Viruses, Viroids, Prions and Lichens
Acellular infectious agents and the algal–fungal partnership of lichens. (Biology › Biological Classification, NEET UG syllabus.)
What is Viruses, Viroids, Prions and Lichens?
An acellular, obligate intracellular parasite consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
Key points
- Differentiate between viruses, viroids, and prions based on their structure and composition.
- Describe the life cycle and mode of infection of viruses.
- Identify diseases caused by viruses, viroids, and prions.
- Explain the symbiotic relationship in lichens and the roles of each partner.
Common exam trap
Confusing viruses with bacteria; viruses are acellular and obligate parasites, bacteria are cellular.
Definitions
- Term
Virus
- Meaning
An acellular, obligate intracellular parasite consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
- Term
Capsid
- Meaning
The protein coat that encloses the genetic material of a virus.
- Term
Virion
- Meaning
A complete, infective virus particle outside a host cell.
- Term
Viroid
- Meaning
An infectious agent consisting solely of a short strand of circular, single-stranded RNA that has no protein coat, primarily causing plant diseases.
- Term
Prion
- Meaning
An infectious agent composed of abnormally folded protein that causes progressive neurodegenerative diseases.
- Term
Lichen
- Meaning
A symbiotic association between a fungus (mycobiont) and an alga or cyanobacterium (phycobiont).
- Term
Phycobiont
- Meaning
The algal or cyanobacterial component of a lichen, which is autotrophic and produces food.
- Term
Mycobiont
- Meaning
The fungal component of a lichen, which is heterotrophic and provides shelter, water, and minerals.
Learning objectives
Differentiate between viruses, viroids, and prions based on their structure and composition.
Describe the life cycle and mode of infection of viruses.
Identify diseases caused by viruses, viroids, and prions.
Explain the symbiotic relationship in lichens and the roles of each partner.
Recognize the ecological significance of lichens as pioneer species and pollution indicators.
Prerequisites
Basic understanding of cell structure (prokaryotic vs. eukaryotic).
Knowledge of basic biological classification (kingdoms).
Understanding of photosynthesis and heterotrophic nutrition.
Common mistakes
Confusing viruses with bacteria; viruses are acellular and obligate parasites, bacteria are cellular.
Assuming viruses have both DNA and RNA; they have one or the other, never both.
Mistaking viroids for viruses; viroids lack a protein coat.
Thinking prions contain genetic material; they are purely proteinaceous.
Not understanding the mutualistic nature of lichens; both partners benefit.
Forgetting that lichens are pollution indicators, specifically for SO2.
Keywords
Virus
Viroid
Prion
Lichen
Acellular
Obligate parasite
Capsid
DNA virus
RNA virus
Bacteriophage
PSTV
BSE
CJD
Symbiosis
Phycobiont
Mycobiont
Pollution indicator
Pioneer species
Practice preview
Which of the following infectious agents lacks a protein coat and contains only free RNA?…
easy
The protein coat enclosing the genetic material of a virus is known as the:…
easy
Lichens are a symbiotic association between which two organisms?…
easy