With the invention of the Cell by Robert Hooke came different cell components, 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 source of energy as it synthesizes Adenosine Triphosphate as the source of power from the food we eat.
Due to this process, 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.
As we all know, the genetic information that determines the characteristics and traits of an individual is stored in the DNA in the form of code made up of four chemical bases Adenine(A), Guanine(G), Cytosine(C), and Thymine(T).
Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. These bases pair up A with T and C with G to form units called base pairs. Each of them is attached to a sugar molecule and a phosphate molecule. All three of these, a base, sugar, and phosphate molecules, are called a nucleotide. These nucleotides are arranged in two long strands that form a spiral called a double helix, and this is the structure of DNA.
DNA has the unique property of replication. DNA can replicate itself or, in simple terms, make copies of itself. This feature of our DNA makes it extremely important to forensic scientists and other biotechnology subfields.
It is studied that mitochondria evolved from proteobacterial ancestors and came into a symbiotic relationship with a eukaryotic cell. As time passed, many genes from mitochondria were transferred to the nucleus. But few of them were retained, which proved to be extremely beneficial as they held the origin of life – DNA. Thus, mitochondrial DNA (mtDNA) is found within the cell’s powerhouse.
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 is used to make molecules called transfer RNA(tRNA) and ribosomal DNA (rRNA), which help assemble amino acids and transform them into functional proteins.
Mitochondrial DNA is always passed on from the mother to the offspring. It is always transferred through the reproductive cell ovum and is never found in sperm. 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 exceptionally high. Therefore, the maternal inheritance of mtDNA observed in early studies could simply have resulted from 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. They found the nucleic acid inside the chick embryo mitochondria by electron microscopy. They published their findings in The Journal of Cell Biology in 1963. The following year, an independent research team published a similar result; they found extranuclear DNA in the mitochondria of yeasts. The entire information on mitochondrial DNA is called mitochondrial genome or mitogenome.
Transcription Of mtDNA
DNA carries the genetic information that codes for a particular protein. Thus, the genetic code for a protein is first copied into the RNA (specifically, mRNA) during protein translation.
This process of creating a copy of DNA into mRNA through the help of the enzyme RNA polymerase is called transcription. Mitochondrial transcription in humans 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.
Mitochondiral Genetic Disorders
Mutation or the defect that DNA experiences at some stage of its life are the same in almost all forms of DNA. Certain dangerous diseases are caused by mtDNA mutation, referred to as mitochondrial genetic disorders.
Cancer: Somatic mutation of mtDNA can result in cancerous growth (exponential growth of non-required cells). 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 condition 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 Other 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 identify the criminal.
- mtDNA can be obtained from the Hair.
- 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 identifying individuals.
- In anthropological genetics, mtDNA helps trace the geographic distribution of genetic variation to investigate expansions, migrations, and other patterns of gene flow. Even though nuclear DNA is essential and easy with the technology of PCR nowadays, mtDNA has a higher copy number than nuclear DNA inside the cell, which proves an advantage.
- Besides forensics, mtDNA is also helpful in food production and agricultural practices. In the early 1030s, 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.
- In the year 2019, Japanese scientists recently discovered how to alter the mitochondrial DNA of plants, which will now help improve and improve the existing agricultural technologies due to the rapid hike in prices of essential resources like food and other natural resources.
The most common form of DNA found from the body fluids and other evidence at the crime scene is nuclear DNA. It is a biopolymer nucleic acid found within the nucleus of eukaryotic cells structured as a double helix and woven through chromosomes. As Francis Crick and James D Watson initially described, the two strands wound around. It encodes the primary structure of proteins consisting of essential amino acid sequences.
As nuclear DNA is found in almost every cell of the body, the most common form of evidence of DNA is this. Hence, technical DNA identification procedures are primarily focused on nuclear DNA.
Difference Between Nuclear And Mitochondrial DNA
Nuclear DNA and mitochondrial DNA differ in major ways;
|NUCLEAR DNA||MITOCHONDRIAL DNA|
|It is located within the nucleus of a eukaryotic cell.||It is found within the mitochondria of the cell.|
|Usually it has two copies per cell.||mtDNA usually contains 100-1000 copies per cell.|
|Chromosomes of nuclear DNA is linear With open ends and includes 46 chromosomes.||Chromosomes of mtDNA are usually closed and circular.|
|It is diploid containing an equal distribution of inherited genes from both father and mother.||It is haploid and is passed on only from the mother, that is the female.|
Contain less mutations range
Contains higher mutational range