Cell theory was proposed by Theodor Schwann, Matthias Schleiden and Rudolf Virchow
The three tenets are
cell theory
1. All living organisms are composed of one or more cells (Schwann and Schleiden, 1838-39)
2. The cell is the basic structural and functional unit of life (Schwann and Schleiden, 1838-39)
3. All cells arise from pre existing cells by cell division (Rudolf Virchow, 1858)
Even though cell theory is universal to a greater extend still there are exceptions.

Notable exceptions are
1. Viruses are obligate intracellular parasites capable of replicating only inside the host cell using the machinery of the host.
  • It consists of genetic material either DNA or RNA surrounded by a protein coat
  • Viruses are devoid of cell, protoplasm or cellular compartments like mitochondrion, chloroplast etc
  • Viruses don’t have a machinery to replicate independently. It depends on the host cell replication machinery to replicate inside the host cell
2. Viroids are small, unencapsidated or naked ssRNA molecules
  • the smallest known pathogen of plants
  • smaller than the smallest virus
  • it consist of linear strand of RNA without a protein coat
  • found in the cytoplasm of higher plants
eg: potato spindle tuber viroid (PSTVd)

3. Virusoids: are infectious sub viral particles consisting of single circular RNA
These are viroids located inside protein coat of true virus.

4. Prions: infectious protein particles causing diseases in man and domestic animals
Prions cause other neurological diseases such as Kuru and Gerstmann-Strausler-Sheinker syndrome, CJD (Creutzfeld-Jakob disease)

5. Coenocytic algae like Vaucheria, and many fungus: The body is a continuous mass of protoplasm without cell wall and with many nucleus.
All the above mentioned particles exhibit characters of life but not cellular in its organisation.
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