Introns And Exons :: essays research papers

Introns and Exons   Ã‚  Ã‚  Ã‚  Ã‚  The finding of the Introns and the exons was one of the most significant discoveries in genetics in the past fifteen years. split genes were discovered when lack of relation between DNA sequences were seen during. DNA- mRNA hybridation. For all new mRNA, they must be transcribed by RNA polymerase enzymes. The transcription begins at the promoter sequence on the DNA and works down, thus the nucleotide sequence of the mRNA is complimentary to the one of DNA. In eukaryotes the mRNA is processed in the nucleus before transport to the cytoplasm for translation. In order for the mRNA to become true functioning RNA it must under go several stages of modification.   Ã‚  Ã‚  Ã‚  Ã‚  At first, when the mRNA is produced, a cap is added enzymaticully to the 5 ¹ end of the RNA by linking a 7-methylguanosine residue by a triphosphate bond this is called the G-cap. The G-cap is necessary for translation. The subunit of the ribosome recognizes the G-cap and then finds the initiation codon to start translation. As the mRNA comes finishes transcription, the Poly A tail is added to the 3 ¹ end. As the two ends are placed the mRNA becomes pre-mRNA.   Ã‚  Ã‚  Ã‚  Ã‚  The pre-mRNA consists of splicing and non-coding regions. pre-mRNA molecules 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 the coding region. The genes are split with coding regions, called exons, short for expressed regions; in between the exons the non-coding region called introns exist. Before the translation of mRNA the introns must be spliced off. Splicing is an complicated process for the cell. It must locate every intron in the primary transcript. An average mRNA consists of eight to ten introns, some even contain sixteen introns. exons, like introns are also spread apart. Some of their codons may be split by introns, so information for a single amino acid could be some distance apart. Splicing takes place in the nucleus but also could take place in the cytoplasm and the mitochondria. After the splicing of the introns, the G-caps and the Poly A tails remain on the mRNA.   Ã‚  Ã‚  Ã‚  Ã‚  A single gene can code for multiple proteins by alternative splicing. A single strand was found to be coding for twenty different proteins, depending on how the exons are assembled. Different splicing combinations are regulated in t issue specific manner.   Ã‚  Ã‚  Ã‚  Ã‚  Most of the transcribed DNA are introns. ninety nine percent of the information contained in the gene transcript is destroyed when the introns are eliminated since exons are only translated.