It is 1953 and molecular biologists Francis Crick and James Watson have been working to understand how genetic material is stored in cells. They see a picture, taken by researchers at Kings College London and figure out that DNA is shaped like a spiral staircase. A letter written by Crick to his son sold earlier this month at auction for $6m. In it he describes the double-helix structure of DNA as beautiful. The scientists knew DNA had some way of copying itself, passing genetic information from old cells to new ones. They decided this was most likely to be achieved if the DNA was formed by two strands. Al Jazeera's Tarek Bazley reports.