In the nucleus, genes are sections of DNA that code for specific proteins in the cell. This is referred to as the genetic code. All of the DNA is not included as part of the genetic code. There are lengths of DNA, called introns, which do not code for proteins. The parts of the DNA form the genetic code are called exons.
To create proteins from DNA, two separate processes occur within the cell. First, transcription creates RNA from the DNA. This is then followed by translation, which produces proteins based on the RNA created previously. Transcription occurs within the nucleus and translation occurs within the cytoplasm of the cell.
During transcription, three types of RNA are produced, based on the DNA in the nucleus. Each has a specific role in translation. Messenger RNA, or mRNA, provides the code for building the proteins. Ribosomal RNA, also called rRNA, is the used to make ribosomes, where translation takes place. Transfer RNA, or tRNA, carries amino acids, the building blocks of proteins, to the ribosome for creating the protein strands.
The presence of exons and introns was initially discovered in 1977 by Phillip A. Sharp, a scientist at MIT. He discovered that the functional mRNA that was produced was only about one third of the length expected based on the DNA. He found that large regions were removed from the mRNA after transcription to make it functional. This happens within the nucleus, before the mRNA is released into the cell to carry out translation.
The removal of introns involves at least four different kinds of small nuclear ribonucleoprotein particles, or snRNPs. The snRNPs recognize a specific sequence of RNA at the boundary between the intron and exon. When the snRNPs have bound to the RNA, they break the bond between the exon and intron and the intron drifts away. Different snRNPs then splice the two remaining exons together. This occurs along the length of the RNA strand to produce the functional mRNA.
Exons are so named because they survive this removal process and are then expressed, or used to produce proteins within the cell. Introns contain the sections of RNA that are removed. Each of the exons does not necessarily equal a single gene, and introns can occur within genes and also between them. This means that removing introns is very important in order to ensure the correct sequence is included in the gene. A mistake of a single base during the removal can mean that the resulting exons cannot be expressed, or used to make the proteins required by the cell.