These shapes can be:
- Helical: The tobacco mosaic virus has a helix shape.
- Icosahedral, near-spherical viruses: Most animal viruses are like this.
- Envelope: Some viruses cover themselves with a modified section of the cell membrane, creating a protective lipid envelope. These include the influenza virus and HIV.
- Other shapes are possible, including nonstandard shapes that combine both helical and icosahedral forms.
Other shapes are possible as well, including nonstandard shapes that combine both helical and icosahedral forms.
Sources
Viruses don’t leave fossil remains, so they are difficult to follow through time. There are molecular techniques used for comparing the DNA and RNA of viruses, to find out more about where they come from.
Also, there are three popular theories that try to explain the origin of viruses.
- Regressive, or reduction hypothesis: Viruses started as independent organisms that became parasites over time. As time passed, they shed genes that did not help them parasitize, that’s how they became fully dependent on the cells they occupy.
- Progressive, or escape hypothesis: Viruses developed from parts of DNA or RNA that “escaped” from the genes of bigger organisms. Thereby, they’ve gained the capacity to become independent and move between cells.
- Virus-first hypothesis: Viruses developed from sophisticated and complex molecules of nucleic acid and proteins either before or at the same time as the first cells appeared on Earth, billions of years ago.
