Question

What evolutionary steps the capsid went through and why? Why a viral capsid does not protect a virus from dehydration and why viruses cannot produce their own energy?

Answer 1

Capsid is a proteinaceous shell encases the viral genome. A comprehensive sequence and structural analysis of major virion proteins indicates that they have evolved on multiple independent occasions, throughout the course of evolution. The Capsid proteins typically do not share the homology with contemporary cellular proteins, making it difficult to understand their provenance and evolution. The possible scenarios may be:-

• Genes encoding Capsid proteins have originated de novo within the genomes of non viral selfish replicons by genetic mechanisms such as over printing and diversification.

Or

• These proteins could have first performed cellular functions, subsequently recruited for virion formation.

The Capsid protein mediates the entrance of viral genetic material into a new host cell upon infection. Capsid is the characteristic feature of all viruses, so it may also coincide with the origin of viruses.

Three major scenarios that explain the origin of viruses are :-

1. Primordial virus world hypothesis:- regards viruses as intermediate between prebiotic chemical systems and cellular life forms.
2. Regression hypothesis:- viruses are degenerated cells that have succumbed to obligate intracellular parasitism.
3. Escape hypothesis:- viruses evolved independently in different domains of life from cellular genes that embraced selfish replication and became infectious.

Typically viruses are composed of inner genetic material that codes for viral proteins and infection and the outer proteinaceous coat that help in protection, infection and transfer but does not have any organelles or mechanism that can produce its own energy.