A new route to evolution: How DNA from our mitochondria works its way into our genomes

Scientists have shown that in one in every 4,000 births, some of the genetic code from our mitochondria — the ‘batteries’ that power our cells — inserts itself into our DNA, revealing a surprising new insight into how humans evolve.

In a study published today in Nature, researchers at the University of Cambridge and Queen Mary University of London show that mitochondrial DNA also appears in some cancer DNA, suggesting that it acts as a sticking plaster to try and repair damage to our genetic code.

Mitochondria are tiny ‘organelles’ that sit within our cells, where they act like batteries, providing energy in the form of the molecule ATP to power the cells. Each mitochondrion has its own DNA — mitochondrial DNA — that is distinct to the rest of the human genome, which is comprised of nuclear DNA.

Mitochondrial DNA is passed down the maternal line — that is, we inherit it from our mothers, not our fathers. However, a study published in PNAS in 2018 from researchers at the Cincinnati Children’s Hospital Medical Center in the USA reported evidence that suggested some mitochondrial DNA had been passed down the paternal line.

To investigate these claims, the Cambridge team looked at the DNA from over 11,000 families recruited to Genomics England’s 100,000 Genomes Project, searching for patterns that looked like paternal inheritance. The Cambridge team found mitochondrial DNA ‘inserts’ in the nuclear DNA of some children that were not present in that of their parents. This meant that the US team had probably reached the wrong conclusions: what they had observed were not paternally-inherited mitochondrial DNA, but rather these inserts.

Now, extending this work to over 66,000 people, the team showed that the new inserts are actually happing all the time, showing a new way our genome evolves.

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