tail = hollow protein tube for nucleic acid passage
  • Contractile sheath surrounds tail
  • Base plate with tail fibers and pins for bacterial attachment
  • Lysozyme at tail bottom hydrolyzes host cell wall

Life Cycles:

Best studied: T phages (T2, T4) that infect E. coli

Infection Process (Common to both cycles):
  1. Attachment/Adsorption: Tail fibers attach to bacterial cell wall receptors
  2. Penetration: Sheath contracts, lysozyme digests cell wall, core tube penetrates
  3. Injection: Viral DNA injected into bacterium; protein coat remains outside
Lytic Cycle (Virulent Phage):
  1. Viral DNA takes control of host machinery
  2. Host synthesizes viral components (DNA & proteins)
  3. ~200 new phages formed in 25 minutes
  4. Bacterial cell bursts (lysis)
  5. New phages released to infect other bacteria
Lysogenic Cycle (Temperate Phage):
  1. Viral DNA incorporates into bacterial chromosome = prophage
  2. Bacterium lives/reproduces normally; viral DNA passed to daughter cells
  3. Induction: Prophage detaches from chromosome (spontaneous or induced by UV/chemicals)
  4. Lytic cycle begins after induction
Feature Lytic Cycle Lysogenic Cycle
Virus Type Lytic/virulent phage Lysogenic/temperate phage
Relationship Master-Slave Host-Guest
Effects Infectious cycle Non-infectious cycle
Viral DNA Takes control Integrated
Bacterial DNA Destroyed Remains intact

Phage Memory Tricks

  • Lytic vs Lysogenic: Lytic = Lysis (bursting); Lysogenic = Latent (hidden)
  • Cycle outcomes: Lytic kills immediately; Lysogenic waits for induction
  • Prophage: PROvirus in phage = integrated viral DNA
  • T-phages: T2 and T4 most studied – “T for Type” and “2+4=6” → E. coli has 6 letters
  • Induction triggers: UV, chemicals, stress – “Anything that stresses bacteria can induce the prophage”

Viral Diseases

Common Human Viral Infections

Disease Virus Transmission Symptoms Prevention/Treatment
Oral Herpes HSV-1 (DNA, enveloped) Oral secretions, contact with sores Blisters in mouth/lips/skin Antiviral drugs, avoid contact
Measles Paramyxovirus (RNA enveloped) Coughing, sneezing Fever, runny nose, red eyes, rash Vaccination
Mumps Paramyxovirus (RNA enveloped) Coughing, sneezing Fever, muscle pain, swollen parotid glands Vaccination
Polio Polio virus (RNA non-enveloped) Oro-fecal route Paralysis of limbs Vaccination, physiotherapy
Hepatitis A HAV (RNA non-enveloped) Oro-fecal route Fever, loss of appetite, nausea, jaundice Vaccination, good hygiene
Hepatitis B HBV (DNA enveloped) Blood, sexual contact, mother to newborn Vomiting, jaundice, dark urine, abdominal pain Vaccination, interferon, blood screening
Hepatitis C HCV (RNA enveloped) Infected blood/products Dark urine, abdominal pain, jaundice Interferon & ribavirin, blood screening
Hepatitis D Viroid Blood or serum Same as Hep B but more severe Same as Hep B
Hepatitis E HEV (RNA non-enveloped) Oro-fecal route Nausea, vomiting, diarrhea, jaundice Good hygiene, no specific treatment
AIDS HIV (RNA enveloped) Blood, sexual contact Opportunistic infections, swollen lymph nodes ART (Antiretroviral therapy)

Disease Mnemonics

  • Hepatitis types: A & E = Alimentary (oral-fecal); B, C, D = Blood/body fluids
  • Measles & Mumps: Both caused by Paramyxovirus – “M&M” both from Paramyxo
  • Polio virus: Smallest known virus – “Polio is a small disease with big impact”
  • Herpes virus: DNA, enveloped – “Herpes has DNA and an envelope to hide in”
  • Vaccine availability: Hep A&B have vaccines; Hep C doesn’t – “A&B are preventable, C is chronic”

HIV and AIDS

Human Immunodeficiency Virus

Retroviruses are associated with tumor production in animals. HIV (causes AIDS) is a human retrovirus.

HIV Structure:

  • Spherical, 100nm diameter
  • Lipoprotein envelope with glycoprotein spikes
  • Spikes: outer gp120 (attachment) + inner gp41 (fusion)
  • Matrix protein shell beneath envelope
  • Conical-shaped capsid
  • Core contains:
    • Two single strands of RNA
    • Reverse transcriptase (RNA → DNA)
    • Integrase (integrates viral DNA into host DNA)
    • Protease (cleaves proteins)

Host Specificity & Transmission:

  • Primary host: Helper T-lymphocytes (CD4 cells)
  • Also affects macrophages and brain cells
  • Transmission:
    • Sexual contact (body secretions enter through micro-tears)
    • Contact with infected blood
    • Mother to child (pregnancy, birth, breastfeeding)
    • Contaminated needles/surgical instruments

HIV Life Cycle:

  1. Attachment: gp120 binds to CD4 receptors on T-cells
  2. Fusion & Entry: Viral envelope fuses with cell membrane
  3. Uncoating: Virus sheds coat, releases RNA into cytoplasm
  4. Reverse Transcription: RNA → single-stranded DNA → double-stranded DNA
  5. Integration: Viral DNA integrates into host DNA (becomes provirus)
  6. Transcription & Translation: Host machinery produces viral components
  7. Assembly & Release: New virions assembled and bud off from host cell

AIDS Stages & Symptoms:

  1. Asymptomatic Carrier (9+ months): Fever, chills, swollen lymph glands, rash
  2. AIDS Related Complex (ARC): Persistent swollen lymph nodes, night sweats, persistent cough/diarrhea, memory loss
  3. Full Blown AIDS: Severe weight loss, opportunistic infections (Kaposi’s sarcoma, fungal/viral infections)

Treatment & Prevention:

  • ART (Antiretroviral Therapy): Controls virus, prolongs life, reduces transmission risk
  • Prevention:
    • Avoid sharing needles/syringes
    • Screen blood before transfusion
    • Practice safe sex
    • Proper sterilization of surgical instruments
    • HIV+ mothers should not breastfeed
  • NOT transmitted by: Mosquitoes, casual contact (handshakes, hugging), toilets, door knobs

HIV Memory Aids

  • HIV structure: gp120 = “1-2-0 attach to CD4 cell”; gp41 = “4-1 fuse to cell”
  • Enzymes: Reverse transcriptase (RNA→DNA), Integrase (integrates), Protease (protein processor)
  • Transmission: “Blood, Sex, Birth” – three main routes
  • AIDS progression: Asymptomatic → ARC → Full-blown (A→B→C stages)
  • ART: AntiRetroviral Therapy = “ART against HIV is an art”

Virology Quiz (50 MCQs)

Question 1 of 50

Test your knowledge of acellular life concepts. Select the correct answer for each question.

Study Guidelines for Students

Effective Learning Strategies

1. Study Schedule & Time Management

  • Consistent Study: Dedicate 1-2 hours daily rather than cramming before exams
  • Pomodoro Technique: Study for 25 minutes, then take a 5-minute break
  • Weekly Review: Spend 30 minutes each week reviewing previous topics
  • Peak Hours: Identify when you’re most alert (morning/evening) for difficult concepts

2. Active Learning Techniques

  • Teach Others: Explain concepts to friends or family to reinforce understanding
  • Create Flashcards: Make digital or physical flashcards for key terms and definitions
  • Draw Diagrams: Sketch virus structures, bacteriophage cycles, and HIV replication
  • Use Mnemonics: Create memory aids like those provided in each section

3. Memorization Strategies

  • Chunking: Group related information (e.g., all DNA viruses together)
  • Association: Link new information to something you already know
  • Visualization: Create mental images of viral structures and processes
  • Repetition with Spacing: Review material at increasing intervals

4. Utilizing This Resource

  • Complete All Sections: Don’t skip any topic – they build upon each other
  • Use Day/Night Mode: Switch to night mode for evening study to reduce eye strain
  • Adjust Font Size: Increase font if you have visual preferences
  • Take the Quiz Seriously: Use it to identify weak areas for further review
  • Review Wrong Answers: Understand why you got questions wrong

5. Exam Preparation Tips

  • Practice MCQs: Regular practice with different question formats
  • Mock Tests: Simulate exam conditions with timed tests
  • Focus on Weak Areas: Spend extra time on topics you find difficult
  • Health & Wellness: Ensure adequate sleep, nutrition, and exercise during study periods

Pro Tip

Combine multiple senses for better retention: Read aloud (auditory), write notes (kinesthetic), and visualize diagrams (visual). This multisensory approach creates stronger memory pathways.