Re: Is mitochondrial DNA similar in most eukaryotes?

The size of the mitochondrial genome varies among eukaryotic kingdoms: in protozoa, it's 20-40 kb (kilobases, thousands of base pairs); in animals, it's 16-19 kb; in fungi, it's 15-80 kb; and in plants, it's anywhere from 16 kb (in algae) to 150-2500 kb (in higher plants). There are several essential genes in the mitochondrial genome that are especially involved in replication, transcription, and translation of the mtDNA, along with the genes for the energy Æ ATP machinery. The major difference between eukaryotes is the existence of introns and intergenic DNA that make up the majority of the mitochondrial genomes, especially in vascular plants. Most of the introns found in these mitochondria are self-extracting, i.e. the intron RNA has enzymatic activity that allows it to remove itself from the messenger RNA without the help of proteins. Not including the non-coding material, there are very few genes that are different among mitochondrial genomes, suggesting that all mitochondria are derived from the same ancestral endosymbiont. Some of these genes have been replaced by nuclear genes which produce proteins that are actively transported into the mitochondria (some of these even encode RNA polymerase subunits that are essential for mitochondrial function).

This and more information on mitochondria can be found in:

Molecular Biology of the Cell by Bruce Alberts, et al. - 3rd ed., Garland Publishing, Inc., New York & London, 1995.

or any other biology textbook.