Viruses:
- They are infectious agents that replicate inside a host's body. They are neither living nor non-living.
- Examples- adenovirus, Hepatitis C virus, etc.
- They usually lack a cell wall. Instead are surrounded by capsid (a protective protein coating)
- They contain either RNA or DNA as the genetic material.
- They utilize the hosts' machinery to replicate and transcribe their genome.
The hypothesis of virus origin:
- According to the progressive hypothesis, viruses are just extraordinary pieces of genetic information.
- These genetic elements gained the power to move in and out of living cells independently.
- According to the regressive hypothesis, viruses are reduced parasitic forms of complex ancestral organisms.
- According to the virus-first theory, viruses were the first to evolve.
- However, none of the above hypotheses are entirely correct. Their origin is intensely contested because of the tremendous diversity among viruses and therefore is challenging to be classified.