Humans have two genomes. One is located in the nucleus. It is three billion DNA base pairs long, tightly wrapped up into chromosomes, and carries 99.94% of our genes that encode more than 90,000 proteins.
The second genome is tiny. Comprising just 16,000 base pairs that code for only 13 proteins, it is tucked away inside the mitochondria, a remarkable cellular organelle that generates the energy that powers the body’s biological processes. Each mitochondrion possesses several copies of this tiny circular genome of “mtDNA”, which has properties more like bacterial DNA than our own thanks to its evolutionary past.
Somewhere around 1.5 billion years ago, one cell engulfed a bacterium and an amazing thing happened: the two began working together. The cell likely provided protection for the bacteria, while the bacteria provided energy for them both. Over the course of evolution, as the two became one, the majority of the genes encoded by the bacteria’s DNA were transferred to the nucleus of the cell, but a select few remained, and these few genes make up the mitochondrial genome as we know it today.
