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).
) - 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.
The human genome
Genetics is the science that studies (among other things) a human being's genetic identity card and maps out how genetic transmission occurs.<top>
Although in practice 'genetic' is often used as a synonym for 'inherited', genetic changes can also occur during embryonic development or later in life. In other words we can develop mutations that we have not inherited and that we do not always pass on to our children.
These 'somatic' mutations are also studied, but they are not a part of classical genetics or the study of inheritance.
In 2003 geneticists succeeded through the 'Human Genome Project' in mapping the precise sequence of the more than three billion nucleotides that make up our DNA. We can therefore say that we know the composition of the human genome.
Our genes are situated on this long chain. These are well-defined pieces of DNA that are responsible for the synthesis of or 'code' for a particular protein. In genetics, 'codes for x' means 'ensures that x is formed'. If DNA is the entire programming language of the human organism, we can see the genes as specific commands.
Thanks to the unravelling of the human genome we now know the basic package of genetic information that is stored in our cells. This is important, because it is precisely these small differences - repetitions, omissions or substitutions - within the basic package that determine what we look like or how we function, and create the differences between people or the similarities between blood relatives.
Nevertheless, it would be an exaggeration to say that our DNA code is an open book to geneticists. Research is still continuing into the precise number of genes and the DNA in between them, what they do (function) and how they work together (interact).