DNA is a twisted ladder with complementary steps, ensuring exact copying of genetic information.
Watson tells the story of the race between research teams to discover DNA's shape. The key clue was an X-ray photograph showing a twisted form. The ladder's two strands always pair up the same way, like a zipper, letting information be copied exactly into every new cell.
When a cell divides, it must copy the entire DNA. The twisted ladder shape makes this possible: the two strands separate, and each becomes a template for a new strand. For example, when you get a cut, the skin heals because new cells copy the old instructions exactly. This faithful copying is the foundation for everything that follows in genetics.
Why it matters The discovery of DNA's structure opened the path to genetic tests used in medicine today.