Splicing Concept
The term Splicing refers to the process by which the pre-mRNA is processed to generate mRNA. This process is based on the introns removal and the linking of the exons present in a sequence.
The genes in the cell
In its DNA, the cell contains coding regions (exons) and non-coding regions (introns) that make up the genes. During transcription, both the exon and intron regions are transcribed into RNA, forming pre-mRNA; after transcription, the introns are removed, and the exons linked, forming functional mRNA that is transported into the cytoplasm and translated into proteins. Splicing occurs in the nucleus and is required for the transport of mRNA into the cytoplasm.
Exons and introns are present in most eukaryotic genes and some bacterial genes. Since introns differ greatly in number and size, they are related to the complexity of the organism. A gene can contain more than 60 introns and its size ranges from 200 to 50000 nucleotides. In general, the introns are larger than the exons and after introns removal the remaining sequence is only about 2.2 kb.
Splicing Mechanism
There are two different types of splicing:
- splicing, which is accomplished by the action of a large protein complex; this complex is called spliceosome and recognizes the GU consensus sequence at the 5’ splicing site (sequence within the exon-intron boundaries) and the consensus sequence AG at the intron 3’ splicing site. The spliceosome consists in 50 small nuclear proteins and RNA associated to form small molecules of ribonucleoprotein (U1, U2, U4, U5 and U6);
- self-splicing, which is catalyzed by RNA itself that has the ability to remove its introns. However, this process is very rare.
Splicing must take place without errors and the sequences must be recognized with the greatest accuracy. The removal or the addition of only one nucleotide may have serious consequences and can lead to protein overproduction or to failure in the production of an essential protein.
Since during the splicing process, the introns are removed, it is possible that this removal gives rise to different sequences because introns cannot be removed successfully from the sequences or exons can be removed along with the introns. This phenomenon is called alternative splicing, and through it, it is possible to obtain different mRNAs variants from the same pre-mRNA. This different mRNAs will be translated into related proteins but with different sequences (isoforms). It is estimated that about 75% of the 25,000 human genes are undergoing this phenomenon. This causes the number of coded proteins (100000) to greatly exceed the number of genes present in the organism genome.
References:
- Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
- Berg J.M., Tymoczko J.L., Stryer L. (2002). Biochemistry (5th edition). W. H. Freeman, New York.
- Brown T.A. (2002). Genomes (2nd edition). Wiley-Liss, Oxford.
- Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
- Griffiths A.J.F., Miller J.H., Suzuki D.T., Lewontin R.C., Gelbart W.M. (2000). An Introduction to Genetic Analysis (7th edition). W. H. Freeman, New York.
- Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
