The nomenclature of introns can be traced to the word intragenic, which refers to a region inside a gene. An Intron, therefore, is any basic structural unit of nucleic acids within a gene. In a similar manner, Intron refers to both the corresponding successiveness in RNA transcripts and the DNA successiveness found inside a gene. The existence of Introns within different genomes is seen to vary widely across the scale of living organisms. For instance, introns are common within the genetic system of the higher vertebrate such as humans. On the other hand, there are those which are not that common within the nuclear genome of many eukaryotic microorganisms. The existence of Introns or their non existence is evolutionary (Garcia-Espana, Mares, Sun, & DeSalle, 2009). This is shown by many orthologus studies which are vastly comparative. The intron loss and gain event according to analyses have been observed, and there is a proposition that there was an intron invasion in the early stage of eukaryotic evolution.
The two well known mechanisms of Intron loss are: genomic deletions and the reverse transcriptase-mediated Intron loss (RTML) (Yenerall, Krupa, & Zhou, 2011). Studies further indicate that the Intron gain mechanism remains controversial and elusive. The seven known Intron gain mechanisms are tandem genomic duplication, Intronisation, Intron transposition, Intron transfer, insertion of a group two Intron, tandem genomic duplication, Intron gain during double standard-strand break repair (DSBR) and transposition insertion (Yenerall, Krupa, & Zhou, 2011). It is quite easy to derive the origin of the recently gained Introns since they lack a host-induced mutation. One overriding thing to note is that out of the seven mentioned methods of Intron gain, not even one could be associated with the Intron gain, and this is what raises the eye brows whether or not the earlier mentioned mechanisms are valid to be used as Introns gain methods.
For quite some time, scientists thought that introns were perhaps useless. However, continued research in to the existence, and reason for existence of introns have shown that introns are overly useful and as Bergman (2001) contends, at some point, Darwinists were opinionative that introns were actually junk parts of a DNA sequence with in mind that introns were actually no code for anything. However, detailed studies have served to reveal that introns serve pertinent biological functions with one function being facilitation of biological diversification. For instance, in an attempt to get to the root of the relationship between Strepsiptera, Diptera and Coleoptera, it was found out that the fore wings of Strepsiptera are similar to the hind wings of Diptera (Hoy, 2003). It could easily be assumed that the two hind wings for Diptera and the fore wings of Strepsiptera would be homologous if a simple mutation would occur to reverse to position of one of the wings only for scientists to realize that these two types of wings (Strepsiptera and Diptera) were inherently different due to the existence of a unique intron insertion in the gene of Diptera which as absent in Strepsiptera (Hoy, 2003).
Additionally, studies have led to the realization that there is alternative reading of the genetic code in which intron exist as exons (Bergman, 2001). Ideally, some DNA have been found to exist as introns when expressed in a particular pathway but as exons when expressed in another way (Bergman, 2001). The importance of this attribute that warrants the seemingly useless introns worth keeping is that existence of such pathways results into more varieties of protein products (Bergman, 2001). At the same time, some introns are pertinent controllers of sequences during the inactivation of x chromosomes causing the “nucleotides to be altered so that the DNA is read in an entirely different way” (Bergman, 2001, p. 6).
References
Bergman, J. (2001). The Functions of Introns: From Junk DNA to Designed DNA. Perspectives on Science and Christian Faith, 53(3), 1-13.
Garcia-Espana, A., Mares, R., Sun, T., & DeSalle, R. (2009). Intron Evolution: Testing Hypotheses of Intron Evolution Using the Phylogenomics of Tetraspanins. PLoS ONE, 4(3), 1-11.
Hoy, M. A. (2003). Insect Molecular Genetics: An Introduction to Principles and Applications. Amsterdam: Academic Press.
Yenerall, P., Krupa, B., & Zhou, L. (2011). Mechanisms of Intron Gain and Loss in Drosophila. BMC Evolutionary Biology, 11(364), 1-11.
