Mitochondrial DNA Definition

Mitochondrial DNA comprises the genetic material of the organelle mitochondrion that codes for all the proteins required in the mitochondrial metabolic reactions.

What is Mitochondrial DNA?

Besides the nuclear DNA, the eukaryotic cell has 2 extranuclear DNA out of which mtDNA and the other one is the chloroplast DNA. The constitution of mtDNA is structurally similar to nuclear DNA. Both are polymers of monomeric nucleotides. These nucleotides comprise a nucleobase, a sugar, and a phosphate group. DNAs are organic compounds that central to the regulation and functioning of the cell.

Mitochondrial DNA History

The mt DNA was discovered by Sylvan Nass and Margit M. K. Nass. Employing the technique of electron microscopy, they discovered it in the mitochondria of the chick embryo. These findings were reported and published in 1963 in The Journal of Cell Biology. Later it was also found in the yeast by a different research team. To distinguish it from nuclear DNA, it was termed mtDNA.

Mitochondrial DNA Structure and Characteristics

The mtDNA differs from nuclear DNA in terms of its genome referred to as mitogenome. This extranuclear DNA may be linear or circular depending on whether the eukaryote is unicellular or multicellular. Linear mtDNA is found in the case, whereas in most mammals, it is a ds circular DNA.

One strand of this mtDNA is referred to as a heavy strand (H-Strand) as it is guanine rich, while the other strand forms a light strand (L-Strand) and is cytosine rich. A total of 37 genes are found in mitogenome out of which 28 genes are found in L-Strand and the rest in H-Strand. These genes code for 2 rRNA, 22 tRNA, and 13 proteins. The mtDNA is semi-autonomous as it can code for the proteins it requires.

Mitochondrial DNA vs Chloroplast DNA

Found in the cytoplasm both cpDNA and mtDNA are semi-autonomous self-reproducing extranuclear DNA. They both show maternal inheritance and both are believed to have endosymbiotic origins. Both this DNA is found as circular DNA in the case of multicellular organisms which also hints about their endosymbiotic origin.

They both have different genomes, the genome of cp DNA is more complex and larger having 120 genes as opposed to 37 genes of mtDNA. Both can encode the required proteins and RNAs. For example, cpDNA codes for photosynthetic proteins, while mtDNA encodes proteins involved in ETC.

This extranuclear DNA also occurs in several copies as there are multiple organelles found in a cell may be ranging to thousands. The cytoplasmic organelles mt and cpDNA randomly segregate during meiosis and mitosis.

Mitochondrial DNA Inheritance

The organelle mitochondrion generates ATP via cellular respiration. It is a double membraned organelle having an outer and inner membrane and the space between the 2 is referred to as the mitochondrial matrix. Many mitoribosomes, enzymes, and genetic material occupy this matrix. This organelle is inherited from the mother’s ovum cytoplasm, unlike nuclear DNA.

The sperm after fusion releases its nucleus into the cytoplasm of the ovum leaving behind its mid-piece that does contain mitochondria. The DNA of sperm and ovum undergo syngamy, and the resulting zygote acquires the mitochondrial DNA.

Mitochondria show maternal inheritance and thus shows the non-Mendelian pattern, and does not adhere to the Mendelian laws. The ovum generally contains about 200,000 mtDNA molecules. Any mutation in these can cause mitochondrial diseases.

Mitochondrial DNA Replication

DNA Replication: The mtDNA replication is also semi-conservative that involves DNA polymerases. Their replication requires some additional nucleus-encoded proteins known as Twinkle a helicase and mitochondrial SSB proteins. This indicates that the replication of mtDNA is regulated by nuclear DNA.

Mitochondrial DNA Transcription: In the case of mt DNA transcription, 3 promoters are involved in the case of humans. These include heavy strand 2 promoters, Heavy strand 1 promoter, and light strand promoter. A polycistronic mRNA that codes for more than 1 polypeptide is produced on transcription of the heavy strand. Proteins and transcriptional factors involved in the initiation of mitochondrial transcription include TFB1M, POLRMT, TFAM, and TFB2M.

Mitochondrial DNA Repair: Any aberrations in the mtDNA are repaired in 2 ways. The repair can either include direct reversal of the process that in the first case caused mutation or by replacing the damaged base. In mitochondrial DNA repair, a unique mechanism is suggested. The damaged or mutated region is degraded and repaired, and replication of integrity is maintained through the degradation of excessively damaged genomes followed by the replication of an intact mtDNA.

Mitochondrial Diseases and Disorders

If mtDNA is exposed to free radicals then it can be damaged or mutated. The free radicals increase following this. It can lead to severe neurodegenerative diseases that progress with age like Parkinson’s disease, coronary artery disease, and Alzheimer’s disease. The mutated mtDNA can cause diseases being inherited. Some examples of mitochondrial diseases include DAD, mitochondrial myopathy, LHON, NARP, Leigh syndrome, and MERRF.

Biological Function of Mitochondrial DNA

MtDNA is semi-autonomous and encodes for proteins required in mitochondrial reactions like the enzymes, complexes, and other proteins in ETC, for example, NADH dehydrogenase, cytochrome c oxidase, cytochrome b, and ATP synthase. The mtDNA in humans encodes micro peptide humanin that is coded by the MT-RNR2 gene. The L-Strand of mtDNA also codes for 16S rRNA, 12S rRNA, MT-RNR1, and MT-RNR2 gene.

Mitochondrial Eve and Research

As mtDNA denotes maternal inheritance, using its genome we can trace our history of the female line and the recent ancestor labeled as Eve. This theory is known as Mitochondrial Eve that also shows the relatedness of different populations.

The mtDNA is essential for different processes and is significant in phylogenetic studies. The study of mtDNA and its genomes comprises mitochondrial genetics. This research and study could have important implications for people suffering from mitochondrial diseases. It has also great scope in forensics for the identification of a person through his mtDNA.