Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes. Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Glycoproteins embedded in the viral envelope are used to attach to host cells. This figure shows three relatively-complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells adenovirus, which uses spikes from its capsid to bind to host cells and HIV, which uses glycoproteins embedded in its envelope to bind to host cells.Įnveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins. Figure: Examples of virus shapes: Viruses can be either complex in shape or relatively simple. Non-enveloped viruses also include those that cause polio (poliovirus), plantar warts (papillomavirus), and hepatitis A (hepatitis A virus). Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells. Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA. Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification. Viruses have simply evolved to make use of these molecules for their own replication. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. They have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.

Animal viruses, such as HIV, are frequently enveloped. Enveloped viruses have membranes surrounding capsids. Isometric viruses have shapes that are roughly spherical, such as poliovirus or herpesviruses. Many plant viruses are filamentous, including TMV (tobacco mosaic virus). Filamentous viruses are long and cylindrical. In general, the shapes of viruses are classified into four groups: filamentous, isometric (or icosahedral), enveloped, and head and tail. Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family. This attachment allows for later penetration of the cell membrane and replication inside the cell.

Morphology Figure: Example of a virus attaching to its host cell: The KSHV virus binds the xCT receptor on the surface of human cells.

H spike definition microbiology