Virology, Subviral Agents, and Viral Pathogenesis

Detailed exploration of the biology of viruses, bacteriophages, animal and plant viruses, prions, viroids, satellites, and associated clinical pathogens.

1. Introduction to Virology and General Properties

Definitions and Nature of Viruses

Viruses are simple, non-cellular, sub-microscopic entities consisting of one or more molecules of either DNA or RNA enclosed in a protective protein coat. They are obligate intracellular parasites that lack any independent metabolic machinery, meaning they can reproduce only within living host cells by subverting the host’s transcriptional and translational systems.

• Virion: A fully assembled, structurally complete, and infectious viral particle.
• Size Range: Viruses are significantly smaller than prokaryotic cells, typically ranging in size from 0.01 to 0.4 micrometres (10 to 400 nm).
• Host Range: Each viral species exhibits a highly restricted host range, meaning it can infect and replicate in only a small, specific group of closely related host species.

Basic Virion Morphology

The structural diversity of virions is extensive, varying widely in size, shape, and chemical composition. However, all viruses share a fundamental nucleocapsid core:

                        [ Simple Virion Anatomy ]

            Naked (Non-Enveloped) Virus        Enveloped Virus

                ┌───────────────┐              ┌───────────────┐
                │  Protein      │              │ Lipid Bilayer │ Envelope
                │  Capsid       │              │ Envelope      │ (Host-derived)
                │  ┌─────────┐  │              │  ┌─────────┐  │
                │  │ Nucleic │  │              │  │ Capsid  │  │
                │  │ Acid    │  │              │  │  ┌───┐  │  │
                │  └─────────┘  │              │  │  │DNA│  │  │ peplomer
                │               │              │  │  └───┘  │  │   /  spike
                └───────────────┘              │  └─────────┘  │  /
                  Nucleocapsid                 └───────────────┘ ───*
  

• Capsid: The protein shell surrounding the viral genome. It is composed of individual structural protein subunits called capsomeres. The primary functions of the capsid are:
  • To shield the viral nucleic acid from physical damage and enzymatic nucleases.
  • To facilitate attachment of the virion to specific receptors exposed on the surface of prospective host cells during infection.
• Nucleocapsid: The complete complex of the viral nucleic acid genome packaged within its protein capsid.
• Enveloped Viruses: Virions surrounded by an outer membranous lipid bilayer that lies external to the nucleocapsid. This envelope is acquired from modified host cell membranes (plasma membrane, nuclear membrane, or endoplasmic reticulum) during the viral budding process. Consequently, the lipid composition of the viral envelope closely mirrors that of the host cell membrane from which it budded.
• Spikes (Peplomers): Glycoprotein structures encoded by the viral genome that are embedded in and protrude from the envelope. These play a critical role in host cell attachment and entry.
• Naked (Non-Enveloped) Viruses: Virions that lack an outer lipid envelope, leaving the nucleocapsid directly exposed to the environment.

2. Genomic Diversity and Architecture

Viral genomes are highly compact, with the largest known viral genome being that of bacteriophage G, measuring 670 kb. Viral genomes exhibit extreme structural diversity, which is summarised in the table below:

Genome Organisation and Packaging

Based on the physical division and packaging of their genomic material, viruses are categorised into three groups:

• Monopartite Viruses: The genome consists of a single, non-segmented nucleic acid molecule contained within a single viral particle. Most viruses are monopartite. All double-stranded DNA (dsDNA) viruses are exclusively monopartite.
• Segmented Viruses: The genome is physically divided into two or more distinct nucleic acid segments (sometimes up to 10 to 12 segments) that are all packaged together within the same individual viral particle (e.g., Influenza virus).
• Multipartite (Multicomponent) Viruses: The genome is divided into two or more distinct nucleic acid segments, but each segment is packaged into a separate virus particle. To establish a successful infection, a host cell must be simultaneously infected by all the different particles carrying the complete genomic complement. ssDNA and (+)ssRNA viruses can be multipartite, but there are no known segmented ssDNA viruses.

Positive-Sense, Negative-Sense, and Ambisense Genomes

• Positive-Sense (+) RNA: Single-stranded viral RNA that can function directly as mRNA in the host cell cytoplasm. It can be immediately translated into viral proteins by host ribosomes.
• Negative-Sense (-) RNA: Single-stranded viral RNA that is complementary to the mRNA sequence. It cannot be directly translated; instead, it must first be transcribed into a positive-sense complementary strand by a virion-associated RNA-dependent RNA polymerase (RdRp).
• Ambisense Genomes: Single-stranded viral nucleic acid molecules where some regions are positive-sense and other regions are negative-sense on the same strand.

3. Symmetry, Taxonomy, and Classifications

Viral Capsid Symmetry

The arrangement of capsomeres in the capsid determines the physical shape and symmetry of the virion:

                              [ Capsid Symmetry ]

              Helical Symmetry (e.g., TMV)       Icosahedral Symmetry

                   ┌───┐ ┌───┐ ┌───┐                /                 │   │ │   │ │   │               /                 ┌─┴─┐ └─┬─┘ └─┬─┘ └─┐            /____               │   ├───┼───┐ │     │           /\    /               └─┬─┘   │   │ └─┬───┘          /  \  /                   └───┘ └───┘   └───┘         /____\/____                 Capsomeres spiral           20 Triangular Faces
                   around central RNA          12 Vertices
  

• Helical Symmetry: Capsomeres are arranged in a helical pattern wrapped around a central, spiral filament of nucleic acid, forming a rigid or flexible rod-shaped nucleocapsid. A classic example is the Tobacco Mosaic Virus (TMV).
• Icosahedral Symmetry: Capsomeres are arranged to form a regular polyhedron with 20 equilateral triangular faces and 12 vertices, resulting in a roughly spherical shape under electron microscopy.
• Complex Symmetry: Virions that possess structural elements that are neither purely helical nor purely icosahedral. An example is the T4 bacteriophage, which possesses an elongated icosahedral head connected to a complex helical tail assembly. Another example is the brick-shaped Poxvirus.

Viral Classification Systems

Viruses are grouped taxonomically based on several criteria:

• Host Range: Animal viruses, plant viruses, bacterial viruses (bacteriophages), insect viruses, etc.
• Virion Morphology and Composition: Size, shape, capsid symmetry, and the presence or absence of a lipid envelope.
• Replication Strategy: The type of nucleic acid, strandedness, and the molecular mechanism of mRNA synthesis.

4. Bacteriophage Biology

Bacteriophages (or phages) are viruses that specifically infect bacterial cells. They were discovered independently by F. Twort in England (1915) and F. d’Herelle in France (1917), who coined the term “bacteriophage”, meaning “eaters of bacteria”.

Anatomy of T4 Bacteriophage

Bacteriophage T4 is a highly complex, virulent dsDNA phage that infects Escherichia coli. Its structure is a premier example of complex binal symmetry:

                               [ T4 Phage Anatomy ]

                                     /                                     /   \   Elongated Icosahedral Head
                                   │     │  (Contains 172 kb linear dsDNA)
                                    \   /
                                     \ /
                                      │     Collar
                                      ║
                                      ║     Contractile Sheath
                                      ║     (Surrounds rigid tail core)
                                      ║
                                     /                                     ┌───┐   Hexagonal Base Plate
                                    │   │
                                   / \ / \  Tail Pins
                                  /   V                                  /                                       /           \  Kinked Tail Fibres (6 total)
  

• Capsid Head: An elongated icosahedral head enclosing a 172 kb linear double-stranded DNA genome.
• Collar: A structural ring connecting the head to the tail assembly.
• Tail Tube & Contractile Sheath: A rigid central hollow core through which the DNA is injected, surrounded by a helical, contractile protein sheath.
• Base Plate: A hexagonal plate at the base of the tail that coordinates host cell recognition and sheath contraction.
• Tail Pins and Fibres: Six tail pins protrude from the base plate, and six kinked tail fibres extend outwards. The tail fibres are responsible for binding to specific lipopolysaccharide or protein receptors on the host outer membrane.

Classification of Phage Genomes

Phages carry diverse genetic materials:

• Single-stranded DNA (ssDNA): $\phi X174$, $fd$
• Double-stranded DNA (dsDNA): T-phages ($T1$ to $T7$), $\lambda$-phage
• Single-stranded RNA (ssRNA, plus-sense): $MS2$
• Double-stranded RNA (dsRNA): $\phi 6$

Note on T-phages: These are historically classified into T-even ($T2, T4, T6$) and T-odd ($T1, T3, T5, T7$) phages, which differ significantly in their tail structure and overall chemical composition.

5. Bacteriophage Life Cycles and Growth Kinetics

Bacteriophages propagate via two distinct pathways: the lytic cycle and the lysogenic cycle.

                          [ Lytic vs. Lysogenic Pathways ]

                                   [ Phage Adsorption ]
                                           │
                                           ▼
                                   [ DNA Injection ]
                                           │
                       ┌───────────────────┴───────────────────┐
                       ▼ (Lytic Pathway)                       ▼ (Lysogenic Pathway)
              [ Synthesis of Phage ]                  [ Integration into Host ]
                Proteins & DNA                          Chromosomal DNA
                       │                                       │
                       ▼                                       ▼
              [ Phage Assembly ]                      [ Prophage Replication ]
                       │                                (As host replicates)
                       ▼                                       │
              [ Host Cell Lysis ]                              ▼ (Induction via UV)
              (Release of virions)                    [ Excision of Prophage ]
                                                      (Enters lytic pathway)
  

The Lytic Pathway (Virulent Phages)

The lytic cycle culminates in the destruction (lysis) of the host cell and the release of numerous infectious progeny virions. It consists of five key steps:

1. Attachment (Adsorption): The tail fibres interact with specific cell wall receptors.
2. Penetration (Injection): The tail sheath contracts, driving the rigid inner tail tube through the outer membrane and peptidoglycan layer. The phage DNA is injected into the cytoplasm, while the empty protein capsid remains outside the cell.
3. Synthesis: The phage genome subverts the host’s transcription and translation machinery, initiating the synthesis of early proteins (enzymes to replicate phage DNA) and late proteins (structural capsid components).
4. Assembly: The viral head, tail, and tail fibres assemble spontaneously and are loaded with the replicated phage DNA.
5. Release (Lysis): Phage-encoded endolysins and holins degrade the bacterial cell wall from within, causing osmotic lysis.

Burst Size: The average number of infectious progeny phage particles released per infected host cell. It is a characteristic value for each virus-host system, typically ranging from 50 to several hundred.

The Lysogenic Pathway (Temperate Phages)

Temperate phages (such as bacteriophage $\lambda$) can choose an alternative pathway where the viral DNA, once injected, is integrated directly into the host’s circular chromosome instead of initiating immediate replication.

• Prophage: The latent, integrated state of the phage genome within the host chromosome.
• Lysogen: The bacterial host cell harboring a prophage.
• Lysogenic (or Phage) Conversion: The phenomenon where the presence of a prophage alters the phenotypic characteristics of the host bacterium, often by expressing extra genes carried on the phage genome (e.g., genes encoding cholera, diphtheria, or botulinum toxins).
• Plasmid prophages: While most temperate phages integrate into the chromosome, some, such as bacteriophage P1, replicate extra-chromosomally as low-copy-number plasmids.

Molecular Integration of Phage Lambda

The circularisation and integration of the linear dsDNA of bacteriophage $\lambda$ into the E. coli chromosome is a highly site-specific recombination event:

                        [ Lambda Site-Specific Recombination ]

                   λ Phage DNA                  E. coli Chromosome
                (attP site: P-O-P')             (attB site: B-O-B')
                        \                               /
                         \                             /
                          \─── Site-Specific Rec. ────/
                                   At O Core
                                       │
                                       ▼
                                  [ Prophage ]
                          (attL: B-O-P' ─── attR: P-O-B')
  

• Attachment Sites: Integration requires specific, non-homologous attachment sequences on both genomes:
  • attB (Bacterial site): Positioned between the gal and bio operons on the E. coli chromosome, represented as B-O-B’.
  • attP (Phage site): Positioned on the circularised $\lambda$ genome, represented as P-O-P’.
• Core Region (O): Both attachment sites share an identical, 15 bp core sequence (O) where the physical strand cleavage, exchange, and ligation take place.
• Recombination Products: The integration reaction, catalysed by the phage-encoded integrase (Int) and host-encoded integration host factor (IHF), yields a linearised prophage flanked by two new hybrid junction sites:
  • attL (Left junction): B-O-P’
  • attR (Right junction): P-O-B’

Lysogeny Maintenance and UV Induction

Lysogeny is actively maintained by the CI repressor protein (encoded by the phage cI gene). The CI repressor binds to the left and right operators ($O_L$ and $O_R$), preventing the transcription of the lytic promoter genes (PL and PR).

                              [ UV Induction Pathway ]

                    UV Light Damage to Host DNA
                                 │
                                 ▼
                    Activation of RecA Protein
                                 │
                                 ▼
               RecA acts as a specific Co-Protease
                                 │
                                 ▼
                   Autolytic Cleavage of CI Repressor
                                 │
                                 ▼
                Promoters PL and PR are derepressed
                                 │
                                 ▼
                  Excision of Prophage & Lytic Cycle
  

• Induction: The transition from the lysogenic to the lytic cycle, triggered by environmental stressors such as UV radiation.
• RecA Co-Protease Action: UV-induced DNA damage generates single-stranded DNA gaps, which recruit and activate the host’s RecA protein. Once active, RecA acts as a highly specific co-protease that stimulates the autolytic cleavage of the CI repressor dimer.
• Excision: Cleavage of the CI repressor derepresses the PL and PR promoters, leading to the immediate synthesis of the phage excisionase (Xis) and integrase (Int). These proteins catalyse site-specific recombination between attL and attR, excising the prophage as a circular molecule that enters the lytic replication cascade.
• Zygotic Induction: This occurs during the conjugation of an Hfr donor cell containing a $\lambda$ prophage with a non-lysogenic F- recipient cell. When the linear prophage DNA is transferred into the recipient cytoplasm, it encounters an environment completely devoid of the CI repressor protein. Lacking repressor molecules to bind its operators, the transferred prophage immediately enters the lytic cycle, replicating and lysing the newly infected recipient cell.

6. Mathematical Kinetics of Viral Growth and Assays

The One-Step Growth Curve

The kinetics of a single round of viral multiplication inside a population of host cells can be plotted as a one-step growth curve, which is divided into three distinct biological intervals:

                         [ One-Step Growth Curve ]

          Log of
          Infectious
          Units
            ▲
            │                          Latent Period
            │              ┌──────────────────────────────────────┐
            │              │                                      │
            │              │          Eclipse Period              │
            │              │    ┌────────────────────────┐        │
            │              │    │                        │        │
            │______________│____│                        │________│\
            │\             │    │                        │        │ \ Progeny
            │ \            │    │                        │        │  \ Released
            │  \           │    │                        │        │             │   \          │    │                        │        │              │    \         │    │                        │        │               │     \________│____│________________________│________│______│_
            └───────────────────────────────────────────────────────────────► Time
               Adsorption  No intracellular virions     No extracellular
                           detected                     virions detected
  

• Eclipse Period: The interval immediately following infection during which no infectious virions (either intracellular or extracellular) can be detected. This corresponds to the phase of uncoating, genome transcription, and translation of viral structural components.
• Latent Period: The time interval from the initial addition of the virus to the cells until the first release of extracellular progeny virions. During this period, no extracellular infectivity is detected because any assembled virions remain trapped inside the intact host cells.
• Maturation Period: The phase within the latent period during which newly synthesised viral genomes and capsids are actively assembled into infectious intracellular virions.

7. Mathematical Problems and Solutions in Virology

Plaque Assays and Titer Calculations

The plaque assay is the standard indirect method used to determine the concentration of infectious viral particles (the titer) in a sample. A plaque is a visible, circular clear zone formed in a dense lawn of host bacteria on an agar plate, representing an area of localized cell lysis initiated by a single infectious virion, or plaque-forming unit (PFU).

To calculate the titer of an undiluted viral stock, the following mathematical formula is applied:

Standard Textbook Example:
Suppose 25 plaques are observed on a plate inoculated with $1.0 \text{ ml}$ of a $10^{-4}$ dilution.

Worked Problem 1: Multi-Step Serial Dilution Titer Calculation

Problem Statement:
A phage stock of unknown concentration is serially diluted through a series of four tubes as follows:

• Tube 1: $0.1 \text{ ml}$ of the original stock is added to $9.9 \text{ ml}$ of dilution buffer.
• Tube 2: $0.1 \text{ ml}$ of Tube 1 is transferred into $9.9 \text{ ml}$ of buffer.
• Tube 3: $0.1 \text{ ml}$ of Tube 2 is transferred into $9.9 \text{ ml}$ of buffer.
• Tube 4: $1.0 \text{ ml}$ of Tube 3 is transferred into $9.0 \text{ ml}$ of buffer.

Finally, $0.1 \text{ ml}$ from Tube 4 is plated onto a lawn of susceptible E. coli cells. After overnight incubation, exactly 180 plaques are counted on the plate. Calculate the titer (number of phage particles per ml) of the original stock suspension.

Step-by-Step Mathematical Solution:
Calculate the dilution factor for each individual step:

• Step 1 (Tube 1):
  $$ \text{Dilution}_1 = \frac{\text{Volume Transferred}}{\text{Total Volume}} = \frac{0.1 \text{ ml}}{0.1 \text{ ml} + 9.9 \text{ ml}} = \frac{0.1}{10.0} = 10^{-2} \text{ (1 in 100)} $$
• Step 2 (Tube 2):
  $$ \text{Dilution}_2 = \frac{0.1 \text{ ml}}{0.1 \text{ ml} + 9.9 \text{ ml}} = 10^{-2} $$
• Step 3 (Tube 3):
  $$ \text{Dilution}_3 = \frac{0.1 \text{ ml}}{0.1 \text{ ml} + 9.9 \text{ ml}} = 10^{-2} $$
• Step 4 (Tube 4):
  $$ \text{Dilution}_4 = \frac{1.0 \text{ ml}}{1.0 \text{ ml} + 9.0 \text{ ml}} = \frac{1.0}{10.0} = 10^{-1} \text{ (1 in 10)} $$

Calculate the cumulative dilution of Tube 4:

Identify the inverse of the dilution (Dilution Factor):

Apply the titer formula using the plated volume ($0.1 \text{ ml}$) and plaque count (180):

Final Answer: The titer of the original stock suspension is $1.8 \times 10^{10} \text{ phage/ml}$.

Multiplicity of Infection (MOI) and Poisson Distribution

The Multiplicity of Infection (MOI) is the ratio of the total number of infectious viral particles (bacteriophages) added to a known total number of host cells during infection:

Because the binding of individual virions to host cells is a random process, some host cells will receive no viral particles, some will receive exactly one, and others will be superinfected with multiple viral particles. The distribution of viruses per cell is described by the Poisson Distribution:

Where:

• $P(x)$ is the probability that a host cell receives exactly $x$ viral particles.
• $\text{MOI}$ is the average number of phages added per host cell.
• $e$ is the base of the natural logarithm ($\approx 2.718$).

Special Cases of the Poisson Equation:

• Probability of a cell remaining uninfected ($x = 0$):
  $$ P(0) = \frac{e^{-\text{MOI}} \cdot (\text{MOI})^0}{0!} = e^{-\text{MOI}} $$
• Probability of a cell receiving at least one viral particle (successful infection):
  $$ P(x \ge 1) = 1 – P(0) = 1 – e^{-\text{MOI}} $$
• Example at High MOI ($\text{MOI} = 5$):
  $$ P(0) = e^{-5} \approx 0.0067 \text{ (0.67% of cells remain uninfected)} $$
• Example at Low MOI ($\text{MOI} = 1$):
  $$ P(0) = e^{-1} \approx 0.3679 \text{ (36.8% of cells remain uninfected)} $$
  $$ P(x \ge 1) = 1 – 0.3679 = 0.6321 \text{ (63.2% of cells are successfully infected)} $$

Worked Problem 2: MOI Calculation and Cellular Targeting

Problem Statement:
A laboratory investigator mixes a $0.1 \text{ ml}$ aliquot of a purified bacteriophage stock suspension having a concentration of $4 \times 10^9 \text{ phage/ml}$ with a $0.5 \text{ ml}$ culture of Escherichia coli cells at a density of $2 \times 10^8 \text{ cells/ml}$.

1. Calculate the exact Multiplicity of Infection (MOI) of this mixture.
2. Determine the percentage of E. coli cells that will remain completely uninfected, according to the Poisson distribution.

Step-by-Step Mathematical Solution:
Calculate the total number of bacteriophages added:

Calculate the total number of host bacterial cells present:

Calculate the Multiplicity of Infection (MOI):

Calculate the probability/percentage of uninfected cells ($P_0$):

Since $e^{-4} \approx 0.01831$:

Final Answer: The Multiplicity of Infection is $4 \text{ phage/cell}$, and exactly $1.83\%$ of the bacterial host cells will remain uninfected.

8. Genetic Analysis of Bacteriophages

Because bacteriophages possess exceptionally short generation times, produce massive numbers of progeny, and can be easily propagated and selected on agar plates, they played a foundational role in the birth of molecular genetics.

Seymour Benzer’s Fine-Structure Mapping of the rII Locus

In the 1950s, Seymour Benzer used the rII locus of bacteriophage T4 to perform the first fine-structure (intragenic) genetic mapping, proving that a gene is a linear sequence of nucleotides rather than an indivisible point on a chromosome.

Phenotypes of Wild-Type vs. rII Mutants:

• Wild-Type T4 ($r^+$): Exhibits lysis inhibition (a physiological delay in cell lysis that occurs when a cell is superinfected with additional phages). On agar lawns, wild-type phages produce small plaques with fuzzy, diffuse edges.
• rII Mutants (Rapid Lysis mutants): Defective in lysis inhibition. They lyse infected cells rapidly, producing large, sharp-edged, clear plaques (known as $r$ plaques).

Host Permissiveness and Conditional Lethality:
Benzer discovered that $rII$ mutants are conditional lethal mutants whose ability to form plaques depends entirely on the host bacterial strain:

The non-permissiveness of E. coli K-12($\lambda$) is due to the presence of the integrated lysogenic $\lambda$ prophage, which blocks the multiplication of T4 phages carrying a defective rII locus.

The Complementation Test (The Cis-Trans Test)

To determine whether two independent rII mutant strains carry mutations in the same gene or in different genes, Benzer developed the complementation test using the non-permissive host E. coli K-12($\lambda$):

                          [ Complementation Test ]

          Scenario A: Mutations in Different Genes      Scenario B: Mutations in Same Gene
                     (rIIA and rIIB)                              (Both in rIIA)

              Phage 1: [ rIIA- ] [ rIIB+ ]                 Phage 1: [ rIIA- ] [ rIIB+ ]
                         │         │                                  │         │
              Phage 2: [ rIIA+ ] [ rIIB- ]                 Phage 2: [ rIIA- ] [ rIIB+ ]
                         │         │                                  │         │
                         ▼         ▼                                  ▼         ▼
               Functional proteins made:                    No functional rIIA made:
                Both A and B present                         No complementation
                         │                                            │
                         ▼                                            ▼
                 Lysis and Plaques                               No Plaques
  

• Double-Infection (Mixed Infection): Non-permissive E. coli K-12($\lambda$) cells are simultaneously infected with equal numbers of mutant phage strain 1 and mutant phage strain 2.
• Complementation (Different Genes): If the mutations reside in different genes (e.g., phage 1 is mutant in rIIA but wild-type for rIIB; phage 2 is wild-type for rIIA but mutant in rIIB), each phage genome provides the functional gene product that the other lacks. Both functional rIIA and rIIB proteins are produced in the shared cytoplasm. Complementation occurs, the phages multiply, and the host cell lyses, forming a visible plaque.
• No Complementation (Same Gene): If both phages carry mutations within the same gene (e.g., both are mutated at different positions within rIIA), neither genome can produce a functional rIIA protein. No complementation occurs, the phages fail to replicate, and no plaques are formed.
• The Cistron: Based on these results, Benzer defined a cistron as the smallest functional genetic unit of complementation (equivalent to the modern term “gene”). If two mutations fail to complement each other in trans, they belong to the same cistron. Benzer resolved the T4 rII locus into two adjacent cistrons: rIIA and rIIB.

Intragenic Mapping (Fine-Structure Recombination)

To map the physical distance between different mutation sites within the same gene (e.g., within rIIA), Benzer utilised intragenic recombination:

                         [ Intragenic Recombination ]

                       Phage 1: ───[ Mutation 1 ]─────────[ Wild-Type ]───
                                        \             /
                                         \  Crossover/
                                          \         /
                       Phage 2: ───[ Wild-Type ]──────────[ Mutation 2 ]───
                                          │         │
                                          ▼         ▼
                    Recombinant 1 (Wild-Type): ───[ Wild-Type ]──────────[ Wild-Type ]───
                    Recombinant 2 (Double Mut):───[ Mutation 1 ]─────────[ Mutation 2 ]───
  

• Permissive Infection: Permissive E. coli B cells are mixed-infected with two different, non-complementing mutant strains that both carry mutations in rIIA.
• Crossover Event: As the genomes replicate, homologous recombination can occur in the short interval between the two mutation sites. A single crossover event generates two recombinant products:
  • A wild-type recombinant ($r^+$) carrying wild-type sequences at both positions.
  • A double-mutant recombinant carrying both mutations on a single strand.
• Differential Plating Selection: Because recombination in a tiny intragenic interval is rare, wild-type recombinants represent a very small fraction of the total progeny. To count them, the progeny phages are plated on two different hosts:
  • Plating on E. coli B: Measures the total number of progeny phages (since both parental mutants and all recombinants can grow).
  • Plating on E. coli K-12($\lambda$): Selects exclusively for wild-type recombinants ($r^+$) (since parental mutants and double mutants cannot grow on this non-permissive host).
• Recombination Frequency Formula: Because a single crossover produces equal numbers of wild-type and double-mutant recombinants, but only the wild-type recombinants are detected on K-12($\lambda$), the actual total number of recombinant progeny is exactly twice the number of plaques counted on K-12($\lambda$):
  $$ \text{Recombination Frequency (RF)} = \frac{2 \times \text{Number of Plaques on } E. coli \text{ K-12}(\lambda)}{\text{Total Number of Plaques on } E. coli \text{ B}} \times 100 $$

9. Animal Viruses: Life Cycles and Replication Strategies

The Baltimore Classification System

The Baltimore System classifies animal viruses into seven distinct classes based on their genome type (DNA or RNA, single- or double-stranded) and the specific pathway they use to synthesise positive-sense mRNA:

                              [ The Baltimore System ]

               Class I (dsDNA) ──────────────────────────┐
                                                         │
               Class II (ssDNA) ──► dsDNA Intermediate ──┼─► (+) mRNA
                                                         │
               Class III (dsRNA) ────────────────────────┤
                                                         │
               Class IV ((+)ssRNA) ──────────────────────┤
                                                         │
               Class V ((-)ssRNA) ───────────────────────┤
                                                         │
               Class VI ((+)ssRNA) ──► ssDNA ──► dsDNA ──┤
                                                         │
               Class VII (dsDNA-RT) ──► RNA Intermed. ───┘
  

• Class I: Double-stranded DNA (dsDNA) genomes (e.g., Adenovirus, Herpesvirus). The viral DNA enters the host nucleus, where cellular RNA polymerase II transcribes it into viral mRNA.
  • Class Ib Exception: Poxviruses are dsDNA viruses but replicate exclusively in the host cytoplasm, relying on their own packaged viral enzymes for transcription and DNA replication.
• Class II: Single-stranded DNA (ssDNA) genomes (e.g., Parvovirus). The ssDNA must first be converted into a dsDNA intermediate in the nucleus by host DNA polymerases before mRNA transcription can proceed.
• Class III: Double-stranded RNA (dsRNA) genomes (e.g., Reovirus). These segmented genomes are transcribed in the cytoplasm into (+)ssRNA transcripts by a virion-associated RNA-dependent RNA polymerase (RdRp).
• Class IV: Positive-sense single-stranded RNA ((+)ssRNA) genomes (e.g., Picornaviruses, Coronaviruses, Togaviruses). The genomic RNA can function directly as mRNA in the cytoplasm and is immediately translated by host ribosomes upon uncoating.
• Class V: Negative-sense single-stranded RNA ((-)ssRNA) genomes (e.g., Rhabdovirus, Paramyxovirus, Orthomyxovirus). The genome is complementary to mRNA and must be transcribed into positive-sense mRNA by a packaged viral RNA-dependent RNA polymerase (RdRp) carried inside the virion.
• Class VI: Positive-sense single-stranded RNA with a DNA intermediate (e.g., Retroviruses like HIV). The (+)ssRNA genome is reverse-transcribed into single-stranded DNA and then double-stranded DNA by a packaged viral reverse transcriptase. This dsDNA integrates into the host genome as a provirus before host RNA polymerase II transcribes it into mRNA.
• Class VII: Double-stranded DNA with an RNA intermediate (e.g., Hepadnaviruses like Hepatitis B). The partially dsDNA genome is repaired in the nucleus to form a fully closed circular DNA, which is transcribed into a pregenomic RNA intermediate. This pregenomic RNA is then reverse-transcribed back into dsDNA inside the newly assembling capsids by a viral reverse transcriptase.

The Animal Virus Productive Life Cycle

The replication cascade of animal viruses consists of five highly coordinated phases:

                        [ Animal Virus Entry Pathways ]

           Direct Membrane Fusion                Receptor-Mediated Endocytosis
             (Enveloped Viruses)                       (Naked or Enveloped)

                Envelope   Spike                           Capsid   Receptor
                  │         │                                │         │
                  ▼         ▼                                ▼         ▼
              ╔═══*─────────*═══╗                        ╔═══*─────────*═══╗
              ║  Host Membrane  ║                        ║  Host Membrane  ║
              ╚════════╦════════╝                        ╚════════╦════════╝
                       │                                          │
                       ▼                                          ▼
              [ Envelope fuses ]                         [ Invagination & ]
              [ with membrane  ]                         [  Endosome Form ]
                       │                                          │
                       ▼                                          ▼
              [ Nucleocapsid   ]                         [ Uncoating via  ]
              [ enters cytosol ]                         [ Acidification  ]
  

1. Adsorption (Attachment)

Protruding viral glycoproteins interact specifically with complementary glycoprotein or carbohydrate receptors on the host cell membrane:

• Adeno-associated virus: Heparan sulfate
• Epstein-Barr virus: CD21
• Herpes simplex virus: Heparan sulfate
• HIV-1: CD4 receptor, along with CCR5 or CXCR4 chemokine co-receptors.
• Influenza virus: Sialic acid residues
• Rabies virus: Acetylcholine receptor

2. Penetration and Uncoating (Entry)

Unlike bacteriophages, animal viruses do not inject their genome; instead, the entire virion enters the cell:

• Direct Fusion (Enveloped Viruses): The viral envelope fuses directly with the host plasma membrane, releasing the naked nucleocapsid directly into the cytosol (e.g., HIV, Herpesvirus).
• Receptor-Mediated Endocytosis: The virion is internalised within a clathrin-coated vesicle that fuses with an endosome.
  • For Enveloped Viruses (e.g., Influenza): Acidification of the endosome triggers a conformational change in viral envelope glycoproteins, driving fusion between the envelope and the endosomal membrane to release the nucleocapsid.
  • For Naked Viruses (e.g., Poliovirus, Adenovirus): Acidification induces pore formation in the endosomal membrane or endosome lysis, releasing the viral genome.

3. Replication and Expression of the Genome

• DNA Viruses: Typically replicate in the host cell nucleus using host transcription factors and DNA polymerases (Exception: Poxviruses replicate in the cytoplasm).
• RNA Viruses: Typically replicate in the cytoplasm using their own encoded RNA-dependent RNA polymerase (RdRp) to copy their RNA templates (Exceptions: Influenza virus transcribes its genome in the nucleus; Retroviruses transcribe via a nuclear DNA provirus).

4. Assembly (Encapsidation)

Viral capsid proteins self-assemble spontaneously around the newly replicated genomes to form mature nucleocapsids.

5. Release

• Naked Viruses: Released predominantly via host cell death and osmotic lysis.
• Enveloped Viruses: Released via budding. The nucleocapsid aligns with viral spikes embedded in the host plasma membrane, pushing outwards to acquire its envelope and pinching off without killing the cell immediately.

10. Molecular Biology of Retroviruses and HIV

Retroviruses are enveloped, positive-sense single-stranded RNA (+ssRNA) animal viruses that replicate through a double-stranded DNA intermediate.

Structural Composition of HIV-1

The Human Immunodeficiency Virus (HIV-1) is an enveloped retrovirus belonging to the Lentivirus genus. It was discovered in 1983 by Luc Montagnier in Paris and Robert Gallo in Bethesda.

                               [ HIV-1 Virion Structure ]

                                     gp120 (External Spike)
                                       │
                                      /                                     ┌──*──┐  gp41 (Transmembrane Anchor)
                                   (       ) Lipid Envelope
                                   │  p17  │ Matrix Protein
                                   │ ┌───┐ │
                                   │ │p24│ │ Cone-Shaped Capsid
                                   │ │ ☼ │ │ Reverse Transcriptase Enzyme
                                   │ │ ☺ │ │ Integrase / Protease Enzymes
                                   │ │ ═ │ │ ssRNA Genome (2 identical copies)
                                   │ └───┘ │
                                   (       )
                                    └──*──┘
  

• Envelope Spikes: Composed of gp120 (external glycoprotein that binds CD4) non-covalently linked to gp41 (transmembrane protein that mediates membrane fusion).
• Matrix (p17): A protein layer directly underneath the envelope that provides structural stability.
• Capsid (p24): A cone-shaped protein core enclosing the viral genetic material and enzymes.
• Genome: Consists of two identical copies of linear, positive-sense, single-stranded RNA, approximately $9.7 \text{ kb}$ in length.
• Enzymes: Packaged within the capsid are three essential viral enzymes:
  • Reverse Transcriptase (RT): Transcribes the viral RNA into DNA. It possesses both DNA polymerase activity (using either RNA or DNA as a template) and RNase H activity (which specifically degrades the RNA strand of an RNA-DNA hybrid).
  • Integrase (IN): Integrates the viral dsDNA into the host chromosome.
  • Protease (PR): Cleaves viral polyproteins into functional mature proteins.

Step-by-Step Mechanism of Reverse Transcription

Upon entry of the HIV-1 nucleocapsid into the cytosol, the viral reverse transcriptase converts the genomic ssRNA into dsDNA through a highly complex, multi-step pathway:

                       [ HIV-1 Reverse Transcription Pathway ]

          5' LTR (R-U5-PBS) ──────────────────────────────── (PPT-U3-R) 3' LTR
                    ▲
                    │ (Hybridises)
               tRNA-Lys3 Primer
                    │
                    ▼
           [ Synthesis of Minus-Strand DNA ] ──► [ RNase H degrades RNA template ]
                    │
                    ▼
           [ First Jump ]: Minus-strand DNA transfers to 3' end R region
                    │
                    ▼
           [ Elongation of Minus-Strand ] ──► [ PPT acts as primer for Plus-Strand ]
                    │
                    ▼
           [ Second Jump ]: Plus-strand transfers to PBS site at 3' end
                    │
                    ▼
           [ Extension of both strands ] ──► Yields dsDNA with identical LTRs
  

1. Primer Binding: A host-derived $\text{tRNA}^{\text{Lys3}}$ molecule, packaged within the virion, hybridises to the Primer Binding Site (PBS) located near the 5′ end of the viral genomic RNA.
2. Minus-Strand Initiation: Reverse transcriptase extends the tRNA primer, synthesising a complementary DNA strand towards the 5′ end of the template (from the PBS through the U5 and R regions). This creates a short RNA-DNA hybrid.
3. RNase H Digestion: The RNase H activity of reverse transcriptase degrades the RNA portion of this newly formed hybrid, leaving a short, single-stranded DNA segment (the minus-strand strong-stop DNA) attached to the tRNA primer.
4. The First Jump: This single-stranded DNA segment hybridises with the complementary R region located at the opposite (3′) end of the viral genomic RNA.
5. Minus-Strand Elongation: Once annealed, reverse transcriptase extends the minus-strand DNA along the entire length of the remaining genomic RNA template. As replication proceeds, RNase H degrades the template RNA, leaving only a short, purine-rich RNA segment called the Polypurine Tract (PPT) intact.
6. Plus-Strand Initiation: The PPT acts as a primer for the synthesis of the positive-sense DNA strand, which extends back towards the 5′ end of the minus-strand DNA, copying the U3, U5, and PBS regions.
7. Primer Removal: The tRNA and PPT primers are completely degraded by RNase H.
8. The Second Jump: The PBS region of the newly synthesised plus-strand DNA hybridises with the complementary PBS region at the 3′ end of the minus-strand DNA.
9. Final Extension: Both DNA strands are extended to their full lengths, creating a linear, double-stranded DNA molecule flanked at both ends by identical regulatory regions called Long Terminal Repeats (LTRs). The LTR is composed of three regions: U3 – R – U5.

The HIV-1 Genome, Splicing Cascade, and Rev Regulation

The integrated HIV-1 dsDNA (the provirus) contains nine genes flanked by 5′ and 3′ LTRs. Transcription is initiated from a single promoter located in the 5′ LTR, producing a single, full-length $9 \text{ kb}$ primary transcript that is subject to complex alternative splicing:

                           [ HIV-1 Splicing Cascade ]

                                 9 kb Transcript
                                        │
           ┌────────────────────────────┼────────────────────────────┐
           ▼ (No Splicing)              ▼ (Singly Spliced)           ▼ (Completely Spliced)
       [ 9 kb RNA ]                 [ 4 kb RNA ]                 [ 2 kb RNA ]
        • Gag & Gag-Pol              • Vif, Vpr, Vpu, Env         • Tat, Rev, Nef
        • Genomic RNA                (Requires Rev export)        (Exported immediately)
  

• Unspliced Transcripts (~9 kb): These serve directly as mRNAs for translating the structural polyproteins Gag and Gag-Pol. They also function as the genomic RNA packaged into progeny virions.
• Singly Spliced Transcripts (~4 kb): These are translated to produce the accessory proteins Vif, Vpr, and Vpu, as well as the envelope glycoprotein precursor Env.
• Completely Spliced Transcripts (<2 kb): These encode the regulatory proteins Tat (transcriptional transactivator), Rev (regulator of virion protein expression), and Nef (negative factor).

The Role of the Rev Protein and the Nuclear Export Switch:
During the early phase of infection, only the completely spliced $<2 \text{ kb}$ transcripts can escape the nucleus via default host transport pathways, leading to the early synthesis of Tat, Rev, and Nef.Once the Rev protein accumulates in the cytoplasm, it is imported back into the nucleus, where it binds to a specific secondary RNA structure called the Rev Response Element (RRE) present only within unspliced ($9 \text{ kb}$) and singly spliced ($4 \text{ kb}$) viral transcripts. Binding of Rev recruits host nuclear export proteins, actively transporting the larger $9 \text{ kb}$ and $4 \text{ kb}$ transcripts into the cytoplasm, initiating the late phase of infection and the synthesis of viral structural proteins and enzymes.

HIV Protein Processing and Functions

The viral proteins are translated as large, inactive polyprotein precursors that must be cleaved into active subunits:

                           [ HIV-1 Polyprotein Cleavage ]

          Gag Polyprotein (p55) ──────► MA (p17) + CA (p24) + NC (p6/p9)

          Pol Polyprotein (p160) ─────► PR (p10) + RT (p66/p51) + IN (p32)

          Env Polyprotein (gp160) ────► SU (gp120) + TM (gp41)
  

• Gag Polyprotein (p55): Cleaved by the viral protease into:
  • Matrix (MA, p17): Lines the inner membrane.
  • Capsid (CA, p24): Forms the inner core.
  • Nucleocapsid (NC, p6/p9): Binds and stabilizes the genomic RNA.
• Pol Polyprotein (p160): Generated as a Gag-Pol fusion protein via a ribosomal frameshifting event. It is cleaved by the viral protease to yield:
  • Protease (PR)
  • Reverse Transcriptase (RT)
  • Integrase (IN)
• Env Polyprotein (gp160): Cleaved in the endoplasmic reticulum by a host cell protease (furin) to yield:
  • gp120 (Surface subunit, SU)
  • gp41 (Transmembrane subunit, TM)

11. Hepatitis Viruses and Plant Virology

Note on Hepatitis D: Hepatitis D is a defective subviral satellite virus containing a circular, single-stranded negative RNA genome. It lacks its own envelope genes and is completely dependent on Hepatitis B virus (HBV) to provide its envelope glycoproteins to construct infectious progeny.

Plant Virology and Tobacco Mosaic Virus (TMV)

Most plant viruses have single-stranded, positive-sense RNA genomes and are rod-shaped or polyhedral.

• Tobacco Mosaic Virus (TMV): A landmark model in virology, discovered by Dmitri Ivanovsky and crystallized by W. Stanley. It is a rigid, rod-shaped, non-enveloped virus with helical symmetry:
  • Its capsid consists of exactly 2,130 identical capsomere protein subunits arranged in a hollow, right-handed helix.
  • The helix has a pitch of 16.3 capsomeres per helical turn, surrounding a single, positive-sense single-stranded RNA genome of ~6,400 nucleotides.

12. Subviral Agents: Prions, Viroids, and Satellites

Prions (Proteinaceous Infectious Particles)

Prions are unique infectious agents composed entirely of protein, containing no nucleic acid. The term was coined in 1982 by Stanley B. Prusiner.

                             [ Prion Conformational Conversion ]

                   Normal Cellular PrP (PrPC)         Infectious Prion (PrPSc)
                        (Rich in α-helices)              (Rich in β-sheets)
                              \                               /
                               \                             /
                                \─── Catalytic Mutation ────/
                                    (Alters secondary shape)
                                               │
                                               ▼
                                    Spongiform Encephalopathy
                                   (Protease-resistant plaques)
  

• PrPC (Normal Cellular Protein): A membrane-bound glycoprotein expressed predominantly in neuronal tissues. Its secondary structure is rich in $\alpha$-helices and is highly sensitive to protease degradation.
• PrPSc (Disease-Causing Form): A misfolded, infectious conformer. Its secondary structure is rich in $\beta$-pleated sheets, rendering it extremely resistant to heat, chemical disinfectants, and proteolytic enzymes.
• Mechanism of Infection: PrPSc acts as a conformational template. Upon physical interaction with normal PrPC, it catalyses its conversion into the pathogenic, $\beta$-sheet-rich PrPSc conformation, leading to the formation of insoluble amyloid fibril aggregates in brain tissue.
• Diseases: Prions cause Transmissible Spongiform Encephalopathies (TSEs):
  • Human: Kuru (associated with ritualistic cannibalism in Papua New Guinea) and Creutzfeldt-Jakob Disease (CJD).
  • Animal: Scrapie (sheep), Bovine Spongiform Encephalopathy (BSE / Mad Cow Disease), and Chronic Wasting Disease (CWD) in deer and elk.

Viroids

Viroids are the smallest known infectious agents, infecting exclusively plants. They were discovered by Otto Diener.

• Structure: Viroids consist solely of a short, single-stranded, covalently closed circular RNA molecule, typically 250 to 400 nucleotides long.
• Key Differences from Viruses:
  • They contain no protein capsid or envelope (they exist as naked RNA).
  • They do not code for any proteins (they are non-coding RNA).
• Replication: Viroids use host cell nuclear or plastid RNA polymerase II for replication via a rolling-circle mechanism.
• Catalytic Activity: Some viroids exhibit self-cleaving, enzymatic hammerhead ribozyme activity.
• Examples: Potato Spindle-Tuber Viroid (PSTV), Coconut cadang-cadang viroid.

Satellites

Satellites are subviral nucleic acids or viruses whose replication is completely dependent on the co-infection of a host cell by a helper virus. They are classified into three major groups:

                                    [ Satellites ]
                                          │
           ┌──────────────────────────────┼──────────────────────────────┐
           ▼                              ▼                              ▼
    [ Satellite Viruses ]       [ Satellite Nucleic Acids ]        [ Virusoids ]
    • Encode their own coat     • Lack capsid genes;               • Class 3 circular
    • Rely on helper for        • Encapsidated by helper           • ssRNA satellites
      genomic replication         coat proteins                    • 350-400 nt long
  

• Satellite Viruses: Encode their own protein capsid but rely on a helper virus for genomic replication (e.g., Adeno-associated virus).
• Satellite Nucleic Acids: Lack any genes encoding capsid proteins. Their genomes are packaged inside capsids made of proteins encoded by the helper virus.
  • Class 1: Large, linear single-stranded RNA satellites ($800 \text{ to } 1,500 \text{ nt}$) containing an open reading frame (ORF) encoding non-structural proteins.
  • Class 2: Small, linear single-stranded RNA satellites ($< 700 \text{ nt}$) without any open reading frame.
• Virusoids (Class 3 Satellites): Small, circular single-stranded RNA satellites ($350 \text{ to } 400 \text{ nt}$) without an open reading frame. They replicate in the host cytoplasm via a rolling-circle mechanism and require a helper virus for replication and encapsidation.

13. Pathogenic Reference Table