Viruses vs viroids vs prions vs lichens: which has protein, which has nucleic acid, and what is the "phycobiont"?
Mehak AroraDropper Sample
29 Sept 2026 · 1 min read
Second attempt, and this is the chapter where I lost marks last time because everything sounds the same. I made a messy table and I want someone to check it, because I get anxious when I can't tell my own notes are right 😅
What I have:
- Virus: obligate parasite, nucleic acid (RNA or DNA) inside a protein coat called capsid.
- Viroid: smaller than a virus, causes potato spindle tuber disease, "free RNA" with no protein coat.
- Prion: abnormally folded protein, causes BSE in cattle and Creutzfeldt-Jakob disease in humans.
- Lichen: symbiosis of algae and fungi.
My confusion is in the details. Do viruses crystallise, and why does that matter? If a prion is just a protein, how can it be infectious without any genetic material? In lichens, which partner is the "phycobiont" and which is the "mycobiont", and who gives what to whom? And I read that lichens are pollution indicators, but of which pollutant?
I know I can read all this from NCERT but my mind goes blank during test. Someone please explain it like I'm sleepy at 6 am 🙂
6 answers & replies
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[+][–]Pooja Iyer12thSample· 21d ago
Your table is right! Just add a few things. Viruses: when W. M. Stanley crystallised them in 1935, the crystals turned out to be largely protein, which made people ask "is it even alive?". Outside the host, viruses behave like non-living chemicals, and inside a host cell they multiply, which is the whole drama. Plant viruses mostly have single stranded RNA, animal viruses have single or double stranded RNA or double stranded DNA, and bacteriophages (viruses that infect bacteria) mostly have double stranded DNA.
[+][–]Zoya Khan12thSample· 21d ago
For prions, it helps to think about what "infectious" has to mean. The infectious agent is an abnormally folded protein, and the idea is that it makes the normal form of that same protein in the host misfold too. So the damage spreads without any DNA or RNA of its own. That is why it is so strange to scientists and why NCERT puts it with these odd agents. The diseases are BSE (mad cow disease) in cattle and CJD in humans, and both damage the brain.
Be gentle with yourself on the notes, they are okay. You just need a way to remember, not more content.
[+][–]Kavya NaikDropperSample· 21d ago
Lichen: the alga is the phycobiont ("phyco" = algae) and the fungus is the mycobiont ("myco" = fungus). The alga is autotrophic, so it photosynthesises and makes food for the fungus. The fungus absorbs water and minerals from the soil or substrate and gives shelter to the alga. So, a proper two-way partnership. Lichens are very sensitive to pollution, especially the air pollutant sulphur dioxide, so they are used as indicators of air pollution. If lichens vanish from a city, the air there is probably bad.
[+][–]Rahul DubeyDropperSample· 20d ago
Here is a short way I finally fixed it in my third attempt, no drama:
- Virus = nucleic acid + protein coat
- Viroid = only RNA
- Prion = only protein
- Lichen = not a single organism, two organisms
Every confusing question is from mixing those four. For dates: Ivanowsky 1892 linked tobacco mosaic to an agent smaller than bacteria, Beijerinck 1898 called the infectious fluid contagium vivum fluidum, Stanley 1935 crystallised the virus, Diener 1971 discovered viroids. I just remember them as a timeline of one story, and the table above is the actual score-giver.
[+][–]Mehak AroraDropperSample· 20d agoAuthor
this four-line version is exactly what I needed, thank you rahul bhaiya. no worries about the dates, I'll just stick to the table 🙂
[+][–]Lakshmi TripathiDropperSample· 20d ago
Small addition from my notes. A common trap: "viroids have a protein coat" is always false. They are free RNA. And "prions contain RNA" is also false. If any option says viruses are made of only protein, that is false too, they always have nucleic acid.
