Intro to viruses
- A virus is an infectious particle that reproduces by "commandeering" a host cell and using its machinery to make more viruses.
- A virus is made up of a DNA or RNA genome inside a protein shell called a capsid. Some viruses have an external membrane envelope.
- Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.
- Viruses reproduce by infecting their host cells and reprogramming them to become virus-making "factories."
What is a virus?
How are viruses different from bacteria?
The structure of a virus
- A protective protein shell, or capsid
- A nucleic acid genome made of DNA or RNA, tucked inside of the capsid
- A layer of membrane called the envelope (some but not all viruses)
- Icosahedral – Icosahedral capsids have twenty faces, and are named after the twenty-sided shape called an icosahedron.
- Filamentous – Filamentous capsids are named after their linear, thin, thread-like appearance. They may also be called rod-shaped or helical.
- Head-tail –These capsids are kind of a hybrid between the filamentous and icosahedral shapes. They basically consist of an icosahedral head attached to a filamentous tail.Diagram of icosahedral (roughly spherical), filamentous (rod-like), and head-tail (icosahedral head attached to filamentous tail) virus capsid shapes.
What is a viral infection?
- Attachment. Virus binds to receptor on cell surface.
- Entry. Virus enters cell by endocytosis. In the cytoplasm, the capsid comes apart, releasing the RNA genome.
- Replication and gene expression. The RNA genome is copied (this would be done by a viral enzyme, not shown) and translated into viral proteins using a host ribosome. The viral proteins produced include capsid proteins.
- Assembly. Capsid proteins and RNA genomes come together to make new viral particles.
- Release. The cell lyses (bursts), releasing the viral particles, which can then infect other host cells.
- Attachment. The virus recognizes and binds to a host cell via a receptor molecule on the cell surface.
- Entry. The virus or its genetic material enters the cell.
- Genome replication and gene expression. The viral genome is copied and its genes are expressed to make viral proteins.
- Assembly. New viral particles are assembled from the genome copies and viral proteins.
- Release. Completed viral particles exit the cell and can infect other cells.