2.2. The molecular parts list[[Student version, December 8, 2002]] 45
Figure 2.17:(Structure rendered from atomic coordinates.) Stereo image of the DNA double helix. To view this
image, begin with your nose a few centimeters from the page (if you’re nearsighted, remove your glasses). Imagine
staring through the page at a distant object. If necessary, rotate the page a few degrees, so that the two dots near
the centers of each panel are aligned horizontally. Wait until the dots fuse. Concentrate on holding the dots fused
as you slowly move the pageawayfrom your nose. When the page is far enoughaway for your eyes to focus on it,
the three-dimensional image will jump off the page at you. The structure is about 2nmwide. The portion shown
consists of twelve base pairs in a vertical stack. Each base pair is roughly a flat, horizontal plate about 0. 34 nmthick.
The stack twists through slightly more than one full revolution from top to bottom. [From (Dickersonet al.,1982).]
[Copyrighted figure; permission pending.]
2.2.3 Big molecules
Just as amino acids can be joined into polypeptide chains, so, chains of nucleotide bases can also
bestrung together to formpolynucleotides.Apolynucleotide formed from nucleotides containing
ribose is called a “ribonucleic acid,” orRNA;the analogous chain with deoxyribose is called a
molecule of “deoxyribonucleic acid,” orDNA.Watson and Crick’s insight (Section 3.3.3) was that
the flat, planar bases of DNA not only fit each other precisely, like jigsaw puzzle pieces (Figure 2.13);
they also can nest neatly in a helical stack (Figure 2.17), with the bases pointing inward and the
sugar and phosphate groups forming two “backbones” on the outside. Cells do not manufacture
RNA strands in complementary pairs, but a single RNA can have short tracts that complement
others along the chain, giving rise to a partially folded structure (Figure 2.18).
Each of your cells contains a total of about one meter of DNA, consisting of forty-six pieces.
Keeping track of such long threads, without their turning into a useless tangle, is not easy. Part of
the solution is a hierarchical packaging scheme: The DNA is wound onto protein “spools,” to form