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by unwinding the double helix  a symbolic "ladder" is formed (rungs and sides)

• the parallel sides of the ladder are formed by repeating units of sugar-phosphate-sugar-phosphate
[white-purple-white-purple]
• the rungs of the ladder are formed by the nitrogen base pairs
• Guanine-cytosine  [green-blue]
• thymine-Adenine  [yellow-red]
• Guanine-cytosine  [green-blue]
• Adenine-thymine  [red-yellow]
• on, and on, and on, possibly as many as 

100 000's of rungs
 
• the sequence of the nitrogen bases makes up the "Genetic Code"