Structural Biochemistry/Nucleic Acid/RNA/RNA modification/Introns

Introns are regions within the primary-transcript where part of the fragments are to be removed. They are named introns for intervening sequences. the regions that are saved are called the exons.

Schematic depiction of removal of introns from a strand of RNA

Introns are removed from the primary transcript, precursor mRNA (pre-mRNA) after the poly A tail and the 5' cap have been added. Introns usually begin with Guanine-Uracil, and end with Adenine-Guanine that is preceded by a pyrimidine-rich tract, which signal splicing. Introns are spliced from the pre-mRNA by spliceosomes, which are made from proteins and small RNA molecules. Some introns are self-splicing which means they have the ability to remove themselves from an RNA molecule. One advantage of having genes being split by introns is that alternate splicing patterns allows the formation of proteins with varying functions without requiring new genes for each such protein.

There are four types of introns: Group I introns, Group II Introns, Nuclear pre-mRNA Introns, and Transfer RNA Itrons. Group I introns are found in some rRNA genes and splices itself out of genes. Group I introns fold into a type of secondary structure that has a nine-looped stem that is required in order to be spliced. Group II introns are found in mitochondria and chloroplasts. They are self-splicing as well, but they cut themselves out differently than Group I introns. They too fold into a secondary structure like Group I introns, but their splicing produces a lariat structure. A lariat structure is formed when an introns folds back on itself after an exon is cut from it. Nuclear pre-mRNA introns are found in the nucleus in protein-encoding genes. Their removal requires the presence of snRNAs and several other proteins. Transfer RNA introns are found in tRNA genes and needs enzymes in order to be spliced out of the genes.

The average human gene has 8 introns and some have more than 100. The size range ranges from 50 to 10,000 nucleotides. They are longer than exons.

Evolutionary Differences in Existence of Introns

Introns are usually found in the genes of higher eukaryotes such as birds or mammals. Lower eukaryotes such as yeast have fewer introns and prokaryotes rarely if ever have introns. Study of genes that have been highly conserved in evolution suggests that introns were present in most organisms long ago but were lost in organisms such as prokaryotes as an evolutionary measure to allow faster replication. The presence of introns is thought to contribute to the development of new genes through exon shuffling. The advantage of the introns is that exons maintained function but are able to interact in new ways. Without introns, the crossover would most likely result in a loss of function.

Discovery of Introns

Introns (intervening sequences), were discovered by Phillip Sharp and Richard Roberts in 1977 discovered that several genes are discontinuous. Electron microscopic studies of mRNA and DNA segments combined showed the presence of introns. If the gene was continuous only one of the strands of DNA would be displaced. However, it was observed that the strands were displaced in some regions but remained as double strands in other regions, thus proving the existence of introns.

References

Info. obtained (Berg, Stryer, et al.)