ELI5 DNA - what is the double helix?

If there's 3.2 billion base pairs in the human DNA, how come there's only about 20,000 genes? What is a gene? What is the double helix? Why is it significant? Who deserves credit for the discovery of DNA? Why do some people think Rosalind Franklin should have been awarded a Nobel Prize?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: 1994x, coolbeans1114, xandarg and nickcagefan2

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