Stiffening the Book's Spine
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This picture shows Life's Instruction
Book and its binding. A short piece of the DNA that makes up the book is shown
(in red and orange) wrapped around a complex of structural proteins called
histones (in all the other colors -- one color for each worker). Since the human
Instruction Book is about five billion bases long and there's around two
hundred bases of DNA on each histone, each
human cell contains about twenty-five
million complexes like this.
Wrapping DNA around histones does two things. First, it makes the Instruction Book much less likely to break, since the histones support the DNA. Second, it stores the DNA in a smaller space, like the way we can wrap a foot of string around a finger to store it in a space only an inch or two long. This wrapping allows human DNA, which is about a yard long, to fit in each body cell -- no more than about one-thousanth of an inch long -- since each cell needs a copy of the body's entire Instruction Book to survive.
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The picture shows something like a cross section of the way DNA is stored
in each cell. We say "something like" because the arrangement of DNA
is more complicated that this picture can show. From the picture, one
might think that DNA's packing is like a string wrapped around a pencil,
with a stack of histones making up the pencil. In fact, the the stack
of histones is itself coiled, so the actual image of DNA is much more
like thread wrapped around a coiled phone cord.
| We're working on a picture of what this looks like. |
Part Three: Improving the Book
Appendix E: A Gallery of Workers