Most eukaryotic genes have introns. Introns make up about 18% of the DNA sequences in our genome. Most of these sequences are junk but introns are functional and up to 80bp of each intron is required for proper splicing. The essential sequences contain the 5′ splice site (~10 bp); the 3′ splice site (~30 bp): the branch site (~10 bp); and enough additional RNA to form a loop (~30 bp). The branch site and the splice sites are where specific proteins bind to the mRNA precursor [Junk in Your Genome: Protein-Encoding Genes]. It turns out that within introns about 0.37% of the genome is essential and about 17% is junk.
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