The simplest viruses have just two components: a nucleic acid core and protein capsid. The nucleic acid core, which may be DNA or RNA, contains the instructions for taking over cells and making more virions, or viral par ticles. The nucleic acid is surrounded by the capsid, a protective protein coat. Each individual protein that makes up the capsid is called a capsomere.

All viruses have at least a capsid and a nucleic acid core. The core consists of one of four types of nucleic acid:

*Double-stranded DNA

*Single-stranded DNA

*Double-stranded RNA

*Single-stranded RNA

One difference between cells and viruses is that cells contain DNA and RNA. However, a single viral particle contains only DNA or RNA. Also, single stranded DNA and double-stranded RNA are commonly found in viruses, but not in cells.

In addition to the capsid and the core, some viruses have an outer membrane layer called an envelope. It’s no coincidence that the envelope of a virus is similar to the plasma membrane of a cell viruses that have envelopes steal them from their cellular victims as they leave the cell! Viral envelopes aren’t exactly the same as plasma membranes because they’ve been changed to suit the needs of the virus by the addition of viral proteins.

Once modified and adopted, the envelope helps the virus enter and exit from host cells. Viruses may also have proteins that stick out of the envelope or off the sur face of the capsid. These proteins, called spikes, help the virus attach to host cells.

Viruses come in three common shapes:

*Helical viruses have a capsid that forms a twisting helix around the nucleic acid core.

*Polyhedral viruses have a regular geometric shape. The most complex polyhedral viruses are icosahedrons with 20 faces.

*Complex viruses have separate patches of proteins, often forming unique structures or extensions on the virus.

Under the microscope, enveloped viruses appear irregular in shape. However, a helical or polyhedral capsid may be located underneath the envelope.

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