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Bryson uses the metaphor of a rope ladder to help describe the structure of DNA and the mechanism of evolution. DNA has a double-helix structure—it looks like a twisted rope ladder. The rungs on the ladder are made by the “bases” A, T, C, and G, and their order determines everything from eye color to the presence of wings. When DNA replicates, it’s as if the ladder rips in half (like a zipper) and each half attaches to more bases to create two ladders. Nearly all the time, the two ladders created will be identical, meaning the rungs always stay in the same order as DNA replicates. Once in a while, however, a base will latch on in the wrong place, creating a different order of genetic code. When this happens, the body that DNA builds will be slightly different, which is how evolution occurs.
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Rope Ladder Symbol Timeline in A Short History of Nearly Everything
The timeline below shows where the symbol Rope Ladder appears in A Short History of Nearly Everything. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 26: The Stuff of Life
DNA’s double helix shape looks like a twisted rope ladder : the rungs (or steps) of the ladder are formed by joining bases—specifically, G with...
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Sometimes, though, a letter gets in the wrong rung on the rope ladder —in technical terms, a “single nucleotide polymorphism” or “Snip” happens—and this can affect the body...
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