James Watson and Francis Crick started with the work of American scientist Linus Pauling, another contestant in the DNA race.
Pauling had already determined that many proteins, one of the main components of cells, had the structure of a helix, a spiral chain in the shape of a twisted ladder.
Watson and Crick decided to start with a helix shape, but how many chains were there?
They next looked at the existing X-ray photos of crystalline DNA that had been taken by British chemist Rosalind Franklin for Wilkins.
Wilkins had studied the photos and thought DNA had two chains, or a double-helix structure, but he chose not to research it further. Watson and Crick agreed with the two-chain theory, but their work was just beginning.
Previous research had also revealed that DNA was made up of compounds called nucleotides.
Nucleotides were composed of sugar, phosphate, hydrogen, and one of four bases, adenine, guanine, thymine, or cytosine. It was also known that DNA contained an equal number of adenine and thymine bases and an equal number of guanine and cytosine bases.
With this information, Watson and Crick made a vital conclusion: DNA must form replicas of itself just like chromosomes do during cell division.
Watson and Crick created their double-helix model with each rung of the twisted ladder consisting of one of the two pairs of bases held together by the hydrogen atoms. When the DNA ladder divides at the middle of each rung, the legs form two new ladders because the same two bases always join to make a new rung.
The model not only made sense of all the previous information that had been gathered about DNA, but also explained how genetic information is passed on from generation to generation.
The order in which the atoms are arranged makes the code by which information is passed on.
The DNA code, which Leeuwenhoek had once called the “great secret,” had been uncovered.