In each cell, there are 23 pairs of long DNA strands which, if put end to end, would measure over 2 meters! The information coded into DNA is essential, and certain kinds of damage done to it can have deadly consequences for our organism. If we consider genes as coded blueprints (around 30 000 genes), DNA is a unique, extensive book indexing all of these blueprints. We then have 23 encyclopedias, each in 2 copies. These copies are so valuable that they cannot leave the central library (the nucleus). The information contained within each gene therefore is “copied” into a molecule called messenger RNA. Messenger RNA is taken out of the cell’s nucleus and led to protein factories. In 1953, researchers Watson and Crick discovered the structure of DNA based on Wilkins’ experimental work, a discovery which would earn all three of them the Nobel Prize in Physiology and Medicine in 1962. DNA looks like a long spiraling ladder carrying all of this information; researchers call it a “double helix”. This information is written in an alphabet made up of four chemical letters called bases: adenine (A), thymine (T), guanine (G) and cytosine (C). A gene is therefore made up of a series of letters, looking something like ATGACACCGTGGA, whose pattern is unique for every gene, like a barcode. The interpretation of the information contained within DNA is achieved through a code: the genetic code, whose decryption is based on the fact that the four DNA bases must be enough to determine all of the twenty amino acids necessary for building proteins. Any spelling errors appearing in the DNA text are referred to as genetic mutations. Mutations can consist of letter inversions, deletions or insertions. Mutations can come from a reading error during DNA replication, through a viral infection or exposure to pollutants. They often lead to illness and occasionally to death.
Sequencing the human genome took about fifteen years. Three billion DNA letter combinations had to be transcribed and the very first sequencing cost one billion dollars. Today, an equivalent sequencing, of your DNA for example, would cost 100 dollars thanks to a microchip.
Mitochondrial DNA is a circular DNA molecule found in mitochondria. This DNA molecule codes a portion of proteins and RNA specific to mitochondrial operations.
Mitochondrial DNA (or mtDNA) is very valuable for genetic analysis, as it only contains 37 genes in humans (compared to 30 000 in human DNA) and is generally better preserved, and obviously much faster to sequence. Also, and contrary to human DNA, which is a combination of half of the mother’s genes and half of the father’s genes (and therefore a quarter of each grandparent, an eighth of each great grandparent, and so on) making filiation extremely difficult to determine past a few generations, mitochondrial DNA is only transmitted through the mother, who herself took it from her mother, etc. This therefore simplifies the study of mother-child filiation and lineage dating immensely. Male mitochondrial DNA comes from the mother and is not passed on to the next generation. Recent studies have shown that all human mitochondria in the world have a common origin dating to around 150,000 years ago, in Africa. More recently, the remains of Tsar Nicholas II and of his family have been identified by comparing the mtDNA of the remains found in Ekaterinburg (Russia) with those of Prince Philip (whose maternal grandmother was the sister of Tsarina Alexandra). This identification is 99% certain.