Mitochondria produce energy-rich ATP molecules and are the only organelles in the animal cell to have their own genetic system.  

The present-day mitochondrion is thought to have evolved from an aerobic bacterium, which formed a mutually beneficial, or symbiotic, association with its predator. 

Over time, many genes from this bacterium were transferred to the nuclear DNA of the host cell and other genes were lost, leaving behind a small but independent mitochondrial DNA.

Some cells, like muscle cells, can contain hundreds of mitochondria, while others, like red blood cells, do not contain any. Each mitochondrion can contain up to 10 copies of the mitochondrial DNA.

Mitochondrial DNA is a closed, circular molecule varying in length from 14,000 to 20,000 bps, in animal cells, as compared to millions of base pairs present in nuclear DNA. 

This mitochondrial DNA encodes only a small number of biomolecules: the 16S and 12S rRNAs, up to 25 tRNAs, and 13 respiratory chain proteins. Nuclear DNA codes for the remaining proteins required for mitochondrial function.

Around 93% of the mitochondrial DNA codes for proteins, unlike nuclear DNA, where only about 1 percent are coding regions. This is partially because introns, which are a regular feature of eukaryotic DNA, are absent in mitochondrial DNA.  

Several sequences of the genetic code are translated differently depending upon the type of DNA. For example, the codon UGA codes for tryptophan in mitochondrial DNA, whereas it is a stop codon in nuclear DNA.

Mitochondrial DNA has a faster rate of evolution than that of nuclear DNA due to the mutation rate in mitochondrial DNA being greater than 10 fold higher. 

This is because mitochondrial DNA is not protected by histones like that of nuclear DNA and is exposed to reactive oxygen species generated during mitochondrial reactions. Additionally, it also has less efficient DNA repair machinery.

Transfer of mitochondrial DNA always happens from mother to offspring. This is known as maternal inheritance. 

Maternal inheritance occurs because, after fertilization, the few mitochondria present in sperm are degraded while the many mitochondria in the ovum remain present in the embryo and are passed onto all the cells in the offspring.