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RNA Editing: Definition & Processes

Instructor: Amanda Robb
In this lesson, we'll be learning about RNA editing. We'll explain what RNA editing is and look at different mechanisms, such as addition, deletion, and substitution through deamination and chemical alternation.

What is RNA Editing?

When we think of the molecule that controls all of our traits, we usually think of DNA. DNA is the genetic material of the cell and holds the ultimate instructions to make you, you. However, in order to access the message in the DNA, it must be transcribed, or copied, to mRNA and then translated to protein. Although most of us know about this central dogma, what we might be less familiar with are the changes to mRNA that occur after transcription. Some cells alter the mRNA in a process known as RNA editing.

RNA editing is the process of modifying RNA nucleotides to change the amino acid sequence. This usually involves editing specific nucleotides and is considered a separate process than the RNA splicing that occurs in eukaryotes, which is more like a large scale cut and paste type of edit. RNA editing can be used to modify the proteins made based on the conditions in the cell. It helps create diversity in protein products from a limited number of genes. Many types of cells from single-celled protozoa all the way to humans perform RNA editing. Today, we're going to look at the mechanisms by which this process happens.

Mechanisms

The mechanisms of RNA editing can be divided into two categories, addition and deletion or substitution.

1. Addition and Deletion

The first way that RNA can be edited is through addition. In addition editing, new nucleotides are inserted into the original sequence. Imagine taking your cursor and choosing a spot to add a new letter when typing a message. This is an example of an addition. Addition can be especially powerful because it can cause a frame shift.

Normally when RNA is read to make protein, it is read in groups of three nucleotides called codons. If you add a nucleotide in the middle of a sequence, everything downstream gets shifted over one, effectively changing the reading frame for the rest of the protein.

Similarly, a deletion, or removal of a nucleotide, can also cause a frameshift. Deletion removes a nucleotide, just as you might delete a letter from a paragraph by pressing the backspace button.

A common example of addition and deletion RNA editing occurs in the protozoan organism trypanosomes, one of which causes the infection of African sleeping sickness. Scientists have extensively investigated RNA editing mechanisms in the mitochondrial DNA in these organisms. They have identified several genes involved in metabolism that are edited through addition and deletion editing. In these organisms, the base uracil (U) is either inserted or deleted to form mRNA that makes a functional protein. A ''guide'' RNA is created by the cell to help place the insertions and deletions in the right place. A group of proteins called the editosome then align the guide with RNA and add or delete bases as needed.

Guide RNA has been shown to help organize additions and deletions in trypanosome mitochondrial RNA
guide RNA

2. Substitution

Another type of RNA editing is substitution. Imagine typing a letter and replacing one letter for another, such as changing an A to a U. This is analogous to substitution in RNA editing, where one nucleotide is replaced with another. However, in order to create a substitution, instead of cutting out the existing nucleotide and replacing it with a new one, the existing nucleotides typically undergo a chemical reaction to change their structure. One common way this is done is through deamination. This type of substitution is called deamination because the adenine nucleotide loses a chemical group called an amine group.

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