Click image to open full size
Viruses: Structure and Life Cycle
Viruses: Structure and Life Cycle
Related Tools
Related Labs
Related Worksheets
Viruses are tiny infectious particles that can reproduce only by entering living cells. They matter because they cause diseases in humans, animals, plants, and bacteria, and they also play important roles in evolution and biotechnology. Unlike cells, viruses do not have ribosomes, cytoplasm, or their own energy supply. Their simple structure makes them dependent on a host for nearly every step of replication.
A virus usually contains genetic material, either DNA or RNA, surrounded by a protein coat called a capsid, and some viruses also have a lipid envelope with surface proteins. Infection begins when the virus attaches to specific molecules on a host cell and delivers its genetic material inside. The host cell machinery is then used to copy viral genes and build viral proteins. New virus particles assemble and leave the cell by lysis or budding, allowing the infection to spread.
Key Facts
- A virus is made of nucleic acid + capsid, and some viruses also have an envelope.
- Viral genome types include DNA or RNA, and they may be single stranded or double stranded.
- Attachment depends on a match between viral surface proteins and host cell receptors.
- Basic lytic cycle steps: attachment -> entry -> replication -> assembly -> release.
- In budding, enveloped viruses leave the cell by taking part of the host membrane as an envelope.
- Viruses are not considered fully living cells because they cannot carry out metabolism or reproduce independently.
Vocabulary
- Capsid
- The capsid is the protein coat that surrounds and protects the viral genetic material.
- Envelope
- The envelope is a lipid membrane around some viruses that is usually taken from the host cell membrane.
- Host cell
- A host cell is a living cell that a virus infects and uses to make more viruses.
- Receptor
- A receptor is a molecule on a cell surface that a virus binds to during attachment.
- Lytic cycle
- The lytic cycle is a viral replication process in which new viruses are made and the host cell is often destroyed.
Common Mistakes to Avoid
- Thinking viruses are cells, which is wrong because viruses lack organelles, ribosomes, and independent metabolism. They are infectious particles that must use a host cell to reproduce.
- Assuming all viruses have envelopes, which is wrong because many viruses are nonenveloped and consist mainly of genetic material inside a capsid. Envelope presence changes how a virus enters and exits cells.
- Confusing attachment with entry, which is wrong because attachment is only the binding of the virus to a host receptor. Entry happens afterward when the viral genome or whole particle gets into the cell.
- Believing antibiotics kill viruses, which is wrong because antibiotics target bacterial structures or processes, not viral replication. Viral infections require prevention, immune response, or antiviral drugs.
Practice Questions
- 1 A virus infects a cell, copies its RNA, makes capsid proteins, assembles 240 new virus particles, and then breaks the cell open to release them. Which stage of the life cycle is the cell in just before release, and how many complete virus particles were produced?
- 2 A nonenveloped virus has a protein capsid and DNA genome. If 150 host cells each release 80 new viruses after one replication cycle, how many new viruses are produced in total?
- 3 Why can a virus infect one type of cell but fail to infect another type in the same organism? Explain using receptors and attachment.