What is DNA?

What is mtDNA?

In addition to the chromosomal DNA present in the nucleus of our cells, there exists another type of DNA which is found in specialized cell structures called mitochondria. Mitochondria are located in the cytoplasm (the fluid that surrounds the nucleus) and are involved in a wide range of processes like providing cellular energy, cell death, synthesis of different enzymes etc.
In contrary to the linear chromosomal DNA, mitochondrial DNA (mtDNA) is a circular molecule. It is thought to have its origin from the circular genome of bacteria which have been incorporated by eukaryotes during evolution (Endosymbiotic theory).

There are several mtDNA molecules (2-10) found throughout the mitochondrial network with a total number of copies ranging from 100 to 10,000 per cell depending on tissue type. Egg cells have much more mtDNA copies, whereas sperm cells contain much less. Among other things, that's why the mtDNA is normally inherited by the female (unlike chromosomal DNA, which is inherited by both parents).
A relatively high mutation rate compared to the chromosomal DNA makes the mtDNA useful for tracking ancestry or in forensic laboratories for identification of human remains.

The mammalian mtDNA contains between 15,000 and 17,000 base pairs and is therefore much smaller than the chromosomal DNA. As mitochondria play a dominant role in energy conversion by oxidation of substrates like glucose, many genes on the mtDNA encode for enzymes of the respiratory chain. The other genes encode for transfer-RNAs (tRNAs), which are the intermediators between the genetic code and the amino acids necessary for protein formation, and ribosomal RNAs (rRNAs), which form part of the structure and function of the ribosomes. Interestingly, most multicellular organism contain the same kinds and amount of these genes although the size of the mtDNA may vary considerably (e.g. some plants have huge mtDNA molecules with more than a million base pairs).
The replication of the mtDNA starts in a non-coding area of the mitochondrial DNA molecule, called the control region or D-loop.

Human mtDNA contains 16,569 base pairs consisting of 37 genes, with 13 genes encoding for proteins of the respiratory chain, 22 genes for tRNAs and 2 genes for rRNAs.