What's the purpose of Introns?

In the various biology courses I have taken at OCC there have been many things that I have found fascinating, confusing, and what felt like straight out of science. One continuous theme throughout those classes was that everything has a reason and that nothing goes to waste.

When the topic of DNA transcription and translation came about one aspect always stood out, introns. The way I studied for a test was that exons were expressed and introns were thrown away. Outside of the test, I could not actually believe that introns were meaningless. When this semester project came around I thought it would be a good idea to tackle this question and see what I can learn.

After searching through pubmed looking for articles, I stumbled upon this article by Jo and Choi called Introns: The Functional Benefits of Introns in Genomes. This was the perfect article to break the ice and dive into the purpose of introns.

Jo and Choi conducted their research to understand the functional roles or benefits of introns. They start by breaking introns into two possible roles: direct or indirect.

            To begin their explanation of introns with direcr roles, they explore how introns are a part of the regulation needed for alternative splicing and gene expression. Alternative splicing produces variant proteins from a single gene. They suggest that introns carry cis-acting elements that allows for the conservative exons to be properly expressed.

            As for the gene regulation, they looked at research Buchman and Berg whose study revealed that proteins produced by simian virus with introns were 400 times greater than simian virus without introns. It showed how introns guarantee a higher level of expression. This idea was termed intron-mediated enhancers (IME). In fact, gene expression increased with IMEs in studies conducted with plants and mammals.

            The authors continue the direct roles of introns in the processes of transcription and translation. IMEs can regulate transcription by modulating the functions of promoters on genes. Introns also contribute to the mRNA transport outside of the cell to have the mRNA translated in the cytoplasm.

            The authors shift gears and explore the indirect roles of introns. They examine how introns can influence creating new genes and natural selection. In the creation of new gens that cited an experiment conducted by Carvunis et al. that looked at proto-genes, non-functional translated code, and how they could become functional genes. The authors infer that introns could be a part of these porot-genes and could lead to new genes due to mutations and alternative splicing.

            In natural selection, introns increase the efficiency of natural selection via recombination and the relaxation of Hill-Robertson (HR) interference. The introns can reduce the HR interference by increasing the recombination possibilities for favorable exons that can lead to new and better genes. Introns can also act as a mutation buffer allowing for the allele to be expressed properly with causing harm to the organism.

            The authors conclude their findings by stating that introns do have functional roles in the cell and organism and that further research is needed to uncover all of the possible roles of introns. One other point that is discussed is the energy cost of copying introns when they may take up to 40% of a gene’s code. But keeping the introns may be in the cell’s best interest due to the direct and indirect roles introns have.

After reading through that first article, I am pleased to learn that introns are in fact not useless and can have a substantial impact on the life of a cell or organism. Now I know that I need to dig deeper to learn more.  Let’s see what I can find next.

Sources:

Jo, B. & Choi, S.S. (2015). Introns: The Functional Benefits of Introns in Genomes. Genomics and Informatics.

Buchman AR, Berg P. (1988). Comparison of intron-dependent and intron-independent gene expression. Mol Cell Biol. 8:4395–4405

Carvunis AR, Rolland T, Wapinski I, Calderwood MA, Yildirim MA, & Simonis N. (2012). Proto-genes and de novo gene birth. Nature. 487:370–374.