In eukaryotes, genes contain exons (coding sequences) separated by introns (non-coding sequences). When a gene is transcribed, the whole sequence, introns and exons, is copied to make pre-mRNA.Before the mRNA can be translated, the introns must be removed. This is called RNA splicing. It takes place in the nucleus and is carried out by spliceosomes, complexes of proteins and small RNA molecules. The spliceosome recognises the boundaries of each intron, cuts it out and joins the exons together to form mature mRNA. The mature mRNA then leaves the nucleus.
Prokaryotic genes do not contain introns, so bacterial mRNA is not spliced.Alternative splicing
The same pre-mRNA can be spliced in different ways. Some exons may be kept in one cell type but removed, along with the introns, in another. This produces different mature mRNAs from one gene, which are translated into different proteins.Example: a gene contains exons 1, 2, 3 and 4. In one tissue all four are kept, giving mRNA 1-2-3-4. In another tissue exon 3 is removed, giving mRNA 1-2-4 and a shorter protein with a different function.Alternative splicing explains how the human genome, with only about 20,000 protein-coding genes, can produce many more different proteins. It is also a way of controlling gene expression, because which version of a protein is made depends on the cell type and conditions.
Prokaryotic genes do not contain introns, so bacterial mRNA is not spliced.Alternative splicingThe same pre-mRNA can be spliced in different ways. Some exons may be kept in one cell type but removed, along with the introns, in another. This produces different mature mRNAs from one gene, which are translated into different proteins.Example: a gene contains exons 1, 2, 3 and 4. In one tissue all four are kept, giving mRNA 1-2-3-4. In another tissue exon 3 is removed, giving mRNA 1-2-4 and a shorter protein with a different function.Alternative splicing explains how the human genome, with only about 20,000 protein-coding genes, can produce many more different proteins. It is also a way of controlling gene expression, because which version of a protein is made depends on the cell type and conditions.