I know that mitochondria of eukaryotes have their own DNA, more similar to that of bacteria than to the rest of the cell's DNA. I also know that a cell can have plenty of mitochondria, and I understand each of them has it's own DNA. Is it the same DNA?
I know that when a human is conceived, the mitochondria of the sperm are destroyed and don't contribute to the new cell. If there were no mutations and the process started with a cell with homogenous mDNA, that would mean that the mDNA not only within one cell but among all of the descendants was the same. But there are mutations because we have different mDNAs. Then shouldn't the mutations cause variety within single cells? It's hard for me to believe that there can even be a single human cell in the world that has homogenous mDNA because, if I understand correctly, an ovum retains many mitochondria during conception, not just one, so at least some of the variety within the ovum should be passed over to the conceived human. But perhaps it's extremely difficult for a cell with variously DNAed mitochondria to survive? Can it be estimated what the percentage of human cells with homogenous or nearly homogenous mDNA is across the world?
I'm asking this because I've heard that the mDNA can be considered another chromosome. I wonder if then every single piece of mitochondrial DNA should be considered another chromosome.
Please note that all I know about biology is what I learned in high school several years ago and the bits I know from Wikipedia and other internet sources. I would like to ask for possibly accessible answers.
EDIT: Reading up on this, I came across the term heteroplasmy. So it does happen that mitochondria of an organism can store different genetic information. But the article seems to suggest that it is an anomaly rather than the normal course of things. That seems strange to me. I understand of course that some mutations have to be harmful, but shouldn't some mutations and some kinds of in-cell variety turn out to be neutral or even advantageous?
I've also just read that a mitochondrion stores more than one copy of its DNA. That seems to allow for variation even within a single mitochondrion.
EDIT 2: I think it's possible that my question is unclear. There seem to be four levels on which mDNA variety can be considered: a species, an organism, a cell, a mitochondrion. I ask the answerers to focus on the third one. I will be happy to learn about the other levels too and I will consider such information on-topic, but my main question is about what happens in a cell.