Cell, the basic structural and functional unit of every organism! This miraculous invention changed the outlook on how people looked at life. The beautiful smallest unit has a tinier substance that holds the secret to every life on earth called the nucleus. And the secret? Well as everybody is aware, DeoxyriboNucleic Acid (DNA) forms the basis of every life. It determines the traits, characters, and habits we inherit, the composition of our organic molecules, strategy of body functioning, the color of our hair, our eyes, the texture of our skin, and many more. But the nucleus is not the only miraculous substance that provides us with DNA.

With the invention of the Cell by Robert Hooke, came different components of the cell and so did Mitochondria, our very own powerhouse of the cell invented by Albert von Kolliker. Located in the cytoplasm, Mitochondria is known to be the main source of energy as it synthesizes Adenosine Triphosphate as the source of energy from the food we eat. It is due to this process that humans are energetic and have the strength to perform various activities depending on their age. But mitochondria have a lot more features than being just a powerhouse.

It is studied that mitochondria evolved from proteobacterial ancestors and came into a symbiotic relationship with an eukaryotic cell. As time passed, many genes from mitochondria were transferred to the nucleus. But few of them were retained, which also proved to be extremely beneficial as they held the origin of life – DNA. Yes! There is something called mitochondrial DNA abbreviated as MtDNA and is found in the genes present in mitochondria.

Features of Mitochondrial DNA

Mitochondrial DNA is comparatively smaller in structure than Nuclear DNA. It appears as small round chromosomes. In humans MtDNA has around 16,500 base pairs, that is the building blocks of DNA. MtDNA consists of 37 genes that are essential for regular mitochondrial functions.

Thirteen of them are structured for making enzymes involved in Oxidative Phosphorylation, the process which uses oxygen and simple sugars to create Adenosine Triphosphate. The rest of them is used for making molecules called transfer RNA (tRNA) and ribosomal DNA (rRNA) which helps in assembling amino acids, transforming them into functional proteins.

Mitochondrial DNA is always passed on from the mother to the offspring, because it is always transferred through the reproductive cell ovum and is never found in sperm, and it never recombines in sexual reproduction. Any defect or mutation in mitochondrial DNA can cause diseases that make the body extremely weak and unable to produce energy ultimately affecting muscles and other organs like the kidney and the brain.

In mammalian sperm cells, the copy number of mtDNA is low, whereas in mammalian oocytes the copy number is extremely high. Therefore, the maternal inheritance of mtDNA observed in early studies could simply have been the result of dilution of the paternal contribution beyond the detection limit of restriction enzyme analysis.

In the structure of MtDNA, most information is encoded on the heavy (H) strand, with genes for two rRNAs, 14 tRNAs, and 12 polypeptides. The light (L) strand codes for eight tRNAs and a single polypeptide. 

The discovery of mitochondrial DNA is credited to Margit M. K. Nass and Sylvan Nass. By electron microscopy, they found the nucleic acid inside the chick embryo mitochondria. They published their findings in The Journal of Cell Biology in 1963. The following year, an independent research team published a similar finding; they found extranuclear DNA in the mitochondria of yeasts. The entire information on mitochondrial DNA is referred to as mitochondrial genome or mitogenome.

Transcription of Mitochondrial DNA

DNA carries the genetic information that codes for a particular protein. Thus, during protein translation, the genetic code for a protein is first copied into the RNA (specifically, mRNA). This process of creating a copy of DNA into mRNA through the help of the enzyme RNA polymerase is called transcription.

In mitochondrial transcription in humans, it is initiated from the three promoters: Heavy strand 1 promoter, heavy strand 2 promoters, and light strand promoter. Transcription on the heavy strand produces a polycistronic mRNA, which is a type of mRNA that encodes for more than one polypeptide in the same RNA molecule.

The types of proteins that initiate mitochondrial transcription are mitochondrial RNA polymerase (POLRMT), mitochondrial transcription factor A (TFAM), mitochondrial transcription factor B1 (TFB1M), and mitochondrial transcription factors B2 (TFB2M). POLRMT, TFAM, and TFB1M (or TFB2M) begin transcription at the mitochondrial promoters.

Mitochnodrial Genetic Disorders

Mutation or the defect that is experienced by DNA at some stage of its life is the same in almost all forms of DNA. Certain dangerous diseases are caused by mutation of MtDNA referred to as mitochondrial genetic disorders.

  • Cancer: Somatic mutation of MtDNA can result in cancerous growth( exponential growth of non-required cells). The presence of mutated MtDNA is usually found in breast cancer, colon cancer, and liver cancer.
  • Labour Hereditary Optic Neuropathy: It is caused due to mutation in MT-ND1, MT-ND4, MT-ND4L, and MT-ND6 forms of MtDNA. These defects cause an inherited form of vision loss followed by a lack of production of ATP in the affected body.
  • Non-Syndromic Deafness: Primer hearing loss without syndrome and is caused due to mutation in MtDNA on MT-RNR1 and MT-TS1.

Forensic Significance and Applications of MtDNA

After receiving evidence from crime scenes, one of the challenges faced by forensic experts is to analyze and draw a conclusive result from a trace amount of samples, which turn out to be not sufficient especially in cases of DNA testing leading to individualization.

Unlike nuclear DNA, mitochondrial DNA is found in abundance due to the presence of infinite mitochondrial cells in human beings, hence linking a sample in a maternal lineage from the list of suspects helps in the identification of the criminal. The fact that MtDNA can be obtained from the most common sample being hair is icing on the cake.

Also, mitochondrial DNA does not suffer as much degradation as nuclear DNA as they are well protected inside cells. Anyone from a maternal lineage could be used as a reference for the identification of individuals. In anthropological genetics, mtDNA is useful to trace the geographic distribution of genetic variation, for the investigation of expansions, migrations, and other patterns of gene flow.

Even though nuclear DNA proves to be important and easy with the technology of PCR nowadays, MtDNA has a higher copy number than nuclear DNA inside the cell, which proves as an advantage.

Other than forensics, mtDNA is also helpful in food production and agricultural practices. In the early 1930s, scientists invented how to alter nuclear DNA in plants which led to the hybridization of crops leading to a faster and more efficient supply of food.

Recently in the year 2019, Japanese scientists discovered how to alter the mitochondrial DNA of plants which will now help in the betterment and efficiency of the existing agricultural technologies owing to the rapid hike in prices of essential resources like food and other natural resources.


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