ALTERNATIVE SPLICING

In Eukaryotes the splicing process of a pre-mRNA can lead to different ripe mRNA molecules and therefore to different proteines. This phenomenon is called alternative splicing. Also viruses have adaptated to this biochemical process when using the protein biosynthesis apparatus.

When the pre-mRNA has been transscribed from the DNA, it includes several introns and exons. In Nematodes, the mean is 4-5 exons and introns, in Drosophila there are more than 100 introns and exons in one transscribed pre-mRNA. But what is an intron and what is an exon is not decided yet. This decision is made during the splicing process. The regulation is widely unknown. However, this leads to a new view of genes. A gene therefore is a twofold information structure:

• A DNA sequence coding for the pre-mRNA
• An additional DNA code or other regulating process, which regulates the alternative splicing.

The alternative splicing is of great importance:

• for science, it means that the old idea of one DNA sequence coding for one polypeptide is no longer correct. External information is needed in order to decide which polypeptide is produced, given a DNA sequence and pre-mRNA.
• Since the ways of regulation are inherited, the meaning of mutation must be changed.
• for Eukaryotes it was a very important step towards higher efficiency.
  • The information can be stored much more economically. Several proteins can be encoded in a DNA sequence whose length would only be enough for two proteins in the prokaryote way of coding.
  • It is unnecessary to change the DNA of a gene for the evolution of a new protein. Instead, a new way of regulation can lead to the same effect, but leaving the code for the established proteines unharmed.
  • New proteins can evolve much faster than in prokaryotes. Furthermore, they are based on hitherto functional amino acid subchains. This gives a higher probability for a functional new protein.
  • Therefore the adaptation to new environments can be much faster - with fewer generations - than in prokaryotes. This might have been one very important step for multicellular organisms with a longer life cycle.