What is the structure of introns?

Introns can make up to 90% of a gene’s length, with some spanning over 10,000 nucleotides. On average, there are about nine introns per gene in the human genome.

Pre-mRNA introns are characterized by three specific short sequences: the 5' splice site (SS), the branch site (BS) the 3' splice site. Following the branch site, there is typically a region abundant in pyrimidine nucleotides, known as the polypyrimidine tract (PPT). Any tRNA genes in both archaea and eukaryotes have an intron located precisely one nucleotide downstream (3') of their anticodon sequence. tRNA introns are found at a specific position between nucleotides 37 and 38 within the mature tRNA molecule, with lengths ranging from 6 to 133 nucleotides. In contrast, the location of introns within archaeal tRNAs can vary. tRNA introns form specific secondary structures, such as stem-loop structures and hairpins.