microRNA

Imagine a tiny editor that can dampen a message after it has already been written out. A microRNA is a very short RNA molecule that does just that to other RNAs, turning down a gene's output without ever touching the DNA.

Mechanistically, a microRNA is about twenty-two nucleotides long and does not code for a protein. It pairs with matching sequences on a messenger RNA and, guided by a protein complex, either blocks that messenger from being translated or marks it for degradation. A single microRNA can target many different messengers, and one messenger can be hit by several microRNAs.

Because they act after transcription, microRNAs add a fast, tunable layer of control on top of the slower decisions made at the DNA level. They help fine-tune development, keep gene networks balanced, and respond to stress.

Their dysregulation is linked to many diseases, including cancer, which has made microRNAs attractive both as biomarkers and as potential drug targets. They are one well-studied member of the broader family of non-coding RNAs.