The process of RNA editing is the alteration of the sequence of nucleotides in the RNA after it has been transcribed from DNA, but before it is translated into a protein. RNA editing occurs by two distinct mechanisms:Substitution editing and Insertion/deletion editing.

Substitution editing is the chemical alteration of individual nucleotides. These alterations are catalyzed by enzymes that recognize a specific target sequence of nucleotides:

*Cytidine Deaminases that convert a C in the RNA to uracil.

*adenosine deaminases that convert an A to inosine,which the ribosome translates as a G.Thus a CAG codon (for Gln) can be converted to a CGG codon (for Arg).

*Insertion/deletion editing is the insertion or deletion of nucleotides in an RNA.

These alterations are mediated by guide RNA molecules that base-pair as best they can with the RNA to be edited and serve as a template for the addition( or removal) in the target.

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