Viruses, Viroids, Prions and Lichens – Class 11 | Chapter – 2 | Short Notes Series PDF

Viruses Viroids Prions and Lichens: In the world of biology, we have to classify things much like a library classifies books. That’s because for every category of book, like math books, there are subcategories, like statistics books. And for every subcategory, there are thousands of books on the topic. Without classification, the library would be a mess. In biology, it’s no different. There are so many topics that we have to use terms to classify them into broad categories. This lesson goes over some of these categories: viruses viroids prions and lichens.

Viruses Viroids Prions and Lichens


Viruses

The term ‘Virus’ means poisonous fluid. The word was coined by Louis Pasteur. Viruses are very small (0.05-0.2 µm), infective, nucleoprotein particles, which can be called as living because of the presence of nucleic acid as genetic material and ability to produce their own copy-viruses. They show only some properties of living beings, otherwise they behave like non-livings. Hence, these are referred to as the connecting link between living and non-living.

On the basis of nature of genetic material, the viruses are of two types

(i)Adenovirus DNA containing, e.g., HIV, etc.

(ii) Retrovirus RNA containing, e.g., Rous sarcoma virus, etc.

On the basis of their host, the viruses can be categorised as

(i)Animal virus (Zoophagineae), e.g., HIV, sarcoma, etc.

(ii)Plant virus (Phytophagineae), e.g., TMV, etc.

(iii)Bacterial viruses (Phagineae), e.g., T4 phage, etc.

Characteristics of Viruses

Characteristics of viruses are as follows

Living

  • They can replicate.
  • In host body, they can synthesise protein.
  • They cause diseases like other living organisms.
  • Similar gene mutation as living organism.

Non-living

  • Do not have protoplasm, and do not perform metabolism.
  • These can be crystallised.
  • They do not respire.
  • In vitro culture is not possible.

Structure of Viruses

  • Viruses are non-cellular and ultramicroscopic.
  • Virus has two components:
    • A core of nucleic acid called nucleoid.
    • A protein coat called capsid.

Viroid’s 

After the discovery of viruses, scientists found another pathogen that was smaller than a virus. It didn’t have a protein layer like a virus and caused potato spindle tuber disease. It was discovered by T.O. Diener in 1971.

Structure of Viroid 

  • Viroid only consists of RNA which is not protected by any layers of protein like a virus.
  • They only infect plants (eg. Potato spindle tuber disease)
  • They are smaller in size than a virus
  • The structure of viroid is never similar
  • As same as viruses, these viroids infect the host cell to reproduce and slowly kill the host during the process.

Prions

These are abnormally folded proteins, which can transmit several diseases. The word prion is derived from “proteinaceous infectious particle”. They were discovered by American biologist Stanley B. Prusiner. 

Structure of Prions

  • They are similar in size to Viroid (Significantly smaller than viroid)
  • Prions generate aberrant protein clumps known as amyloids, which accumulate in infected tissue and are linked to tissue damage and cell death.
  • Prions are abundant in one’s body but the one that is dangerous are structurally different.

The most common disease caused by Prions is bovine spongiform encephalopathy (BSE), Cr–Jacob disease (CJD), etc.


Lichens

Lichens are symbiotic (Mutual) relationship between algae and fungi in which algae prepare food and fungi provides shelter. In this relationship, algae are known as photobionts, and fungi are known as mycobionts. Lichens have properties different from those of their component organisms.

Structure of lichen

  • Physically, the most of the observed part of lichens is the filaments of its fungal symbiote
  • The algae part is situated below this fungal filament and is separately situated
  • The thick filaments help in attaching them to the surrounding. 

Types of lichens

  1. Fruticose– Grows outside of a standing branch tree
  2. Foliose– This species has two defined upper and lower sides.
  3. Squamulose– A lichen that is composed of small, often overlapping “scales” called squamulose.
  4. Leprose– This lichen has the physical appearance of dust.
  5. Gelatinous– This lichen has the physical appearance of jelly
  6. Crustose– a crust that strongly appears.

JOIN OUR TELEGRAM CHANNELS
Biology Quiz & Notes Physics Quiz & Notes Chemistry Quiz & Notes

Follow on Facebook

By Team Learning Mantras