Viruses are tiny infectious particles that can reproduce only inside living cells. They matter because they cause many diseases, but they are also useful tools in genetics, vaccine design, and biotechnology. A viral infection begins when a virus recognizes and attaches to specific molecules on a host cell surface.
This specificity helps explain why different viruses infect different tissues and species.
After attachment, the virus enters the cell or injects its genetic material, then redirects the host cell's machinery to copy viral genes and make viral proteins. New viral parts are assembled into complete particles called virions. Finally, the new viruses leave the cell by bursting it open, budding through the membrane, or being exported.
Each stage of the cycle is a possible target for immune defenses, vaccines, or antiviral drugs.
Understanding Biology: How Viruses Infect Cells
Attachment is more than a virus sticking to a cell. Viral surface proteins must fit particular receptor molecules, much like a key fitting a lock. Some viruses need a second helper molecule before entry can begin.
A cell may carry the right receptor yet lack the helper, so it remains hard to infect. Receptors are not made for viruses. They normally help cells receive signals, bind nutrients, or interact with nearby cells.
Viruses exploit these useful cell features. Small changes in a viral surface protein can change which cells it enters. This helps explain why a virus can sometimes spread into a new animal species or reach a different tissue in the same body.
Entry depends on the structure of the virus and the cell. Enveloped viruses have an outer lipid membrane. Their membrane can merge with the cell membrane, releasing the viral contents inward.
Other viruses are taken into a small membrane bubble called an endosome. Conditions inside the endosome can become acidic. That change may reshape viral proteins and open the particle.
This is called uncoating. The genome must be released at the right place and time. If it is exposed too early, cell enzymes may destroy it.
If it stays inside its protective coat, the cell cannot read its instructions. Many antiviral medicines work by blocking one of these precise early steps.
Once inside, different viral genomes create very different problems for the host cell. DNA viruses often use the cell nucleus, where much of the cell's DNA work happens. Many RNA viruses copy themselves in the cell fluid, called cytoplasm.
Some bring their own enzymes because human cells do not normally copy RNA from RNA. Retroviruses use an enzyme to make DNA from their RNA, then place that DNA into the host genome. Viral genes direct ribosomes to build viral proteins.
Ribosomes are the cell structures that normally build the cell's own proteins. Copying is often imperfect, especially for RNA viruses. New variants can appear as errors accumulate, which can affect how quickly a virus spreads or escapes immune recognition.
The immune system can interrupt infection before large numbers of new particles form. Antibodies can cover viral surface proteins and prevent cell entry. Infected cells can display pieces of viral protein on their surface.
Killer T cells detect these pieces and destroy infected cells. Fever and inflammation can slow infection, though they can cause symptoms themselves. Vaccines train immune cells to recognize a virus before a real exposure.
In school investigations, it is important to separate infection from illness. A person can carry a virus without obvious symptoms, while still sometimes passing it on.
It is also useful to remember that a virus population does not grow like an animal population. It grows in bursts when infected cells produce many particles, making timing and cell availability important.
Key Facts
- A virus must bind a specific host receptor before it can infect a cell.
- The main stages of viral infection are attachment, entry, uncoating, replication, assembly, and release.
- Multiplicity of infection can be estimated by MOI = number of virus particles / number of target cells.
- Viral genomes may be DNA or RNA, single-stranded or double-stranded, depending on the virus.
- Lytic release breaks the host cell open, while budding releases viruses through the cell membrane.
- Viral population growth can be estimated by final virions = infected cells x burst size.
Vocabulary
- Virus
- A virus is a noncellular infectious particle made of genetic material surrounded by a protein coat and sometimes a lipid envelope.
- Receptor
- A receptor is a specific molecule on a cell surface that a virus can bind to during attachment.
- Capsid
- A capsid is the protein shell that protects a virus's genetic material.
- Replication
- Replication is the process of copying viral genetic material inside a host cell.
- Virion
- A virion is a complete virus particle that can infect another cell.
Common Mistakes to Avoid
- Thinking viruses are cells, which is wrong because viruses do not have cytoplasm, ribosomes, or independent metabolism.
- Assuming any virus can infect any cell, which is wrong because viral attachment usually requires a matching receptor on the host cell.
- Confusing entry with replication, which is wrong because entry brings the virus or its genome into the cell, while replication makes new copies of the viral genome.
- Saying all viruses burst cells to escape, which is wrong because some viruses leave by budding or secretion without immediately killing the host cell.
Practice Questions
- 1 A lab dish contains 2.0 x 10^6 human cells and is exposed to 1.0 x 10^7 virus particles. Calculate the MOI using MOI = number of virus particles / number of target cells.
- 2 If 5,000 infected cells each release 200 new virions, how many total virions are produced? Use final virions = infected cells x burst size.
- 3 A virus binds strongly to lung cell receptors but not to liver cell receptors. Explain how receptor specificity affects which tissues the virus can infect.