Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (int-ron denotes their intervening role), which may be involved in gene regulation, but are removed from the pre-mRNA during processing.Intron sequences in mRNA do not encode functional proteins. This is because eukaryotes had introns long ago, before splitting with prokaryotes. They are rare within the nuclear genomes of single-celled eukaryotes. The eukaryotic pre-mRNA possesses introns (intervening sequencing) that interrupt the exons (the coding regions). Introns are less conserved. Pre-mRNA Splicing. Here we see the structure of a pre-mRNA (or hrRNA) and a mature mRNA following mRNA processing (splicing, the addition of a 5′-cap and a poly-A tail). What this means is that they change their sequence very often over time. Introns were discovered in 1977 with the introduction of DNA sequencing. Introns are rare within prokaryotic cells. Introns and exons are parts of genes. Eukaryotic cells have different regions or segments within their DNA and RNA. Introns are very much common in the genome of higher vertebrates such as human beings, mammals, amphibians, fish and mice but unlikely to be seen in the genome of certain varieties of eukaryotic micro-organism. Introns are more important as like the exons, new gene or genotype combinations are constructed by introducing exon in new between or between unknown introns. It was believed that in ancient times the introns are also present in prokaryotes but removed entirely to make the cell simpler. Introns or the intervening sequence are considered as the non-coding part of the genes, while the exons or the expressed sequence are known to be as the coding part for proteins of the genes. Intron Discovery. Therefore, the immature pre-mRNA must be processed before it leaves the nucleus. Introns are found only in eukaryotic organisms. The nuclear group I introns, exemplified by the rRNA introns of Tetrahymena thermophila and Physarum polycephalum, may well have arisen by … And eukaryotes evolving from these early ancestors should keep some of these same introns. Introns are interruptions in the text of eukaryotic genes. Exons code for proteins, whereas introns do not. Eukaryotic cells possess a nucleus bound by a membrane, and their genes consist of alternating introns and exons. The pre-mRNA is cut, introns are spliced out (removed) and two flanking exons are joined together. For example, the human genome has groupings called introns and exons. This process is called splicing. At least for some of the oldest ones. Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (int-ron denotes their intervening role), which may be involved in gene regulation, but are removed from the pre-mRNA during processing. 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