Question

Why are viruses not included in the three domain classification system?

Answer 1

Hint: This three domain classification divides the cellular life forms into archaea, eukaryotic domain and bacteria. This classification is proposed by Woese. These three domains are important things on the earth. Virus may or may not be the living organism. So he doesn't include the virus in his three domain classification system. In this classification five kingdoms are available.

Complete answer:
First of all, compared to other organisms or species, viruses are different organisms. It doesn’t have any cells. Virus characteristics don’t match with any of the kingdoms in the three domain classification system. So viruses are not included in this classification.
Viruses cannot perform any metabolic function and they don't have any organelle and they cannot respire. It performs metabolic function only at the living host. It contains a protein coat called the capsid. It performs reproductive functions accurately and superbly while at the living host called humans, animals and plants.
It does not grow or respond to the surrounding environment and stimuli. It multiplies while it is in the living organism. It can't make food, take food and produce waste. It is the non cellular particles. It has different shapes like helical, polyhedral and complex in shapes.

Note:
This classification is only based on the living organism. So the scientist considers the virus as non living one. So he is not adding viruses in this classification system. Virus is a very small organism compared to bacteria and archaea. It is much smaller than the smaller cells. Viruses have spikes to attach to other living organisms. Few viruses are surrounded by the protective coating or layer.