Mitochondrial DNA is a double stranded circular molecule, which is inherited from the mother in all multi-cellular organisms, though some recent evidence suggests that in rare instances mitochondria may also be inherited via a paternal route. Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided. There are about 2 to 10 transcripts of the mt-DNA in each mitochondrion. Compared to chromosomes, it is relatively smaller, and contains the genes in a limited number.
The size of mitochondrial genomes varies greatly among different organisms, with the largest found among plants, including that of the plant Arabidopsis, with a genome of 200 kbp in size and 57 protein-encoding genes. The smallest mtDNA genomes include that of the protist Plasmodium falciparum, which has a genome of only 6 kbp and just 2 protein- encoding genomes. Humans and other animals have a mitochondrial genome size of 17 kbp and 13 protein genes.
Mitochondrial DNA consists of 5-10 rings of DNA and appears to carry 16,569 base pairs with 37 genes (13 proteins, 22 t-RNAs and two r-RNA) which are concerned with the production of proteins involved in respiration. Out of the 37 genes, 13 are responsible for making enzymes, involved in oxidative phosphorylation, a process that uses oxygen and sugar to produce adenosine tri-phosphate (Fig. 4.56). The other 14 genes are responsible for making molecules, called transfer RNA (t-RNA) and ribosomal RNA (r-RNA). In some metazoans, there are about 100 – 10,000 separate copies of mt-DNA present in each cell.
Unlike nuclear DNA, mitochondrial DNA doesn’t get shuffled every generation, so it is presumed to change at a slower rate, which is useful for the study of human evolution. Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts and has been implicated in a number of genetic diseases, such as Alzheimer’s disease and diabetes. Changes in mt-DNA can cause maternally inherited diseases, which leads to faster aging process and genetic disorders.
Mitochondria convert the potential energy of food molecules into ATP by the Krebs cycle, electron transport and oxidative phosphorylation in presence of oxygen. The energy from food molecules (e.g., glucose) is used to produce NADH and FADH2 molecules, via glycolysis and the Krebs cycle. The protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, cytochrome c oxidase) use the released energy to pump protons (FT) against a gradient.