This photomicrograph shows Cyanobacteria (green) found
in a common pond. Image source: Wayne LanierLast blog I talked about mitochondria. These are the parts of a cell that ultimately turn our food into energy. They also have a very interesting past.
A billion years ago or so, mitochondria were free living bacteria. Then our ancestors hijacked them and now they do our bidding. And mitochondria aren't the only cells that got hijacked. So did the chloroplast’s ancestors.
Chloroplasts are the part of a plant cell that turns sunshine into sugar. Every green plant that we’ve looked at has them. And chloroplasts were almost certainly once free living cyanobacteria.
Both mitochondria and chloroplasts still have many bacterial qualities including having their own DNA. But they don't have a lot of their old DNA left. Most of it has migrated to where the rest of our DNA is kept—the nucleus. Or at least that's the theory.
Do scientists have any proof that DNA can move in a cell from compartment to compartment? As a matter of fact they do. At least with the chloroplast.
Scientists used their ability to put DNA specifically into a chloroplast or mitochondrion to design an experiment to look for cells where DNA had migrated. What they did was put some DNA into a chloroplast that could only be read in the nucleus. (Remember, chloroplasts and mitochondria are different enough that nuclear DNA doesn't work there and vice versa.)
The DNA they put in made the plant resistant to a poison IF the DNA could be read. One way the plant could survive was if the DNA they put in the chloroplast ended up moving from there to the nucleus. And it did.
In fact, it was pretty common in their experiment. The DNA moved in something like 1 in 16,000 pollen cells. A rate like this suggests that, for example, different cells on the same leaf might have different amounts of chloroplast DNA in their nuclei.
So DNA can move from the chloroplast to the nucleus. And probably from the mitochondrion to the nucleus too. The evidence is less direct for this but there is plenty of DNA in the nuclei of lots of different plants and animals that looks very mitochondrion-like.
This all fits in with our understanding that DNA is not as stable as a lot of people think. DNA changes between generations and within an organism. Chromosomes can get rearranged, genes copied or deleted, small DNA changes can happen and who knows what else. And these changes are a big part of the motor that drives evolution.