Edexcel A Level (IAL) Biology -6.5 Comparison of Bacterial & Viral Structure- Study Notes- New Syllabus

Comparing Bacteria and Viruses + Lytic vs Latency

🌱 Introduction

This topic compares how bacteria differ from viruses in structure and life cycle. You also need to know how different example viruses (Ebola, TMV, HIV, lambda phage) fit into these structures. Finally, you must understand two important viral pathways: lytic and latency.

🔍 Structure of Bacteria vs Viruses

Bacteria (Prokaryotic Cells)

• Living, cellular organisms.
• Much larger than viruses.
• Have cell wall, cell membrane, cytoplasm, ribosomes, DNA loop.
• Some have plasmids, flagella, capsules.
• Reproduce by binary fission (asexual).
• Contain their own machinery for metabolism and protein synthesis.

Viruses

• Non-cellular infectious particles.
• Cannot reproduce on their own; must infect a host cell.
• Core structure includes:
  • Nucleic acid (DNA or RNA)
  • Capsid (protein coat)
  • Envelope (lipid membrane) in some viruses.
• Much smaller than bacteria.
• No ribosomes, no cytoplasm, no metabolism.

🧩 Key Structural Differences 

• Size: Viruses are far smaller.
• Genetic material: Bacteria always have DNA; viruses may have DNA or RNA.
• Protective layer: Bacteria have cell walls; viruses have capsids and some have envelopes.
• Living status: Bacteria are alive; viruses are not fully alive.
• Reproduction: Bacteria divide; viruses hijack host machinery.
• Complexity: Bacteria have organelle-like structures; viruses are extremely simple.

🧪 Examples of Virus Structures

Ebola Virus

• RNA virus
• Has helical capsid
• Has envelope
• Filamentous shape
• Causes severe haemorrhagic fever.

Tobacco Mosaic Virus (TMV)

• RNA virus (single-stranded)
• Rigid, helical capsid
• No envelope
• Infects plant cells.

HIV (Human Immunodeficiency Virus)

• RNA retrovirus
• Icosahedral capsid
• Has envelope with glycoprotein spikes
• Integrates its genetic material into host DNA.

Lambda Phage (λ Phage)

• DNA virus
• Classic bacteriophage structure:
  • Icosahedral head
  • Tail sheath
  • Tail fibres
• Infects E. coli.

🌀 Viral Life Cycles: Lytic vs Latency

Lytic Cycle

• Virus enters host cell → takes over machinery → makes new virus particles → cell bursts (lysis).
• Results in rapid increase in viral numbers.
• Cell dies immediately.
• Example: many bacteriophages (including lytic phase of lambda phage).

Key features:

• Fast
• Destructive
• Symptoms appear quickly
• No dormant phase

Latency (Lysogenic Pathway)

• Viral nucleic acid becomes part of host genome.
• Lies dormant (no symptoms).
• Replicated along with host DNA during cell division.
• Can switch to lytic cycle later when triggered.

Key features:

• Slow
• Hidden/dormant stage
• Host cell not destroyed immediately
• Seen in HIV and temperate phages (e.g., λ phage)

📋 Summary Table

🧠 Quick Recap
Bacteria = living prokaryotes, viruses = non-living particles.
Viruses: nucleic acid + capsid + optional envelope.
Ebola = RNA + envelope, TMV = RNA no envelope, HIV = RNA retrovirus + envelope, λ phage = DNA with head and tail.
Lytic = fast, cell bursts.
Latency = viral DNA hides in host genome and activates later.