[katieHaylor (OP)]

Greg says:

Since mitochondrial DNA is only inherited from the mother (in animals), does that mean it has not and cannot evolve?  Mitochondrial DNA is so much smaller [than nuclear DNA], are researchers close to a complete understanding of its functions and proteins?

Can mitochondrial DNA be edited using CRISPR-Cas 9, and if so, are there any research projects underway? Given the importance of mitochondria as the cell's powerhouse and possible lack of evolution, it seems ripe for experimental research.

What do you think?

[evan_au]

does that mean (mitochondrial DNA) has not and cannot evolve?

The factors driving evolution are a population which has a source of variation (eg mutation), and a selection pressure.

Mitochondria represent a population:
- within a single cell, with up to 2000 mitochondria in a liver cell.
- Each mitochondria typically has several copies of its genome.
- And different cells have different mitochondria populations, as do different individuals.

There are sources of genetic variation:
- Mitochondria are the cells powerhouse, and are subject to some reactive chemical reactions which could mutate mitochondrial DNA.
- When a mammalian cell divides, mitochondria are thought to be randomly allocated to the daughter cells. This is another source of variation.

There are selection pressures on the mitochondria:
- A daughter cell which receives badly mutated mitochondria will die, and be replaced by another cell which (hopefully) has a better set of mitochondria.
- When a cell's energy demands drop, the quantity of mitochondria reduce. This represents a selection pressure.
- When an egg cell is maturing, typically several will ripen at once, but one "wins the race". This is presumably one which has a good set of mitochondria
- At the level of an individual, someone who inherits a mitochondrial disease will be at a disadvantage
- There is some evidence that mitochondria can recombine their DNA into new forms. I guess this might be similar to horizontal gene transfer between bacteria.
- Mitochondria have relatively few genes, compared to the bacteria from which they are thought to have evolved, and far less than the cell's nuclear DNA. It is thought that over time, many mitochondrial genes have transferred to nuclear DNA; in this form, they will be able to recombine during production of eggs and sperm.
 
See: https://en.wikipedia.org/wiki/Mitochondrion#Replication_and_inheritance

Can mitochondrial DNA be edited using CRISPR-Cas 9

I understand that the answer here is "no".
But I am sure that researchers are working to develop techniques that work on mitochondrial DNA.

only inherited from the mother

Some researchers have suggested that one reason men don't live as long as women is that a mutation that harmed a woman would be selected against, while a mutation that only harmed men would persist in the mitochondrial genome.