Mitochondrial DNA
DNA that is found in the cytoplasm of cells, outside of the nucleus, is referred to as cytoplasmic DNA. Examples of cytoplasmic DNA can be found in organelles such as mitochondria and chloroplasts (the organelles responsible for photosynthesis in plants). Mitochondria are responsible for the energy of the cell. Therefore, they have DNA that codes for enzymes necessary for energy-generating processes such as respiration and fatty acid biosynthesis. Mitochondrial genes also produce RNA, since most RNA cannot pass through the mitochondrial membrane.
Mitochondrial genomes are usually circular, double-stranded, supercoiled molecules of DNA, although linear genomes (like those shown in this animation) do exist in some protozoa and fungi. Mitochondrial DNA differs from nuclear DNA in its sequence, quantity in the cell, and mode of inheritance.
Mitochondrial DNA has a greater percentage of guanine- cytosine base pairs than nuclear DNA. As noted in the upcoming DNA Chemistry section, adenine-thymine pairs are connected by two hydrogen bonds, while guanine- cytosine pairs are connected by three hydrogen bonds. Thus, mitochondrial DNA has more hydrogen bonds than nuclear DNA.
Mitochondrial DNA is located in a structure called a nucleoid. Each mitochondrion contains several nucleoid regions, which in turn contain several copies of the mitochondrial genome. Finally, each cell can have thousands of mitochondria. Thus, each animal somatic cell possesses thousands of copies of mitochondrial DNA, as compared to only two copies (maternal and paternal) of nuclear DNA.
In animals, nuclear DNA is usually inherited evenly from both parents. Mitochondrial DNA, in contrast, is typically inherited from the mother because the male gamete contributes little or no mitochondria to the progeny. Thus, mitochondrial genomes do not undergo the shuffling and recombination that the maternal and paternal nuclear genomes do, and the sequence of mitochondrial DNA remains consistent through generations unless mutations of copying accidents occur. This fact makes mitochondrial DNA very useful in evolutionary studies and some identification cases.