Latest update: 29/06/2017


The human organism is controlled by a code that we call DNA, which is short for DeoxyriboNucleic Acid.
It is present in (almost) all our cells.

DNA is a chain (in humans) of billions of nucleotides (or bases) arranged in a specific sequence. There are only four different bases: adenine (A), cytosine (C), guanine (G) and thymine (T).

These are joined in a constantly changing sequence to a chain of sugars and phosphates, thereby forming a long chain.
They occur in pairs and every base is joined to another base (always the same one):

  • A is always joined to T (and T to A);
  • C always to G (and G to C).

In other words DNA forms a double chain that looks like a rolled-up spiral ladder. Each pair of bases forms a single rung on this ladder. This is the so-called double helix structure of DNA (see the diagram).
The bonded nucleotides - always in groups of three (see genes) - are responsible for making all the amino acids in our body. Amino acids join together to form proteins, and these are in turn needed to build or repair cells and tissues.

In short: the long DNA code controls our whole organism, so those four letters form the basis for all human life.

Mitochondrial DNA

In addition to nuclear DNA we also have mitochondrial DNA (mtDNA). This is found in the cytoplasm of cells, and specifically in mitochondria. These are a particular type of cell organelle, and there are about 100 to 1,000 of them in each cell. They are also referred to as little power plants, because they convert sugars and fats into the energy (ATP) that our body needs to function properly.

We only receive mtDNA from our mother; the father's mtDNA is lost during fertilisation because it is broken down. The egg provides the cytoplasm, and this contains the mother's mitochondria.

MtDNA is ring-shaped, and is therefore not organised into chromosomes. It accounts for less than one percent of the total quantity of DNA, with only 16,500 nucleotides, as compared to three billion in nuclear DNA.
Nevertheless, the several hundred mitochondria in each cell each contain many of these circular DNA strands, which have 37 active genes. MtDNA is therefore crucial to our metabolism and (if there is any defect in its structure) it can give rise to complex disorders.