Gene structure: exon, intron, promoter
A gene consists of several parts: a promoter that switches it on and off, exons that are kept, and introns that are cut back out. After transcription the introns are removed and the exons joined into the finished messenger RNA. Only this spliced text is the template for the protein.
The parts of a gene
In front of the gene proper sits the promoter, a switch where reading begins. Then exons and introns alternate, framed by untranslated regions at the start and end. The exons hold the coding information, the introns separate it. This structure is the rule, not the exception, in almost all human genes.
Splicing
Reading first produces a raw transcript that still contains all introns. A machinery of proteins and RNA cuts the introns out and joins the exons seamlessly. That genes come in pieces was discovered in 1977; the terms exon and intron were coined by Walter Gilbert a year later. Only the spliced messenger RNA leaves the nucleus.
Why the location matters
A variant in an exon can change the amino acid sequence of the protein. A variant at a splice site can cause an exon to be lost or an intron to stay in. A variant in the promoter does not change the protein itself but how much of it is made. That is why the position of a variant is often as important as the letter change itself.
What Genome measures. Whether a variant sits in an exon, an intron or the promoter region often decides its effect. Genome locates every marker within this structure, so it is clear what it can influence.
Related topics
Sources
- 1Berget, Moore & Sharp, 1977 Spliced segments at the 5′ terminus of adenovirus 2 late mRNA. PNAS 74:3171–3175. doi.org/10.1073/pnas.74.8.3171
- 2Gilbert, 1978 Why genes in pieces?. Nature 271:501. doi.org/10.1038/271501a0