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.
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.