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Exons: Definition & Overview

Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Scientists once thought that each gene produced a single protein. Now we know that isn't true. Indeed, alternative splicing can create several products from one gene. This can be done by changing which exons are retained or spliced. But what is an exon?

Definition

When DNA is copied into messenger RNA, some information that is not needed for the final product is included. This form of RNA is considered immature. This information needs to be removed to create a functional product. Once the information is removed, by a process called RNA splicing, the mRNA is considered mature.

Exons are the name for the nucleotide sequences that remain in a mature mRNA. Introns are the name for the regions that are removed (spliced out). The term 'exon' refers to both the DNA sequence within a gene and to the corresponding sequence in mRNA (also known as transcripts). During RNA splicing, introns are removed and exons are covalently joined to one another. Once all introns are removed, the resulting mRNA or noncoding RNA gene-product is considered mature.

History

Walter Gilbert, and American biochemist, coined the term 'exon': 'The notion of the cistron… must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger - which I suggest we call introns (for intragenic regions) - alternating with regions which will be expressed - exons.'

Walter Gilbert
Image of Walter Gilbert

Although this definition was meant to refer to sequences in protein-coding transcripts only, it has been expanded to include sequences that are important for other forms of RNA. These include ribosomal RNA and transfer RNA. More recently, the term has been applied to sequences in RNA molecules originating from different parts of the genome. The key is that regions of the immature transcript are removed. Anything left behind that has a function is considered an exon.

Creation of a Mature Transcript

Below is a diagram showing the relationship between DNA, immature RNA, and mature RNA. Notice that DNA is double-stranded. However, only one strand, the template strand, is actually copied into RNA. Since the DNA is copied exactly, it contains introns and exons. The introns are cut out of the transcript and the exons are joined together by a spliceosome. This happens in the nucleus.

Relationship between DNA, immature transcripts, and mature transcripts
Image of splicing

Alternative splicing

The theory used to be: one gene to one protein. Then, alternative splicing was discovered. This is exactly as it sounds. In certain proteins the splicing process can create different proteins. How? Because the splicing pattern can be changed to create a new protein. For example, exons can be extended by duplication. They can also be skipped entirely. On the other hand, introns can be retained.

So now you are probably asking what regulates which version of a protein will be created. In some instances alternative splicing occurs in a tissue-specific manner and/or under specific cellular conditions. So, how does a tissue or special conditions control splicing?

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