James Watson and Francis Crick discovered the DNA structure in 1953 basing on X-ray pictures (diffraction of X-rays on the DNA molecules) made by Rosalind Franklin.

DNA is a double helix. It consists of two spirally wound chains. DNA is a polymer where each chain is built by nucleotides.

In the helix bases are inside the structure and outside there is the sugar phosphate backbone.

DNA chains have different directions. It is stated by the number of carbon atoms from the sugar at the end of the strand.

Two strands of the helix keep together by hydrogen bonds between corresponding bases. Thymine always binds with adenine and cytosine always with guanine. As a result there is as much thymine as adenine and cytosine as guanine in each DNA molecule. This dependence is called the Chargraff rule.

The fact that there is only one way of binding the bases has a great meaning for DNA function of the information carrier.

There are 10 nucleotides in the one full twist. The length of human genome (haploid set) is about 3.000.000.000 nucleotides, so about 1 meter. Considering the size of cell it must be densely packed to be contained in the nucleus.