Скачать или смотреть EXCEPTIONS OF CELL THEORY | Cell and subcellular organelle | Chapter 3 F.Sc.1st Year Biology | Lec#6

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Exceptions to Cell Theory
While cell theory is widely accepted, there are notable exceptions. For example:
1. Acellular Entities:
 Viruses challenge cell theory because
 They are not made of cells and cannot carry out life processes
independently.
 They require a host cell to replicate.
 They are considered by many scientists to be at the border of living and
non-living entities.
 Prions and viroids similarly, show properties of living organism but are
not composed of cells. They are made of only DNA, RNA or proteins.
2. Semi-Autonomous Organelles:
Eukaryotic organelles mitochondria and chloroplasts have their own DNA and can
replicate independently of the cell's nucleus.
Contradiction. This suggests they may have originated from free-living
prokaryotic cells.
3. Coenocytic structures
Some organisms, such as certain fungi and algae, have structures where multiple
nuclei coexist within a shared cytoplasmic mass.
Contradiction. These structures blur the boundaries of individual cells as defined
by traditional cell theory.
4. Muscle cells (myocytes)
These in vertebrates can fuse to form multinucleated fibres.
Contradiction. These challenging the concept of a single cell as the basic unit
in complex tissues.