Exon- Definition, Structure, Splicing, Process of Splicing


An exon is a segment of DNA or RNA that codes for a part of a protein or a functional RNA molecule. Exons are found in the genes of eukaryotes, which are organisms with a nucleus and other membrane-bound organelles. Exons are separated by non-coding regions called introns, which are removed during the process of RNA splicing.

Exons can vary in size and number depending on the gene and the organism. In humans, the average exon length is about 170 base pairs (bp), and the average number of exons per gene is about 9. However, some exons can be as short as 2 bp or as long as 11,555 bp, and some genes can have more than 100 exons.

Exons have different roles and functions depending on their location and sequence. Exons can be classified into three types:

  • Coding exons contain the nucleotide triplets that specify the amino acids of a protein. Coding exons usually start with a start codon (AUG) and end with a stop codon (UAA, UAG, or UGA).
  • Untranslated exons are located at the 5` and 3` ends of a messenger RNA (mRNA) molecule and do not code for any amino acids. Untranslated exons regulate the stability, transport, and translation of the mRNA.
  • Non-coding exons are found in genes that produce non-coding RNAs, such as ribosomal RNA (rRNA), transfer RNA (tRNA), microRNA (miRNA), and long non-coding RNA (lncRNA). Non-coding exons may have structural or regulatory functions in these RNAs.

The figure below shows an example of a gene with three coding exons (red) and two untranslated exons (grey). The introns (blue) are spliced out to form a mature mRNA that can be translated into a protein.

Figure 1: Exon structure in a protein-coding gene.