Introns are segments that do not code for specific proteins. 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. Introns are the common attribute found in the genes of the multicellular eukaryotes like humans, while exons are found in both prokaryotes and eukaryotes. However, within the cells of multicelled animals and plants almost every gene has introns. The pre-mRNA is cut, introns are spliced out (removed) and two flanking exons are joined together. Introns were discovered in 1977 with the introduction of DNA sequencing. Introns are rare within prokaryotic cells. They create extra work for the cell, but they also have important functions. Intron Discovery. At least for some of the oldest ones. This is because eukaryotes had introns long ago, before splitting with prokaryotes. 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. Introns are found only in eukaryotic organisms. What this means is that they change their sequence very often over time. For example, the human genome has groupings called introns and exons. Introns and exons are parts of genes. A great way to remember this is by considering introns as intervening sequences and exons as expressed sequences. Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (intron denotes their intervening role), which may be involved in gene regulation but are removed from the pre-mRNA during processing. And eukaryotes evolving from these early ancestors should keep some of these same introns. This process is called splicing. Introns are interruptions in the text of eukaryotic genes. 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. This event takes place in nucleus before transport of mature mRNA in cytoplasm. 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. Therefore, the immature pre-mRNA must be processed before it leaves the nucleus. 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). If introns were lost from prokaryotes (IE theory), then we might predict that introns in eukaryotes should be in around the same place. Exons code for proteins, whereas introns do not. Introns belong to the non-coding DNA. 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. Eukaryotic cells possess a nucleus bound by a membrane, and their genes consist of alternating introns and exons. The eukaryotic pre-mRNA possesses introns (intervening sequencing) that interrupt the exons (the coding regions). 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