Structure of Viruses
Viruses are tiny, infectious particles that can reproduce only by infecting a host cell. They are much smaller than bacteria and consist of a single- or double-stranded nucleic acid (DNA or RNA) surrounded by a protein shell called a capsid. Some viruses also have an outer envelope composed of lipids and proteins. They vary in shape.
A typical virus consists of two basic parts: 1) nucleic acid genome and 2) protein capsid together called the nucleocapsid. Many animal viruses also contain 3) lipid envelope along with some additional parts such as the neck, tail sheath, tail fibers, pins, and endplate to form a complete virion.
The capsid is made from the proteins that are encoded by viral genes within their genome. The shape of the capsid may vary from one type of virus to another. The shape of the capsid serves as one basis for classification of viruses.
ome viruses have an envelope of phospholipids and proteins. The envelope is made from portions of the hosts cell membrane. It surrounds the capsid and helps protect the virus from the hosts immune system. The envelope may also have receptor molecules that can bind with host cells. They make it easier for the virus to infect the cells.
Types of Viruses
1. Helical Viruses: Helical capsids are made up of a single type of protein subunit stacked around a central axis to form a helical structure. The helix may have a hollow center, which makes it look like a hollow tube. This arrangement results in rod-shaped or filamentous virions. These virions can be anything from short and very rigid, to long and very flexible. The well-studied tobacco mosaic virus (TMV) is an example of a helical virus.
2. Icosahedral Viruses: Icosahedral capsid symmetry gives viruses a spherical appearance at low magnification, but the protein subunits are actually arranged in a regular geometrical pattern, similar to a soccer ball.
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”. Because they can’t reproduce by themselves (without a host), viruses are not considered living. Nor do viruses have cells: they’re very small, much smaller than the cells of living things, and are basically just packages of nucleic acid and protein. Still, viruses have some important features in common with cell-based life..
