Explaining the DNA structure
The DNA double helix contains two sequences of nucleotide code letters than run along the molecule. Untwist the spiral and DNA can be seen as two parallel strands. That's why its sometimes called the double helix. Unzip the strands and you have two linear sequences of the letters A C G and T. The precise order of the letters along the molecule carries the coded instructions. These are the equivalent of binary 1s and 0s in a computer, one strand is a complementary image of the other an a will always pair with T and C with G so if you know the sequence of one strand you can work out the sequence of the other. It dawned on Watson and Crick that this complementary copy of the code was the key to understanding how genetic information could be passed on. So the essence of life could be explained with chemistry, there were no mysterious life forces.
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Image depicting DNA helix model and table.
The structure of DNA, that Watson and Crick discovered, suggests how genetic information is passed on.
DNA as a double helix.
DNA base pairs
Animation of 2D DNA model becoming three dimensional.
Because it contains the directions for assembling the components of the cell, DNA is often thought of as the "instruction book" for assembling life.
1953 picture of Francis Crick (L) and James Watson (R) walking along the backs of King's College in Cambridge.
Image of DNA helix model (closeup).
DNA as a regular, repeating structure -- a helix.