What is the name of the process that removes sections of non-coding DNA called introns from the mRNA molecule?

In some genes the protein-coding sections of the DNA ("exons") are interrupted by non-coding regions ("introns"). RNA splicing removes the introns from pre mRNA to produce the final set of instructions for the protein.

Duration: 1 minutes, 37 seconds

As DNA is transcribed into RNA it needs to be edited to remove non-coding regions, or introns, shown in green. This editing process is called splicing, which involves removing the introns, leaving only the yellow, protein-coding regions, called exons.

RNA splicing begins with assembly of helper proteins at the intron/exon borders. These splicing factors act as beacons to guide small nuclear ribo proteins to form a splicing machine, called the spliceosome. The animation is showing this happening in real time. The spliceosome then brings the exons on either side of the intron very close together, ready to be cut. One end of the intron is cut and folded back on itself to join and form a loop. The spliceosome then cuts the RNA to release the loop and join the two exons together. The edited RNA and intron are released and the spliceosome disassembles.

This process is repeated for every intron in the RNA. Numerous spliceosomes, shown here in purple, assemble along the RNA. Each spliceosome removes one intron, releasing the loop before disassembling. In this example, three introns are removed from the RNA to leave the complete instructions for a protein.

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This animation is available on YouTube .

• ID: 16938
• Source: www.learnaboutsma.org
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Only about 1 percent of DNA is made up of protein-coding genes; the other 99 percent is noncoding. Noncoding DNA does not provide instructions for making proteins. Scientists once thought noncoding DNA was “junk,” with no known purpose. However, it is becoming clear that at least some of it is integral to the function of cells, particularly the control of gene activity. For example, noncoding DNA contains sequences that act as regulatory elements, determining when and where genes are turned on and off. Such elements provide sites for specialized proteins (called transcription factors) to attach (bind) and either activate or repress the process by which the information from genes is turned into proteins (transcription). Noncoding DNA contains many types of regulatory elements:

• Promoters provide binding sites for the protein machinery that carries out transcription. Promoters are typically found just ahead of the gene on the DNA strand.

• Enhancers provide binding sites for proteins that help activate transcription. Enhancers can be found on the DNA strand before or after the gene they control, sometimes far away.

• Silencers provide binding sites for proteins that repress transcription. Like enhancers, silencers can be found before or after the gene they control and can be some distance away on the DNA strand.

• Insulators provide binding sites for proteins that control transcription in a number of ways. Some prevent enhancers from aiding in transcription (enhancer-blocker insulators). Others prevent structural changes in the DNA that repress gene activity (barrier insulators). Some insulators can function as both an enhancer blocker and a barrier.

Other regions of noncoding DNA provide instructions for the formation of certain kinds of RNA molecules. RNA is a chemical cousin of DNA. Examples of specialized RNA molecules produced from noncoding DNA include transfer RNAs

(tRNAs) and ribosomal RNAs (rRNAs), which help assemble protein building blocks (amino acids) into a chain that forms a protein; microRNAs (miRNAs), which are short lengths of RNA that block the process of protein production; and long noncoding RNAs (lncRNAs), which are longer lengths of RNA that have diverse roles in regulating gene activity.

Some structural elements of chromosomes are also part of noncoding DNA. For example, repeated noncoding DNA sequences at the ends of chromosomes form telomeres

. Telomeres protect the ends of chromosomes from being degraded during the copying of genetic material. Repetitive noncoding DNA sequences also form satellite DNA, which is a part of other structural elements. Satellite DNA is the basis of the centromere, which is the constriction point of the X-shaped chromosome pair. Satellite DNA also forms heterochromatin, which is densely packed DNA that is important for controlling gene activity and maintaining the structure of chromosomes.

Some noncoding DNA regions, called introns, are located within protein-coding genes but are removed before a protein is made. Regulatory elements, such as enhancers, can be located in introns. Other noncoding regions are found between genes and are known as intergenic regions.

The identity of regulatory elements and other functional regions in noncoding DNA is not completely understood. Researchers are working to understand the location and role of these genetic components.

Scientific journal articles for further reading

Maston GA, Evans SK, Green MR. Transcriptional regulatory elements in the human genome. Annu Rev Genomics Hum Genet. 2006;7:29-59. Review. PubMed: 16719718.

ENCODE Project Consortium. An integrated encyclopedia of DNA elements in the human genome. Nature. 2012 Sep 6;489(7414):57-74. doi: 10.1038/nature11247. PubMed: 22955616; Free full text available from PubMed Central: PMC3439153.

Plank JL, Dean A. Enhancer function: mechanistic and genome-wide insights come together. Mol Cell. 2014 Jul 3;55(1):5-14. doi: 10.1016/j.molcel.2014.06.015. Review. PubMed: 24996062.

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• Before mRNA is used as instructions to make a protein, it can be cut into smaller sections and re-arranged in a process called splicing.
• Splicing occurs at the end of the transcription process, as part of pre-mRNA processing.
• During splicing, coding-regions of mRNA (exons) are kept and non-coding regions of mRNA (introns) are cut out and removed.
• mRNA Splicing is an important step in the transcription process, as without removing the introns the correct protein cannot be formed.
• mRNA Splicing is also part of the regulation of gene expression and protein levels in the cell.

An illustration showing the process of RNA splicing.
Image credit: Genome Research Limited

What is alternative splicing?

• The process of splicing can create different variations of the same mRNA by keeping different combinations of exons, this is called alternative splicing.
• Alternative splicing means that a single gene can code for more than one type of mRNA molecule, and therefore more than one protein. This is thought to be a ‘space saving’ mechanism, as multiple proteins can be created by a single stretch of DNA.

An illustration showing the key stages in alternative splicing, whereby multiple different protein products can be created from the same stretch of DNA and pre-mRNA.
Image credit: Genome Research Limited

Article written by Olivia Edwards, PhD Student at the Wellcome Sanger Institute

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