The Rogues' Gallery

So Why Do Mitochondria Still Have DNA

Bob Novella February 26, 2016 5 Comments

A new biological simulation solves a long-standing mystery why mitochondria still have DNA.

Mitochondria are my favorite organelles. Organelles are the specialized nanomachines floating within eukaryotic cells and carrying out the critical tasks of cellular life. Examples include the Golgi apparatus, lysosomes, the endoplasmic reticulum etc.

Some organelles are extra special, like our own mitochondria or the chloroplasts found in plant cells. They did not evolve with the cell from the beginning like the other organelles. It is thought that 1.5 billion years ago, they were independent single-celled organisms that early eukaryotic cells absorbed. I’m not saying they were eaten. These ancestral mitochondria somehow permanently ensconced themselves within the host cell. They then evolved together into quite a wonderful symbiotic relationship. This is called the endosymbiotic theory and it likely changed the course of evolution on the planet by being one of the key evolutionary developments leading to multicellular life

This is because mitochondria are powerful biological batteries in a sense. Using oxygen, they create the energy currency of biology. This takes the form of ATP or Adenosine Tri-Phosphate. ATP transports chemical energy within cells and anything important that happens likely needs ATP. This runs the gamut from a simple eye-blink to the whole of metabolism itself. People typically create and use 40kg or 88 pounds of it in a single day. An athlete can use an astounding 70kg or 154 pounds in the same period of time.

Mitochondria have their own genes, which is just one of the primary reasons why we think they were absorbed into what became modern eukaryotic cells. They have not remained static over all this time however. They could have had as many as 2000+ genes initially, but over the eons, many of those genes were outsourced into the nuclear DNA of our genome. Some genes remain however. Humans for example have 37 genes remaining in each of our mitochondria. Why? Why didn’t they transfer permanently into the probably safer nucleus like the others? That has been a mystery for many decades.

Ben Williams, a postdoctoral fellow at the Whitehead Institute for Biomedical Research, Iain Johnston, a research fellow at the University of Birmingham and others decided to try to answer this question in a way that had never been done before. They accumulated much of what’s been learned about mitochondrial DNA into a computer simulation. That’s more than 2000 different mitochondrial genomes from plants, animals, fungi, and protists like amoebas. Using this, they created an algorithm that determined when each gene likely jumped ship.

This analysis allowed them to conclude a couple of things

The genes that are retained are critical for constructing the mitochondria’s internal structure. These are responsible for keeping the genes together and resist breaking apart. The creation of ATP essentially turns each mitochondrion into a hazardous environment. The free radicals that are created are a byproduct of metabolism, but they also can wreak havoc. Without those critical genes right there in the mitochondrial genome they many never have survived for long.

The local DNA also can help regulate the production of energy to an exquisite degree. The cell is able to control each individual mitochondrion as needed. Instead of having to make a sweeping change to all the 100s or 1000s of mitochondria in a cell, the changes can be made on an individual basis resulting in far better control and fine tuning of energy production.

This pattern of gene outsourcing closely matched many of the different organisms the researchers studied. They all transferred similar genes over similar timeframes. This means that certain aspects of evolution can follow the same path multiple times instead of being random. This makes evolutionary prediction much easier.

Says Johnston:

“If we can harness data on what evolution has done in the past and make predictive statements about where it’s going to go next, the possibility for exploring synthetic biology and disease are massive”

In the future, this algorithm that they developed could help elucidate other endosymbiotic organelles like the chloroplasts of plant cells. We can also learn about devastating mitochondrial diseases.

So the next time you’re running, blinking, or just digesting, give a special thought to those little bacteria we absorbed long ago that make it all possible.