what is the cause of introns in transcription?

Answer 1

Brian B is partially correct, but I think it goes a bit deeper than that. The introns of a pre mRNA are spliced out as mentioned before, but to what is the cause and function of introns is more complex.

For example bacteria have no introns for the most part. Why would eukaryotes want to carry around the extra DNA and take the energy to replicate it? The answers to this are still being worked out, but one thing that introns do is that they enable a gene to be transcribed in slightly variable forms. This is called alternate splicing. The gene may skip an exon giving the finished mRNA a different sequence and hence a slighty different protein. This gives the cell more options in its regulation.

If you consider the human genome codes for only ~30,000 genes, which was far lower than anyone expected, alternate splicing allows for the complexity that one would expect for an organism as complex as a human being. (throw in post translational modification of proteins and that really gives you the complexity)

So introns allow for gene complexity and are therefore evolutionarly advantageous.

Hope this helps

Stooleo

Answer 2

Basically there are genes that code for RNAs, which are transcribed to snRNP (small nuclear ribonuclearproteins), and snRNAs. The snRNPs and the snRNAs forms RNA-snRNP complexes called spliceosomes. These spliceosome complexes recognize specific cleavage sites in mRNA that form the introns.

For example:

AGCUGucccgagaauauGCUAAC

Where capital letters represent exons in mRNA, and lower case letters represent introns, the spliceosome in this case recognizes Gu...uG and, makes the appropriate cuts in the mRNA.