# Virus structure

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u1-virus-structure/

This sub-topic covers the key structural features of viruses, contrasts viral structure with prokaryotic and eukaryotic cells, and explains core distinctions between enveloped and non-enveloped virus types for CIE A-Level exams.

**Prerequisites:** [Prokaryotic and eukaryotic cell structure](https://www.owlsprep.com/study/cie-9700-u1-prokaryotic-eukaryotic-cell-structure/)

## Learning objectives

- Describe the general structure of a virion
- Distinguish between enveloped and non-enveloped viruses
- Compare viral structure to prokaryotic and eukaryotic cells
- Identify functions of key viral components

## General Viral Structure and Key Components

**Virion** — A complete, infectious viral particle that exists outside of a host cell. All virions have two core components: genetic material and a protein coat called a capsid.

**Capsid** — The protein coat that surrounds and protects the viral genetic material, built from repeating protein subunits called capsomeres. Capsids can be helical, icosahedral, or complex in shape.

All virions contain genetic material, which can be either single-stranded or double-stranded DNA or RNA. A defining feature of viruses is that they never contain both DNA and RNA, unlike all living cells.

**Worked example:** Name and state the function of four key structural components of a typical bacteriophage.

1. 1. Head (capsid):
2. Function: Protects the viral genetic material and holds it in place.
3. 2. Viral DNA:
4. Function: Carries all genetic instructions required to produce new viral proteins and assemble new virions inside the host bacterium.
5. 3. Tail fibers:
6. Function: Bind to specific receptor molecules on the surface of the host bacterial cell to enable attachment.
7. 4. Contractile sheath:
8. Function: Contracts after attachment to inject the viral DNA into the host bacterial cytoplasm.

> **Exam tip:** CIE frequently asks to label diagrams of bacteriophages, so memorise the position and function of each core structure.

## Enveloped vs Non-Enveloped Viruses

Beyond the core capsid and genetic material, some viruses have an additional outer layer called an envelope. This structural difference impacts how viruses enter host cells and interact with the immune system.

**Viral Envelope** — A lipid bilayer membrane derived from the host cell (from the cell membrane, nuclear membrane, or endoplasmic reticulum) that surrounds the capsid. Envelopes are embedded with viral glycoprotein spikes that aid host cell attachment.

**Worked example:** State one similarity and two differences between non-enveloped tobacco mosaic virus (TMV) and enveloped influenza virus.

1. Similarity: Both viruses have a protein capsid surrounding their genetic material, and both have RNA as their genetic material.
2. Difference 1: TMV is non-enveloped, so its outermost layer is the protein capsid. Influenza has an outer lipid envelope surrounding its capsid, with embedded glycoprotein spikes.
3. Difference 2: TMV infects plant cells and spreads through physical damage to plant cell walls, while influenza enters host animal cells by fusing its envelope with the host cell membrane.

> **tip**
>
> Glycoprotein spikes on envelopes are the main targets for recognition by the host immune system. This is why seasonal flu vaccines are updated annually to match new spike variants.

## Comparing Viruses to Cellular Life

A key examinable point is that viruses are acellular, meaning they do not have a cell structure. This sets them apart from all prokaryotic and eukaryotic organisms.

- Viruses lack cytoplasm, ribosomes, and all other organelles
- Viruses cannot carry out independent metabolic reactions (e.g. respiration)
- Viruses cannot replicate on their own, they require host cell machinery
- Viruses have only one type of nucleic acid (DNA or RNA), while all cells have both

**Worked example:** Give three structural differences between a bacterium (prokaryotic cell) and a virus.

1. 1. Bacteria are cellular (have a cell structure), while viruses are acellular with no cell structure.
2. 2. Bacteria contain both DNA and RNA, while viruses contain only DNA or RNA, never both.
3. 3. Bacteria have their own ribosomes for protein synthesis, while viruses do not have ribosomes or any other organelles.

**Check your understanding**

Test your understanding of core differences:

1. Which feature is shared by all viruses and all prokaryotic cells?

   - A. Ribosomes
   - B. Nucleic acid
   - C. Lipid envelope
   - D. Cell wall

   *Why:* All viruses contain nucleic acid (DNA or RNA) and all prokaryotic cells contain DNA and RNA, so this is the shared feature. Viruses do not have ribosomes or cell walls, and only some viruses have envelopes.

## Common pitfalls

- **Wrong:** Stating that viruses contain both DNA and RNA
  - Why it fails: All living cells have both DNA and RNA, but viruses only carry one type of nucleic acid in their virion
  - Correct: Always state that viruses have either DNA or RNA, never both
- **Wrong:** Confusing the capsid and the viral envelope
  - Why it fails: Many students mix up which layer is always present vs which is optional
  - Correct: Remember: capsid = protein coat that is always present; envelope = lipid outer layer only found in some viruses
- **Wrong:** Calling viruses 'living cells'
  - Why it fails: CIE 9700 follows the standard classification that viruses are not living or cellular
  - Correct: Refer to viruses as acellular infectious particles, not cells
- **Wrong:** Claiming the entire bacteriophage virion enters the host bacterium
  - Why it fails: Most students generalise entry from animal viruses to bacteriophages incorrectly
  - Correct: Recall that only the bacteriophage DNA enters the host; the capsid remains outside the bacterial cell

## Cheatsheet

| Component | Description | Core Function |
| --- | --- | --- |
| Virion | Complete infectious viral particle | Transmit virus between host cells |
| Capsid | Protein coat of capsomeres | Protect viral genetic material |
| Envelope | Host-derived lipid bilayer | Aid host cell attachment and entry |
| Genetic material | Either DNA or RNA | Store instructions for new virions |
| Bacteriophage tail fibers | Protein appendages | Bind to host bacterial receptors |

## What's next

Understanding virus structure is a foundational concept for multiple later topics in CIE A-Level Biology. This sub-topic reinforces the core distinction between cellular and acellular life, a key theme of Unit 1 cell structure. You will build on this knowledge when studying how viruses cause disease, how the immune system recognises viral pathogens, and how vaccines target viral surface proteins in later units of the syllabus.

- [Comparison of eukaryotic and prokaryotic cells](https://www.owlsprep.com/study/cie-9700-u1-comparison-of-eukaryotic-and-prokaryotic/)
- [Biological Molecules](https://www.owlsprep.com/study/cie-9700-u2-overview/)
- [Water and inorganic ions](https://www.owlsprep.com/study/cie-9700-u2-water-and-inorganic-ions/)

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