Viruses are tiny infectious particles that range from 20 to 400 nm in diameter, and they can be examined only under an electron microscope. 

Viruses can be classified by (i) morphological virus types, or (ii) host range.

(i)   Four main morphological virus types

(ii)   Host range (the type of host cells they can infect)

• Bacterial viruses / Bacteriophages (e.g., T4 phage, lambda phage)

• Animal viruses (e.g., influenze virus, Human Immunodeficiency Virus - HIV)

• Plant viruses (e.g., TMV)

Viruses are not cells but simply nucleic acids enclosed in a protein capsid. 

💡Basic Structure

The basic structure of a virus consists of a protein capsid and a nucleic acid genome. Together, they make up a nucleocapsid. The unnarrated video below shows the structure and functions of these basic structures.

💡Additional Structures: The Viral Envelope

In addition to the nucleocapsid, some viruses have a phospholipid bilayer membrane that surrounds the the capsid. This viral envelope is derived from the host cell membrane and often have glycoproteins embedded in it. 

💡Additional Structures: The Tail

Viruses that infect bacteria - bacteriophages - each has a head (the nucleocapsid) and a tail (= tail sheath + base plate + tail fibres). The tail is neat little 'machine' that helps the bacteriophage infect a bacterial cell through an 'attach-and-inject' mechanism! 

🖊️Check Your Understanding

Attempt Qn 4 of the learning goals.

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