A2.3:  Viruses

This topic is HL only

Theme:  Unity and Diversity

Unity:

• All viruses are small with fixed sizes, contain nucleic acid as genetic material, have a protein capsid for protection, lack cytoplasm, and possess few or no enzymes
• Every virus exhibits obligate parasitism, completely depending on host cells for energy supply, nutrition, protein synthesis, and other essential life functions, making them incapable of independent survival
• Viruses share the same genetic code as all living organisms, enabling them to hijack host cellular machinery for replication
• Structural features common to all viruses may represent convergent evolution toward the same solution for parasitic existence, showing how similar environmental pressures produce similar biological outcomes
• All viruses must utilize host ribosomes, enzymes, and metabolic pathways to complete their life cycles, demonstrating their fundamental reliance on the cellular machinery of living organisms

Diversity:

• Viruses exhibit tremendous variation in shape and structure
• Viral genomes can be composed of DNA or RNA, which may be single-stranded or double-stranded, linear or circular, providing different strategies for genetic storage and replication
• Some viruses acquire lipid envelopes from host cell membranes while others remain non-enveloped, representing different approaches to protection and host cell entry
• Viruses employ different reproductive strategies, including immediate lytic cycles that rapidly destroy host cells and lysogenic cycles that integrate into host genomes for long-term persistence
• The extreme diversity of viruses suggests they evolved independently multiple times from different cellular components, representing convergent evolution toward parasitic lifestyles rather than descent from a common viral ancestor
• Different viruses evolve at dramatically different rates

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• How can viruses exist with so few genes?
• In what ways do viruses vary?

​
Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• What mechanisms contribute to convergent evolution? (A2.3)
• To what extent is the natural history of life characterized by increasing complexity or simplicity? (A2.3)​

Key Terms to Know: all are higher level only

Bacteriophage
Capsid
Convergent Evolution
Coronavirus
Deoxyribonucleic Acid (DNA)
Double-Stranded
Envelope

Evolution
Genetic Code
Human Immunodeficiency Virus (HIV)
Influenza
Lambda
Lysogenic Cycle

Lytic Cycle
Nucleic Acid
Obligate Parasitism
Ribonucleic Acid (RNA)
Single-Stranded
Virus

AHL A2.3.1— Structural features common to viruses.

• List structural features common to all viruses. ​

AHL A2.3.2— Diversity of structure in viruses. 

• Outline the diversity of structure in viruses, including genetic material and the presence or absence of envelopes. 
• Distinguish between negative-sense RNA, positive sense RNA, retroviruses and DNA viruses.
• Compare the structure of bacteriophage lambda, coronaviruses and HIV.

AHL A2.3.3— Lytic cycle of a virus. 

• Outline the phases of the lytic cycle of a bacteriophage lambda virus.
• State that in the lytic cycle, a virus relies on a host cell for energy supply, nutrition, protein synthesis and other life functions.  ​

AHL A2.3.4— Lysogenic cycle of a virus.

• Compare the lytic system to the lysogenic cycle of a virus.  
• State that a temperate virus can alternate between a lytic and lysogenic cycle. 
• Outline the phases of the lysogenic cycle of a virus.

AHL A2.3.5— Evidence for several origins of viruses from other organisms.  

• Outline evidence that viruses evolved after the origin of cells.
• Outline the progressive and regressive hypotheses for the origin of viruses. 
• Outline how convergent evolution could result in the commonality of some structures shared by viruses.

AHL A2.3.6—  Rapid evolution in viruses. 

• Outline three reasons for rapid evolution in viruses. 
• Discuss the consequences of rapid virus evolution on treating diseases caused by viruses (influenza and HIV).