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According to cell theory all organisms are made up of cells and cells are the structural and
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functional unit of life. However, there are some exceptions to the cell theory and one of the most
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fascinating examples is acellular organisms. Acellular organisms are organisms that do not
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have cells. The most well-known acellular organisms are viruses. Other acellular organisms include
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viroids, virusoids, prions and satellite viruses. Let's begin with viruses. Viruses are incredibly
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small particles, size ranging from 5 to 300 nanometers, typically much smaller than bacteria
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and they lack many of the key features of a cell like cell membrane, cytoplasm, nucleus or other
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membrane-bound organelles. They are parasitic entities or strict parasites that can only
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replicate inside living cells by hijacking the host cellar machinery. Viruses are made up of
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two main components, a genetic material that can be either RNA or DNA and a protein cord
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The genetic material contains the instructions for building new viruses and the protein cord
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protects machinery and produces new viruses. Viruses can cause a wide range of diseases including common cold influenza AIDS and COVID The second one viroids are infectious pathogenic RNA particles discovered in 1971 by TH Diner
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It is made up of only short strand of circular RNA. They are the smallest known infectious agents
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Viroids reproduce by infecting host cells and hijacking the host cells machinery to replicate
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their RNA. Viroids can cause a variety of diseases in plants such as potato spindle
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tuber viroid and citrus exocortis viroid. The next subcellular agent is virusoid
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Virusoids are similar to viroids. They are single-stranded RNA molecules but virusoids are dependent on a helper virus for replication and encapsulation or they are
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non-replicating pathogenic single-stranded RNAs. Helper viruses are viruses that can replicate on
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their own and provide the virusoid with the proteins and enzymes for its replication
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An example of a helper virus is the subterranean clovermottel virus which has an associated
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virusoid packaged inside the viral capsid. A virusoid genome does not code for any proteins
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but instead serves only to replicate virusoid RNA At one time scientists believed that any infectious particle must contain
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nucleic acids, either DNA or RNA. Then in 1982, Stanley prisoner, a medical doctor studying scrapie, a fatal degenerative disease in sheep
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discovered that disease was caused by a proteinaceous infectious particle or prions. These infectious protein molecules can misfold and cause other proteins to misfold
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as well. This can lead to a variety of neurodegenerative diseases such as Alzheimer's
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disease and Markov disease. A prion is actually a misfolded form of a normal protein like this
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PRPC found in the cell. This misfolded protein PRPSC, SE stands for scrapie, which may be caused
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by a genetic mutation or occurs spontaneously and can be infectious, stimulating other endogenous
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normal proteins to become misfolded, forming plagues. Now prions are linked with many diseases
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like bovine spongiform encephalopathy or Markov disease, Crucible-Jakob disease and even Alzheimer's disease
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And finally, the satellite viruses. Satellite viruses are subviral pathogens that are entirely dependent upon the replication machinery of their helper viruses Satellite viruses are particles that contain nucleic acid genome that is encoding a structural protein that encapsulates the satellite genome Satellite
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viruses have been characterized in animals, protists and plants. An example of satellite
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virus is satellite tuberculosis virus which encodes a capsid protein that forms an
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ecosohedral capsid that packages only the nucleotide satellite RNA. The helper virus
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tubarco necrosis virus, encodes an RNA polymerase that replicates its genome and that of the
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satellite. In humans, hepatitis delta satellite virus is associated with a human helper virus
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that is hepatitis B virus. Infection with hepatitis delta satellite virus only occurs
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in individuals infected with hepatitis B virus, suggesting the essentiality of a helper virus for
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the replication of satellite virus. Only this much is currently known about subcellular particles or
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as cellular organisms. The definition of life is evolving with each discovery of the subcellular
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organisms. Hope you are clear with the subcellular agents. Thank you so much for your attention
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Take care. Stay blessed. Thank you so much. You are with biologyexams4a.com
