Viruses: Structure
A virus particle, also known as a virion, is essentially a nucleic acid (DNA or RNA) enclosed in a protein shell or coat. Viruses are extremely small, approximately 15 - 25 nanometers in diameter.Viruses: Genetic Material
Viruses may have double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. The type of genetic material found in a particular virus depends on the nature and function of the specific virus. The genetic material is not typically exposed but covered by a protein coat.
The viral genome can consist of a very small number of genes or up to hundreds of genes depending on the type of virus. Note that the genome is typically organized as a long molecule that is usually straight or circular.
Viruses: Capsids
The protein coat that envelopes viral genetic material is known as a capsid. A capsid is composed of protein subunits called capsomeres. Capsids can have several shapes: polyhedral, rod or complex. Capsids function to protect the viral genetic material from damage.In addition to the protein coat, some viruses have specialized structures. For example, the flu virus has a membrane-like envelope around its capsid. The envelope has both host cell and viral components and assists the virus in infecting its host. Capsid additions are also found in bacteriophages. For example, bacteriophages can have a protein "tail" attached to the capsid that is used to infect host bacteria.
Virus Replication
Viruses are intracellular obligate parasites which means that they cannot replicate or express their genes without the help of a living cell. A single virus particle (virion) is in and of itself essentially inert. It lacks needed components that cells have to reproduce. When a virus infects a cell, it marshals the cell's ribosomes, enzymes and much of the cellular machinery to replicate. Unlike what we have seen in cellular replication processes such as mitosis and meiosis, viral replication produces many progeny, that when complete, leave the host cell to infect other cells in the organism.
Viruses may contain double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. The type of genetic material found in a particular virus depends on the nature and function of the specific virus. The exact nature of what happens after a host is infected varies depending on the nature of the virus. The process for double-stranded DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA viral replication will differ. For example, double-stranded DNA viruses typically must enter the host cell's nucleus before they can replicate. Single-stranded RNA viruses however, replicate mainly in the host cell's cytoplasm.
Once a virus infects its host and the viral progeny components are produced by the host's cellular machinery, the assembly of the viral capsid is a non-enzymatic process. It is usually spontaneous. Viruses typically can only infect a limited number of hosts (also known as host range). The "lock and key" mechanism is the most common explanation for this range. Certain proteins on the virus particle must fit certain receptor sites on the particular host's cell surface.
How Viruses Infect Cells
The basic process of viral infection and virus replication occurs in 6 main steps.
- Adsorption - virus binds to the host cell.
- Penetration - virus injects its genome into host cell.
- Viral Genome Replication - viral genome replicates using the host's cellular machinery.
- Assembly - viral components and enzymes are produced and begin to assemble.
- Maturation - viral components assemble and viruses fully develop.
- Release - newly produced viruses are expelled from the host cell.
Viruses may infect any type of cell including animal cells, plant cells and bacterial cells. To view an example of the process of viral infection and virus replication, see Virus Replication: Bacteriophage. You will discover how a bacteriophage, a virus that infects bacteria, replicates after infecting a bacterial cell.