Deoxyribonucleic acid (DNA), also known as DNA in German, is a biomolecule (biologically active compounds or molecules found in living things) with the property of carrying genes and their inherited properties. It is found in all organized entities with the abilities of metabolism, reproduction, irritability, growth and evolution, as well as in some types of viruses. The structure of DNA is in the form of a double helix (a kind of helix, but in which the winding motif appears twice). The double helix is circulated parallel to each other by the two DNA strands. These two DNA strands are called polynucleotides because they are composed of so-called nucleotides. Components of a nucleotide are one of the four nitrogenous nucleic bases, which are either adenine, cytosine, guanine or thymine, often abbreviated by their initial letters. In addition, nucleotides are composed of the carbohydrate deoxyribose and a phosphate residue. Through molecular bonding, the nucleotides are linked together in an alternating sugar–phosphate chain. According to the principle of the double helix, adenosine (nucleoside, which has the nucleic base adenine) always forms a hydrogen bond to thymidine (nucleoside, which has the nucleic base thymine). Guanosine (nucleoside with the nucleic base guanine), on the other hand, forms a hydrogen bond to cytidine (nucleoside with the nucleic base cytosine). DNA is capable of self-renewal, which is called DNA replication. In this process, the two DNA strands are separated from each other. This is catalyzed by the enzyme helicase and the DNA to be replenished from the respective strand is newly generated (DNA synthesis). An enzyme from the DNA polymerase group is responsible for this process, as well as an RNA primer that serves as a starting point for the polymerase. This process is essential, especially during cell division. In certain cases, damage to the DNA can occur. This is caused by so-called mutagens, which are of chemical (e.g. by X-rays or ultraviolet rays) or physical origin. They lead to changes in the DNA sequence. Depending on the mutagen, different forms of DNA damage occur. Most damage is caused by oxidations, which include free radicals or hydrogen peroxides. These can cause harmful base modifications (changes to a nucleic base), but they can also cause the far more dangerous and often cancer-causing point mutations such as deletions (loss of a DNA sequence) or even insertions (new gain of one or more base pairs within a DNA sequence), as well as chromosomal translocations (chromosomal abnormality caused by rearrangement).
Mitochondrial DNA
Inside mitochondria is mitochondrial DNA, also called mtDNA or mDNA, which is double-stranded like DNA but closed into a ring. The mitochondria regenerate the energy-rich molecule adenosine triphosphate (universal and immediately available energy carrier in cells and important regulator of energy-producing processes) via the respiratory chain. Furthermore, they fulfill essential tasks for the cell. Mitochondrial DNA contains only 37 genes, 13 of which code for proteins localized on the respiratory chain. The rest is transcribed into tRNA as well as rRNA, which enable the coding of the 13 mentioned genes. The mtDNA is exclusively maternally inherited, i.e. from the mother. Mitochondrial DNA exists in both plants and animals. It is of evolutionary origin and descended from circular genomes of bacteria.
Evolutionary History
In 1869, Friedrich Miescher, a physician from Switzerland, isolated a microscopic substance from a pus extract that came from the nucleus of a lymphocyte. This he called nuclein. In 1878, the German biochemist isolated nucleic acid from nuclein and later its four nucleic bases. In 1919, Lithuanian biochemist Phoebus Levene discovered the sugar deoxyribose and the phosphate residue of DNA. In 1937, William Astbury used X-rays to visualize the regular DNA structure for the first time. The fact that DNA plays an important role in heredity was confirmed by the geneticists Alfred Day Hershey and Martha Chase in 1952 based on their discovery that DNA is genetic material. A year later, James Watson, together with Francis Crick, presented in the journal Nature what is now considered the first correct double-helix model of DNA structure.In doing so, the foundations of their molecular double-helix model came from an X-ray taken in May 1952 by Rosalind Franklin.
