What are microrna?

The ‘junk DNA’ that controls at least 30 per cent of our genes

When scientists sequenced the human genome, they expected to find hundreds of thousands of genes. As it turned out, we only have approximately 25,000, and most of the rest of our genetic code is ‘junk’. Or so we thought.

Genes are sections of genetic code that carry the instructions to build proteins; they are the manual for the human body. When cells want to make a protein, they make lots of temporary copies of its gene. These copies pass out of the cell nucleus and into molecular machines, which read the sequence and assemble the protein. For the cell to work properly, getting the timing right is crucial.

When the cell wants to turn production off, it could simply stop making copies of the gene, but old copies hang around in the cell and can carry on making protein. This is where micrornas (mirnas) come in. These short stretches of genetic code come from the ‘junk’ part of our genome. There are around 2,200 of them, and they fine-tune protein production.

As with normal genes, the cell makes temporary copies of their code, but these copies don’t tell the machines to make protein. Instead, they stick to the copies of protein-coding genes and stop them passing through the machinery. They can also ‘tag’ the copies for destruction, telling the cell to get rid of the unwanted code. mirnas keep protein production in check, making sure cells have exactly the right amount of protein at the right time.