Central Dogma of Molecular Genetics
Exons and Introns
Exon - sequences found in the gene that are also represented in the mRNA product
Introns - sequences present in the gene that are not represented in the mRNA product
The discovery of introns was made using a technique called R-loop analysis. This analysis involves the hybridization of an RNA molecule to a DNA molecule and an analysis of the result with electron microscope. In those experiments which discovered introns, mRNA was hybridized to a genomic clone of the mouse beta-globin gene. These experiments are performed by melting the DNA and mixing it with the mRNA. Because the mRNA-DNA complex is more stable, the complementary strand of the gene is displaced and a R-loop forms. In earlier experiments with microbial systems, one displacement loop of single-stranded DNA of the clone was observed. But when a eukaryotic gene was analyzed extra loops were detected. It was later shown by sequence analysis that this extra DNA was not represented in the mRNA.
Introns have now been located in many eukaryotic genes and the number can range from a low of one up to more than 50. The only consensus information among introns are the sequences 5'-GT-3' at the 5' border and 5'-AG-3' at the 3' border. No other sequences are common among introns which suggests that these sequences are in some manner important to the process that removes introns from heteronuclear RNA (hnRNA), the initial product of transcription.
Copyright © 1997. Phillip McClean