Re: How is the DNA from mitocandria passed on and why does it have its own DNA?

Mitochondrial DNA (mtDNA) is carried inside the mitochondria which continuously divide on their own. Most cells contain hundreds of mitochondria, such that even cell division randomly distributes half of the the mitochondria in each daughter cell. In cases of uneven cell division, like meiosis, most of the mitochondria end up in the larger daughter cell, like the ovum (egg cell). Sperm contain mitochondria which power their flagellar tails, however since mitochondria are carried in the cytoplasm and very little cytoplasm is transferred from the sperm to the egg, few, if any, mitochondria are inherited from the father. This has prompted geneticists to use mtDNA as an evolutionary clock to trace all humans back to one ancestor.

The two chief differences between mtDNA and nuclear DNA are that the mitochondrial genome is contained on a single circular chromosome rather than multiple linear chromosomes, and that there are 5 to 10 copies per mitochondrion of the mitochondrial chromosome rather than just two copies of each chromosome in the nucleus. There are also differences in the replication and transcription of mtDNA, such as using different tRNAs and ribosomes, which are biochemically more similar to bacteria than eukaryotes (organisms with nuclei). This has prompted most biologists to view these organelles as having evolved from endosymbiotic bacteria. Endosymbionts are bacteria that live inside other organism's cells and serve some function: a more recent example would be the Rhizobacteria that enter the root cells of legumes and allow them to live in nitrogen-poor soil. The Rhizobacteria "fix" nitrogen from the atmosphere, converting it into nitrates that the plant can use. Mitochondria evolved specifically from a-purple bacteria which became an integral part of eukaryotic life before the evolution of higher organisms.

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.