Introns are deleted before a gene is transcribed from DNA to mRNA?

Introns are deleted before a gene is transcribed from DNA to RNA because introns are non coding regions between exons their presence will interrupt the translation process.

Explanation:

Introns are the non coding regions on mRNA that are transcribed from a sequence of DNA. The introns are deleted by RNA splicing and only exons are coded so that correct protein is formed otherwise introns would get coded along and wrong protein will be formed. Introns are present in eukaryotic cell.

Introns play a very important role in controlling gene expression. Multiple variant of a particular protein is controlled by introns. An error during splicing would cause a different protein to be code.

Introns are important because all the cells in eukaryotes have same group of genes and to prevent them in getting expressed in all the cells where they are not required.