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Introns and Exons

The genes of many organisms, including animals, are often constructed in a very interesting way. Specifically, the regions that ultimately become part of the mRNA transcript, called exons, are intermingled with intron sequences, which are lost as the mRNA is made. The process by which introns are separated from exons during mRNA production is called splicing.

This animation illustrates splicing in a 'typical' gene, which consists of three exons and two introns. The number of introns and exons in a real gene can actually vary tremendously, although the average number of introns across all human genes is approximately four.

During transcription, all of the genetic information in a gene is initially copied into an RNA molecule called a primary transcript. The transcript then has a chemical 'cap' added to its 5' end, and a 'polyA tail' which is a long string of adenines, added to its 3' end. These structures increase the stability of the mRNA as it moves through the cytoplasm and also help it to interact with the components necessary for protein synthesis.

Next, the primary transcript is 'spliced'. That is, the introns are cut out of the sequence, and the exons are joined together. The mature mRNA is now ready to go to the cytoplasm.