Introns and Exons     The finding of the Introns and the exons was one of the most significantdiscoveries in genetics in the past fifteen years. split genes were discoveredwhen lack of relation between DNA sequences were seen during. DNA- mRNAhybridation. For all new mRNA, they must be transcribed by RNA polymeraseenzymes. The transcription begins at the promoter sequence on the DNA and worksdown, thus the nucleotide sequence of the mRNA is complimentary to the one ofDNA. In eukaryotes the mRNA is processed in the nucleus before transport to thecytoplasm for translation. In order for the mRNA to become true functioningRNA it must under go several stages of modification.     At first, when the mRNA is produced, a cap is added enzymaticully to the5¹ end of the RNA by linking a 7-methylguanosine residue by a triphosphate bondthis is called the G-cap. The G-cap is necessary for translation. The subunitof the ribosome recognizes the G-cap and then finds the initiation codon tostart translation. As the mRNA comes finishes transcription, the Poly A tail isadded to the 3¹ end. As the two ends are placed the mRNA becomes pre-mRNA.     The pre-mRNA consists of splicing and non-coding regions. pre-mRNAmolecules are much longer than the mRNA molecule needed to code for its protein.The regions that do not code for amino acids; aa, are scattered all along thecoding region. The genes are split with coding regions, called exons, short forexpressed regions; in between the exons the non-coding region called intronsexist. Before the translation of mRNA the introns must be spliced off.Splicing is an complicated process for the cell. It must locate every intron inthe primary transcript. An average mRNA consists of eight to ten introns, someeven contain sixteen introns. exons, like introns are also spread apart. Someof their codons may be split by introns, so information for a single amino acidcould be some distance apart. Splicing takes place in the nucleus but alsocould take place in the cytoplasm and the mitochondria. After the splicing ofthe introns, the G-caps and the Poly A tails remain on the mRNA.     A single gene can code for multiple proteins by alternative splicing. Asingle strand was found to be coding for twenty different proteins, depending onhow the exons are assembled. Different splicing combinations are regulated in tissue specific manner.     Most of the transcribed DNA are introns. ninety nine percent of theinformation contained in the gene transcript is destroyed when the introns areeliminated since exons are only translated.
