IB DP Biology Viruses Study Notes
IB DP Biology Viruses Study Notes
IB DP Biology Viruses Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- How can viruses exist with so few genes?
- In what ways do viruses vary?
Standard level and higher level: 4 hours
Additional higher level: 1 hour
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
A2.3.1 – Structural Features Common to Viruses
🦠 What Are Viruses?
Viruses are non-cellular infectious agents. They are not considered living organisms because they cannot reproduce or carry out metabolism on their own.
🔬 Common Structural Features of All Viruses
| Feature | Explanation |
|---|---|
| Very Small Size | Typically 10–400 nm – much smaller than cells |
| Fixed Size | Viruses do not grow; they are assembled from fixed components |
| Genetic Material | Can be DNA or RNA, but never both, may be single- or double-stranded |
| Capsid | A protective protein coat that encloses the genetic material |
| No Cytoplasm | Lack cytoplasm – no internal fluid or organelles for metabolism |
| Few or No Enzymes | Some (like retroviruses) carry a few enzymes (e.g., reverse transcriptase), but most have none |
| Not Truly Alive | Do not carry out homeostasis, metabolism, or reproduction without a host |
🧠 Why Are Viruses Not Classified as Living?
Viruses do not meet all the criteria of living organisms:
- No metabolism
- No response to stimuli
- No cell structure
- Cannot reproduce independently
They must infect a host cell to replicate – acting like molecular parasites.
All viruses are:
– Very small
– Have either DNA or RNA
– Enclosed in a protein capsid
– Lack cytoplasm and internal organelles
– Do not grow, and may carry few or no enzymes
A2.3.2 – Diversity of Structure in Viruses
🧬 Why Are Viruses So Diverse?
Viruses vary greatly in their shape, structure, and genetic material. This diversity allows them to infect many types of organisms from bacteria to plants and animals.
| Feature | Variation in Viruses |
|---|---|
| Genetic Material | Can be DNA or RNA (never both) Can be single-stranded (ss) or double-stranded (ds) |
| Capsid Shape | Helical, icosahedral (spherical), complex or irregular |
| Envelope | Some are enveloped (have a membrane from host) Others are naked (no envelope) |
| Size & Host Range | Range from ~10 nm to 300+ nm; infect bacteria, animals, plants, fungi, archaea |
🔍 Examples of Viral Diversity
| Virus | Shape | Envelope? | Genetic Material | Infects | Special Features |
|---|---|---|---|---|---|
| Bacteriophage Lambda | Complex (head & tail) | No | Double-stranded DNA | Bacteria (e.g. E. coli) | Tail fibers for attachment |
| Coronavirus | Spherical (helical RNA inside) | Yes | Single-stranded RNA | Respiratory cells in humans | Spike proteins for cell entry |
| HIV (Retrovirus) | Spherical | Yes | 2 copies of ssRNA | Human immune cells (CD4⁺ T cells) | Contains reverse transcriptase enzyme |
📌 Key Differences Between Virus Types
| Feature | Bacteriophage λ | Coronavirus | HIV |
|---|---|---|---|
| Envelope | No | Yes (lipid envelope) | Yes (lipid envelope) |
| Genetic Material | dsDNA | ssRNA | 2 ssRNA (retrovirus) |
| Host | Bacteria (e.g. E. coli) | Human respiratory cells | Human immune cells (CD4⁺) |
| Shape | Complex (head + tail) | Spherical with spikes | Spherical with glycoprotein spikes |
| Special Features | Tail fibers, base plate | Spike proteins (S proteins) | Reverse transcriptase for RNA → DNA |
🟨Summary:
Viruses differ in:
– Genetic material: RNA or DNA, single or double stranded
– Shape: helical, icosahedral, complex
– Envelope: some have host-derived membranes, others are naked
– Hosts: viruses target specific organisms
Examples:
Bacteriophage λ – dsDNA, no envelope, infects bacteria
Coronavirus – ssRNA, enveloped, infects humans
HIV – ssRNA x2, enveloped, infects immune cells, uses reverse transcriptase
A2.3.3 – Lytic Cycle of a Virus (Using Bacteriophage Lambda as Example)
🦠 What Is the Lytic Cycle?
The lytic cycle is a viral replication process in which the virus hijacks a host cell, replicates rapidly, and destroys the host by bursting (lysis) to release new viruses.
Viruses lack organelles or enzymes to carry out metabolism or reproduction. So, they rely on the host cell for:
- Energy (ATP)
- Raw materials (amino acids, nucleotides)
- Protein synthesis (host ribosomes)
- Replication machinery (host enzymes)
🔬 Phases of the Lytic Cycle (Bacteriophage λ)
| Phase | What Happens |
|---|---|
| 1. Attachment (Adsorption) | Bacteriophage attaches to specific receptors on the E. coli cell surface using its tail fibers. |
| 2. Penetration | The virus injects its double-stranded DNA into the host cell, leaving the capsid outside. |
| 3. Replication | The host’s enzymes and machinery replicate viral DNA and synthesize viral proteins. |
| 4. Assembly (Maturation) | New phage components (head, tail, fibers) are assembled into complete viruses (virions). |
| 5. Lysis (Release) | Viral enzymes break open the bacterial cell wall, releasing hundreds of new phages. |
Lytic cycle = quick infection & destruction
Virus injects DNA → host cell makes virus parts
Parts are assembled → host bursts (lysis)
New viruses spread to other cells
Bacteriophage λ is a common example of this fast, deadly cycle.
📌Tip:
Lytic cycle is different from the lysogenic cycle, where viral DNA integrates into the host genome and remains dormant for a while.
In the lytic cycle, the infection is immediate and aggressive.
A2.3.4 – Lysogenic Cycle of a Virus (Example: Bacteriophage Lambda)
🧬 What Is the Lysogenic Cycle?
The lysogenic cycle is a “silent” viral infection in which the virus inserts its DNA into the host cell’s chromosome and remains inactive (latent) for a period of time without immediately destroying the cell.
- No new viruses are made initially
- The viral DNA is replicated along with the host’s DNA
- This cycle allows the virus to stay hidden inside the host population
🦠 Bacteriophage Lambda as an Example
| Stage | What Happens |
|---|---|
| 1. Attachment | The phage attaches to the surface of a bacterial cell (e.g. E. coli) using tail fibers. |
| 2. Injection | The viral DNA is injected into the host cell. |
| 3. Integration | The viral DNA becomes part of the bacterial DNA. It is now called a prophage. |
| 4. Replication | Every time the bacterium divides, it copies the prophage DNA along with its own DNA. |
| 5. Activation (optional) | A trigger (e.g. UV light, stress) causes the prophage to exit the bacterial genome and enter the lytic cycle. |
Lysogenic = long-term stealth infection
Virus inserts DNA into host genome (prophage)
Host lives on, copying viral DNA silently
Under stress → enters lytic cycle → cell bursts
✔️ Example: Bacteriophage λ
📌 Key Differences from Lytic Cycle
| Lytic Cycle | Lysogenic Cycle |
|---|---|
| Immediate virus production | No virus made at first |
| Host cell is lysed (bursts) | Host cell remains alive |
| Short duration | Can last many generations |
| Virulent pathway | Dormant/latent pathway |
A2.3.5 – Evidence for Several Origins of Viruses from Other Organisms
🌍 Why Are Viral Origins Complex?
Viruses show huge structural and genetic diversity, and unlike living organisms, they don’t share a single clear ancestor. This suggests that viruses may have originated multiple times in evolutionary history.
🧪 Three Main Theories for the Origin of Viruses
| Theory | Description | Evidence/Key Ideas |
|---|---|---|
| 1. Virus-First Hypothesis | Viruses existed before cellular life and may have been among the first self-replicating entities. | Suggests viruses predate cells and gave rise to some of the first genes. |
| 2. Regressive (Degeneracy) Hypothesis | Viruses came from parasitic cells that lost unnecessary genes over time. | Some viruses have gene remnants of cellular machinery. |
| 3. Escape (Vagrancy) Hypothesis | Viruses originated from escaped bits of genetic material (e.g. plasmids or transposons). | Explains similarities between viral and host genes. |
🧬 Convergent Evolution in Viruses
Convergent evolution = Different origins, similar structures due to similar selective pressures.
- Viruses are all obligate intracellular parasites (they must infect cells to reproduce).
- Despite different origins, they all evolved:
- a protein capsid for protection
- use of nucleic acid (DNA or RNA) as genetic material
- lack of cytoplasm and independent metabolism
Viruses are so diverse that one origin theory may not explain all viruses.
Some may have started as primitive life (virus-first),
Others from degenerate cells, or escaped host DNA.
Shared features (capsid, small size) = convergent evolution, not common ancestry.
🧩 The Genetic Code Connection
- All viruses use the same genetic code as living organisms.
- This shows that viruses evolved within the context of life on Earth – they’re not completely alien, even if their origin is unusual.
A2.3.6 – Rapid Evolution in Viruses
🧬 Why Do Viruses Evolve So Rapidly?
Viruses show high rates of evolution due to several key biological features:
| Factor | Explanation |
|---|---|
| Short generation time | Viruses replicate rapidly sometimes within hours. |
| High mutation rates | Many viruses (especially RNA viruses) lack proofreading enzymes. |
| Large population sizes | Billions of virions in one host increase genetic variation. |
| Selection pressure | Host immunity, antivirals, and vaccines select for new variants. |
🔁 Two Mechanisms of Viral Evolution
| Mechanism | Description | Impact |
|---|---|---|
| Antigenic Drift | Gradual accumulation of point mutations | Small surface protein changes (e.g., HIV, flu) |
| Antigenic Shift | Sudden genetic recombination from coinfection | Major new strains, potential pandemics |
🦠 Case Study 1: Influenza Virus
Genome: Segmented RNA virus
- Undergoes both antigenic drift and shift
- Drift → seasonal flu variation → vaccines updated yearly
- Shift → recombination of human + animal strains → pandemics (e.g. 1918, 2009 H1N1)
🧫 Case Study 2: HIV (Human Immunodeficiency Virus)
Genome: ssRNA (retrovirus)
- Evolves by antigenic drift (high mutation rate)
- Reverse transcriptase lacks proofreading
- Envelope proteins (like gp120) mutate rapidly
Result: Evades immune system, no lasting vaccine, drug resistance develops fast
⚠️ Consequences of Rapid Viral Evolution
| Impact | Examples |
|---|---|
| Vaccines lose effectiveness | Seasonal flu updates, no HIV vaccine yet |
| Drug resistance evolves | HIV to antiretrovirals, flu to oseltamivir |
| Harder to control outbreaks | Variants spread before immunity builds |
| Frequent epidemics and pandemics | Influenza shifts, HIV drift → global health challenges |
Viruses evolve quickly because of short life cycles, high mutation rates, and large populations.
This leads to vaccine resistance, drug resistance, and frequent emergence of new strains – like new flu or HIV variants.
Rapid evolution = serious challenge in public health.