Question: Why is it that cells all have the same DNA, but the muscle cells have mitochondria that work and some that don't; so they must have different DNA? Can the mitochondrial DNA that doesn't work as well be inherited by a person's child?

Thiloka Ratnaike answered on 4 Jun 2019:

Hi Alice, this is a really great question and a very complicated/confusing field so I am glad you asked!
We can split your question into 2 parts:
1) Do cells contain different DNAs?
– Short answer: yes, there are 2 types: nuclear DNA and mitochondrial DNA.
All human cells apart from red blood cells contain mitochondria. Red blood cells need all the space they can get for carrying oxygen around so they don’t have a nucleus either. The rest of the body’s cells contain mitochondria in varying numbers. Each mitochondrion contains several (10s-100s) copies of mitochondrial DNA. Mitochondrial DNA is tiny compared to nuclear DNA, but codes some of the machinery which help the mitochondria to work and create energy.
Each cell contains many mitochondria so the numbers of mitochondrial DNA quickly escalate up.
However, each cell only contains 1 nucleus with 1 set of nuclear DNA.

2) How can defective mitochondrial DNA cause problems?
This is a really technical field and there are lots of researchers around the world working on this problem.
You inherit your mitochondrial DNA from your mum, because only the mitochondria in the mum’s egg is passed on when creating an embryo. Even if your mum doesn’t have a mitochondrial DNA problem causing her a disease, sometimes the mitochondria in her egg may contain a mitochondrial DNA that has a defect, and this can be passed into the embryo as it grows and this defective mitochondrial DNA is replicated and may go onto cause that baby problems or problems when they are older. It is sometimes completely by chance – 1 in a million chance and impossible to predict who is going to get the disease as a result of problem with the mitochondrial DNA.
Sometimes when people know they have a mitochondrial disease as a result of mitochondrial DNA problems- they can get help from Geneticists to think about their chances of having an affected baby and how to reduce this risk.
To further complicate things- the nuclear DNA also codes parts of the mitochondrial energy-producing machinery. SO, if the nuclear DNA has a problem that is specific to mitochondria- this can also lead to mitochondrial disease!!
An excellent question Alice, and a very tough field which is full of challenges, but very worthwhile if we can come up with some answers for the patients affected by these diseases.