• A virus particle consists of DNA or RNA within a protective protein coat called a capsid.
  • The shape of the capsid may vary from one type of virus to another.
  • Some viruses have an envelope of phospholipids and proteins.
  • The envelope is made from portions of the host’s cell membrane.
  • It surrounds the virus and helps protect the virus from the host's immune system.
  • The capsid of the virus shown in Figure below is icosahedral.
  • Viral structures are built of repeated identical protein subunits.
  • This makes the icosahedron the easiest shape to assemble.
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