Human Mitochondrial DNA versus Nuclear DNA
The mitochondrion is a membrane-bound organelle found in most eukaryotic cells. It is also known as ‘the powerhouse of the cell’ because it generates most of the ATP used by the cell. There many compartments in mitochondria that within which specific processes take place – the outer and inner membranes, the inter-membrane space, cristae and matrix. Oxidative phosphorylation and ATP synthesis occur in the inner mitochondrial membrane. The citric acid cycle occurs in the mitochondrial matrix. Other functions of the mitochondria are participation in apoptosis, calcium signalling and steroid synthesis apart from regulating the redox potential of the cell. The mitochondrion is a special organelle because it is the only other organelle to contain DNA, other than the nucleus.
The human mitochondrial DNA is 16 kbps long and circular. This DNA encodes 37 genes. 13 genes encode the subunits of the respiratory complexes I, II, III and IV; 22 genes encode the 20 amino acids, there are two extra genes for serine and leucine, and two genes encode rRNA. Mitochondrial DNA is inherited only from the mother in sexually reproducing organisms. Thus property of the mitochondrial DNA is used to create haplogroups that trace the matrilineal origin of humans to ancestors in Africa. The most recent common ancestor is known as the mitochondrial Eve.
It differs from nuclear DNA in the following ways:
1. DNA is not packed into chromatin.
2. DNA is not bound by a nuclear envelope.
3. Most mitochondrial DNA does not contain introns.
4. Sometimes, the universal rule of translation of codons is not followed.
In these manners, the mitochondrial DNA resembles the DNA of bacteria. It is considered that the mitochondrion is the vestige of bacterial endosymbionts that were incorporated into the cytoplasm of the host cells some time in the planet’s evolutionary past.