The RNA of viruses do not code for any protein and have a very important secondary structure. As viroids do not contribute to the production of any protein, it is difficult to see how they affect the host cell. The most probable hypothesis is that they disturb the normal expression of the genes at the level of the RNA messenger (mRNA) thus affecting all the cellular metabolism.
They only affect crops having complex levels of organisation. If some of them cause few or no apparent damage to the plant, others are responsible for the development of serious diseases.
Viroids differ from viruses on six points :
- They exist inside the cells as RNA particles only, without capsid nor envelope.
- They have only one circular RNA strand which contains very little nucleotides.
- Their RNA do not code for any protein.
- Unlike certain viruses, viroids do not need the help of a virus to infect a cell.
- Unlike a virus of which the RNA can be copied in the cytoplasm or the nucleus, the viroids RNA is copied in the nucleus only..
- Special techniques must be used to detect their presence in the plant tissues.
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