Organisms use ribonucleic acid (RNA) molecules for protein synthesis and as worker molecules in their own right. The RNA itself is a copy of a gene sequence in the organism's genome. Generally, the RNA is an accurate copy of the gene, but sometimes it is altered in a process known as RNA editing. The editing can insert or delete letters of the sequence, or it can change one letter in the sequence to another. As the sequence of letters in the RNA changes, its ability to code for a particular protein, or its function, changes.
Three types of RNA are affected by RNA editing. The first, and most prone to editing, is messenger RNA (mRNA). The function of mRNA is to act as a template for the construction of proteins. Ribosomal RNA (rRNA) and transfer RNA (tRNA) are not templates but make the proteins the mRNA codes for. All of these molecules are copies of particular genes in the DNA genome of the cell.
Each RNA strand is usually an accurate copy of the deoxyribonucleic acid (DNA) of a gene. In situations where RNA editing occurs, the sequence of the gene does not match the sequence of the RNA. The editing process happens after the cell copies the DNA genes but before the individual RNAs fulfill their function.
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The RNA editing process occurs in one of two different ways. Both ways alter the sequence of the RNA. RNA is made of four bases, uracil (U), cytosine (C), guanine (G), and adenine (A). The editing can either add or remove bases, or it can change one base into a different base.
Changing one base into another is known as substitution. Substitution is performed by proteins that target a specific sequence. Insertion and deletion, which is the other method, is performed by molecules made from RNA.
RNA editing may seem unnecessary, as the genome is supposed to code for all the proteins the organism needs to survive. There are situations, however, where altering a sequence of RNA is necessary. Some organisms may have genes that do not code for entire proteins, and the RNA copies of these genes therefore need tweaking before the cell can make the required protein. Editing may also correct sequences copied from mutated parts of genes.
Another possibility is that RNA editing is a clever way to produce more than one protein product from one gene. Cells may also make different types of one product as a way of keeping up protein diversity, which increases adaptability. Finally, RNA editing may be a way for the cell to test tweaked forms of proteins without losing the original, usable gene sequence.