1.2.7—Virus structure and envelope

A virus is a non-cellular infectious particle, not a prokaryotic or eukaryotic cell. Its basic structure is genetic material—a DNA or RNA core—inside a protein capsid; some viruses also have a phospholipid envelope.

The capsid protects the viral genome. If an envelope is present, it is derived from host-cell membrane phospholipids and carries attachment proteins. Those proteins bind a suitable host-cell receptor, allowing the virus to deliver its genome and use the host cell’s machinery to make new virus particles. Without a host cell, the particle cannot reproduce independently.

• Nucleic-acid core: contains either DNA or RNA.
• Capsid: protein coat surrounding and protecting the core.
• Envelope: present in some viruses only; a phospholipid outer layer acquired from a host cell.
• Scale and boundary: virus particles are much smaller than typical cells and have no cytoplasm, ribosomes or independent metabolism.

Do not call a virus a cell or a living organism, and do not assume every virus has an envelope. Its ability to reproduce by using host-cell machinery is the key boundary from cellular organisms.