Splice

The Splice represents a crucial process during transcription in the cell nucleus of eukaryotes, during which the mature mRNA emerges from the pre-mRNA. Introns that are still contained in the pre-mRNA after transcription are removed and the remaining exons are combined to form the finished mRNA.

What is splicing

The central dogma of molecular biology states that the flow of genetic information takes place from the information carrier DNA via the RNA to the protein. The first step in gene expression is what is known as transcription. RNA is synthesized using the DNA as a template. The DNA is the carrier of the genetic information, which is stored there with the help of a code consisting of the four bases adenes, thymine, guanine and cytosine. The RNA polymerase protein complex reads the base sequence of the DNA during transcription and produces the corresponding “pre-messenger RNA” (pre-mRNA for short). Instead of thymine, uracil is always incorporated.

Genes are made up of exons and introns. Exons are those parts of the genome that actually encode genetic information. In contrast, introns represent non-coding sections within a gene. The genes stored on the DNA are traversed by long sections which do not correspond to any amino acids in the later protein and do not contribute to translation.

A gene can have up to 60 introns, with lengths between 35 and 100,000 nucleotides. On average, these introns are ten times longer than exons. The pre-mRNA produced in the first step of transcription, also often referred to as immature mRNA, still contains both exons and introns. This is where the splicing process begins.

The introns must be removed from the pre-mRNA and the remaining exons must be linked together. Only then can the mature mRNA leave the cell nucleus and initiate translation.

The splicing is mostly done with the help of the spliceosome (German: spliceosome). This is made up of five snRNPs (small nuclear ribonucleoprotein particles). Each of these snRNPs consists of a snRNA and proteins. Some other proteins that are not part of the snRNPs are also part of the spliceosome. Spliceosomes are divided into major and minor spliceosomes. The major spliceosome processes over 95% of all human introns, the minor spliceosome mainly handles the ATAC introns.

For the explanation of splicing, Richard John Roberts and Phillip A. Sharp were awarded the Nobel Prize in Medicine in 1993. Thomas R. Cech and Sidney Altman received the Nobel Prize in Chemistry in 1989 for their research on alternative splicing and the catalytic effect of RNA.

Function & task

During the splicing process, the spliceosome is formed anew from its individual parts. In mammals, the snRNP U1 first attaches itself to the 5‘-splice site and initiates the formation of the remaining spliceosome. The snRNP U2 binds to the branching point of the intron. Thereupon also binds the tri-snRNP.

The spliceosome catalyzes the splicing reaction by means of two successive transesterifications. In the first part of the reaction, an oxygen atom from the 2‘-OH group of an adenosine from the "branch point sequence" (BPS) attacks a phosphorus atom of a phosphodiester bond in the 5'-splice site. This releases the 5‘ exon and circulates the intron. The oxygen atom of the now free 3'-OH group of the 5'-exon now binds to the 3'-splice site, as a result of which the two exons are connected and the intron is released. The intron is brought into a streamlined conformation, called a lariat, which is then broken down.

In contrast to this, spliceosomes do not play a role in self-splicing. Here the introns are excluded from translation by the secondary structure of the RNA itself. The enzymatic splicing of tRNA (transfer RNA) occurs in eukaryotes and archeae, but not in bacteria.

The splicing process must take place with the utmost precision at the exon-intron boundary, since a deviation by just a single nucleotide would lead to the incorrect coding of amino acids and thus to the formation of completely different proteins.

The splicing of a pre-mRNA can turn out differently due to environmental influences or tissue type. This means that different proteins can be formed from the same DNA sequence and thus the same pre-mRNA. This process is known as alternative splicing. A human cell contains around 20,000 genes, but is able to produce several hundred thousand proteins due to alternative splicing. About 30% of all human genes have alternative splicing.

Splicing has played a major role in evolution. Exons often encode individual domains of proteins, which can be combined with one another in different ways. This means that a large variety of proteins with completely different functions can be produced from a few exons. This process is called exon shuffling.

Illnesses & ailments

Some hereditary diseases can be closely related to splicing. Mutations in the non-coding introns usually do not lead to errors in the formation of proteins. However, if a mutation occurs in a part of an intron which is important for the regulation of splicing, this can lead to faulty splicing of the pre-mRNA. The resulting mature mRNA then encodes faulty or, in the worst case, harmful proteins. This is the case, for example, with some types of beta-thalassemia, an inherited anemia. Other representatives of diseases that develop in this way are, for example, Ehlers-Danlos syndrome (EDS) type II and spinal muscular atrophy.