SPLIT GENES(mosaic genes or interrupted genes)

Split genes (mosaic genes or interrupted genes) are the non-contiguous eukayotic genes with  informative or coding sequences, interspaced with or interrupted by long stretches of non-informative or non-coding "silent" or "spacer" sequences. Thus, split genes appear to be complex and fragmented genes, split up into alternating coding and non-coding regions. The non- coding regions are commonly called junk DNA or introns .The coding units are called exons, and the intervening non-coding regions are called introns or inserts, or intervening sequences (the terms exons and introns were coined by Walter Gilbert in 1978). Introns and split genes are very rare, or absent, in prokaryotic, mitochondrial and chloroplast DNAs, but relatively numerous in most eukaryotic nuclear genomes.They are present in some animal viruses also. Intron size varies from less than 100 to over 10,000 base pairs.

All exons may undergo transcription and translation. So, they find their expression in the pimary, intermediate and final products (i.e. in pre-mRNA, mature mRNA and proteins).On the other hand, introns are only transcribed to the pre-mRNA and then sliced out during the post-transcriptional chemical processing of immature mRNA. This is called mRNA splicing

Thus, introns appear only in the primary mRNA transcript and never in mature mRNA and proteins. On the whole, eukaryotic genes appear to be fragmented or interrupted, and they are expressed only after the splicing or excision of their introns during mRNA processing Split genes were discovered in 1977 by Richard. J. Roberts and Phillip Sharp (1993 Nobel Prize recipients), while studying the genetic material of adenoviruses (viruses which cause common cold, conjunctivitis, etc.). This discovery was a surprise jolt to the colinearity of genes and their products.

Features of split genes

Some of the features, common to all split genes, are the following:

(i) The organization of split genes would be consistently the same in all tissues of an organism.

(ii) Exons have precisely the same order in the split gene and also in its primary and secondary mRNA transcripts.

(iii) Introns, all by themselves, do not have any coding function. But, exon-intron combination codes for maturases. Maturases (Lazowska et al 1980, Cech 1985) are the proteins encoded by exon-intron combination. They catalyse the excision of introns from the primary transcript and the subsequent splicing of exons.

(iv) The presence of introns is neither essential nor deleterious to the organism.

(v) All split genes begin with an exon, and also end with an exon. The primary mRNA, transcribed from a split gene, is known as heterogeneous nuclear RNA (hnRNA) or precursor mRNA (pre-mRNA). t might be an exact replica of the split gene, with altemating exons and introns. Before its transformation to mature mRNA, it undergoes a chemical tailoring". Its introns are very carefully sliced away by excision enzymes (exo and endonucleases). Soon, the exons are spliced together by maturases (RNA ligases). If mutanon takes place in introns, maturases will not be produced, or modified maturases without splicing powers will be produced.

Significance of split genes

The significance of split genes is not definitely understood. Still, they are obviously advantageous to organisms in the following ways:

(I) Split genes provide an ingenious device for accommodating more DNA than what is actually required for the functioning of genes.

(ii) They enable the storage of the genetic information within minimum DNA.

(iii) The different exons of the same split gene can code for the different active sitesot a protein molecule (e.g., antibody genes).

(iv) Several proteins with the same amino acids can be formed from the same split gene that has many exons.

(v) Exons and introns allow flexibility in the synthesis of a variety of gene products.