Central Dogma of Molecular Genetics

Restriction-Modification Systems of Bacteria

Cloning Vectors

cDNA Cloning

Clone Library Screening

DNA Sequencing

Southern and Northern Analysis

Exons and Introns

Polymerase Chain Reaction (or PCR)

Study Questions

Cloning and Molecular Analysis of Genes WWW Links

Genetic Topics

Exons and Introns

The molecular analysis of genes began in full force in the late 1970s. One of the most startling discoveries from these early analyzes was the occurrence of sequences in the gene that did not appear in the final mRNA product. Two terms were coined to explain this result.

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