Common structural features of viruses

• Viruses are acellular: they have no cytoplasm and do not have the full cell machinery needed for independent life.

• All viruses share only a small set of common features:

  • small, fixed size

  • nucleic acid as genetic material (DNA or RNA)

  • capsid made of protein

  • few or no enzymes

• Core exam idea: viruses are obligate intracellular parasites — they must use a host cell for energy supply, nutrition, protein synthesis, and other life functions.

• Do not describe viruses as fully living cells: they lack cell structure such as cytoplasm and cannot carry out metabolism independently.

This diagram shows the basic components shared by many viruses. It is useful for revising the difference between the genetic material, the capsid/protein coat, and the envelope found in some viruses. Source

Diversity of virus structure

• Viruses are highly diverse in shape and structure.

• Viral genetic material may be:

  • DNA or RNA

  • single-stranded or double-stranded

• Some viruses are enveloped: they are surrounded by part of the host cell membrane.

• Others are non-enveloped and consist mainly of genetic material + capsid.

• Named examples from the syllabus:

  • bacteriophage lambda

  • coronaviruses

  • HIV

• Useful comparison for exams:

  • Bacteriophage lambda → infects bacteria, complex head-and-tail shape, commonly used for lytic and lysogenic cycles.

  • Coronavirus → enveloped RNA virus with surface spike proteins.

  • HIV → enveloped RNA retrovirus with enzymes needed for replication in host cells.

• Exam wording to use: viruses show unity and diversity — a few shared features, but major variation in genome type, envelope presence, and overall morphology.

This image shows the structure of a coronavirus virion, including the envelope, spike proteins, and nucleocapsid. It is useful for revising what an enveloped virus looks like and how it differs from a phage. Source

This diagram shows the structure of HIV, including its envelope, capsid, RNA genome, and viral enzymes. It helps distinguish HIV as an enveloped RNA virus and supports comparison with other virus types in exam questions. Source

Lytic cycle

• The lytic cycle is a viral reproductive cycle in which the virus takes over the host cell, produces many new viruses, and causes cell lysis to release them.

• Use bacteriophage lambda as the model example.

• Typical stages:

  • Attachment: virus binds to specific receptors on the host cell.

  • Entry/injection: viral nucleic acid enters the host.

  • Replication and protein synthesis: host machinery is used to copy viral nucleic acid and make capsid proteins.

  • Assembly: new virus particles are put together.

  • Lysis and release: host cell bursts, releasing many new virions.

• Key exam idea: the virus depends on the host cell for ATP/energy, raw materials, ribosomes, and biosynthetic machinery.

• Lytic infection is usually associated with rapid production of viruses and destruction of the host cell.

• In data questions, identify lytic infection by evidence of increasing virus numbers and cell death/lysis.

This diagram shows the lytic cycle, including attachment, injection of viral DNA, replication, assembly, and lysis. It is ideal for memorizing the order of events in a typical exam question on phage infection. Source

Lysogenic cycle

• In the lysogenic cycle, viral nucleic acid becomes incorporated into the host genome and is replicated with the host DNA.

• In bacteriophage lambda, the integrated viral DNA is called a prophage.

• Key sequence:

  • virus infects host cell

  • viral DNA integrates into host DNA

  • prophage is copied each time the host cell divides

  • viral genes may remain dormant/latent for a period

• A lysogenic virus can later switch to the lytic cycle.

• Main exam contrast:

  • Lytic cycle → immediate replication + host cell destruction

  • Lysogenic cycle → integration, latency, host cell survives initially

• Students should be able to compare the two cycles clearly and identify them from a diagram.

This diagram shows the lysogenic cycle, especially the formation of a prophage and its replication with the host cell. It is useful for understanding how a virus can remain latent before entering a lytic phase. Source

Lytic vs lysogenic: must-know comparison

• Lytic: viral genes are expressed immediately, new virions are assembled quickly, host cell lyses.

• Lysogenic: viral DNA is integrated into host DNA, copied during host cell division, may stay inactive.

• Lytic gives rapid spread but kills the host cell.

• Lysogenic allows persistence of viral DNA in a host lineage.

• Switching from lysogenic to lytic increases viral production when conditions favor release.

• In exam responses, always mention dependence on host machinery in both cycles.

Origins of viruses

• The syllabus states that evidence suggests several origins of viruses, not one single origin.

• Why this idea is supported:

  • viruses are extremely diverse in genome type and structure

  • their diversity suggests multiple evolutionary origins

  • their shared parasitic lifestyle may be due to convergent evolution

• Important phrase: viruses may share an extreme form of obligate parasitism.

• Their shared features do not necessarily prove one common origin; some similarities may have evolved because similar selective pressures favored a similar parasitic mode of existence.

• The genetic code is shared between viruses and living organisms, linking viruses to cellular life.

• Good evaluation point: virus evolution may involve a mixture of descent from other organisms and convergent evolution of similar features.

Rapid evolution in viruses

• Some viruses evolve very quickly because they can have:

  • very short generation times

  • high mutation rates

  • rapid replication producing large populations

  • selection pressures from host immune responses and drug treatments

• Two required examples:

  • Influenza viruses

  • HIV

• Influenza viruses evolve rapidly, helping explain why immunity may not last and why vaccine formulations may need updating.

• HIV evolves rapidly within hosts, making treatment difficult and contributing to drug resistance.

• Exam consequence to mention: rapid viral evolution makes disease control, vaccine design, and long-term treatment more difficult.

• Strong exam phrasing: rapid evolution can lead to antigenic change, treatment failure, and the need for continual monitoring.

Why viruses can exist with so few genes

• Viruses can have very small genomes because they use the host cell’s machinery instead of carrying genes for all life processes themselves.

• They do not need genes for a full metabolism, a cytoplasm, or complete systems for protein synthesis.

• Their genes mainly code for essential functions such as:

  • capsid/envelope proteins

  • proteins for entry into host cells

  • replication-related functions in some viruses

• This is a key answer to the guiding question: viruses survive with few genes because they are obligate parasites that outsource most functions to the host.

Exam traps and high-yield reminders

• Do not say viruses have cytoplasm or carry out full metabolism independently.

• Do not confuse lysis with lysogeny:

  • lysis = host cell bursts

  • lysogeny = viral DNA becomes integrated and can stay dormant

• Remember: not all viruses are enveloped.

• Remember: viral genetic material can be DNA or RNA.

• If asked about evidence for virus origins, emphasize diversity, shared genetic code, and possible convergent evolution.

• If asked why rapidly evolving viruses are hard to treat, connect to mutation, selection, and resistance/antigenic change.