Molecular genetics · BEGINNER LESSON
Intron
An intron is a stretch of RNA found in a gene that is removed from the initial RNA copy during splicing, so it does not remain in the mature messenger RNA (mRNA). Introns are defined by the spliceosome, the RNA-processing machine that recognizes splice sites and cuts and joins RNA pieces.
What an intron is
An intron is a region within a gene that appears in the first RNA copy made from DNA, called precursor messenger RNA or pre-mRNA, but is removed before the mature mRNA is used to make protein. The parts that stay and are joined together are called exons. Intron status is determined by splicing, not just by whether a sequence encodes protein.
[2] Intron - National Human Genome Research Institute (NHGRI) [3] Do not panic: An intron-centric guide to alternative splicing - PMC
How introns are recognized
Splicing is carried out by the spliceosome, a large RNA-protein complex. For intron definition, the spliceosome assembles across an intron by using sequence signals from the 5′ splice site (5′SS), the branch point sequence (BPS), and the 3′ splice site (3′SS). The sources also note that RNA secondary structure can help bring these elements together, and that disruption of such structures can reduce splicing.
[1] Intron definition, exon definition and back-splicing revisited - Nature Reviews Molecular Cell Biology [3] Do not panic: An intron-centric guide to alternative splicing - PMC
What intron size can change
In yeast, short introns often favor intron definition. In vertebrates, where introns are often longer and exons shorter, exon definition is thought to be more common. The same spliceosome complexes can define introns or exons, and the source notes that exon definition in long exons can also promote back-splicing, producing circular RNAs as a by-product.
Limits and caveats
An intron is not simply any noncoding DNA or RNA segment. Some introns can affect gene expression or RNA processing, and some retained introns may appear in mature RNA under certain conditions. Alternative splicing means whether a segment is treated as an intron can depend on the transcript isoform being made.
[2] Intron - National Human Genome Research Institute (NHGRI) [3] Do not panic: An intron-centric guide to alternative splicing - PMC
SEE THE IDEA
How an intron is processed
The first RNA copy contains both introns and exons.
This complex reads splice signals and assembles across the RNA.
The intron is cut out and the exons are joined.
The processed RNA keeps the exons that can be translated.
pre-mRNA -> spliceosome recognition -> intron excision -> mature mRNA
An intron is measured by whether it is removed during splicing, not by whether it can encode protein.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.