Progressive increases in the complexity of crystal structures. Scales for linear dimensions are as labeled on associated bars. (a) Rocksalt (NaCl). This is a crystal in space group with lattice constant a = 5.640 Å; thus, its unit-cell volume (Vuc) is 180 Å3 and the volume of the asymmetic unit (Vau) is Vuc/192 = 0.93 Å3. The image is adapted from Fig. 10 of Bragg (1913b) where A and B may represent Na and Cl, respectively, or vice versa. (b) Crambin. This protein molecule is from a crystal in space group P21 with a = 40.96, b = 18.65, c = 22.52 Å and β = 90.77°; thus its Vau = Vuc/2 = 8601 Å3. The image here was adaped from a drawing made by Irving Geis based on the coordinates of Hendrickson & Teeter (1981). All atoms are shown, including hydrogen atoms. The six sulfur atoms, from which the structure was determined by what is now known as SAD phasing, are in yellow. (c) Complex of a 70S ribosome with an mRNA and three tRNAs. This particle is from a crystal in space group P212121 with a = 213.3, b = 453.0 and c = 631.4 Å; thus its Vau = Vuc/4 = 15 252 240 Å3 for the two ribosome particles per asymmetric unit. The image is reproduced from Fig. S1 of Selmer et al. (2006). Reprinted with permission from AAAS. The image shows only the RNA portions from one ribosome (50S bluish, 30S yellowish and tRNAs brown, magenta and dark gray). Phosphate backbones are drawn as ribbons, and ribose and base rings are drawn as colored planes. The 54 ordered protein molecules of this 70S ribosomal structure are omitted from the figure. (d) Human adenovirus capsid. This virus structure is from a crystal in space group P1 with lattice constants a = 854.0, b = 855.2, c = 865.2 Å, α = 119.6, β = 91.7 and γ = 118.1°; thus its Vau = Vuc = 450 275 173 Å3 for one viral particle. The image is reproduced from Fig. 1A of Reddy et al. (2010). Reprinted with permission from AAAS. It shows a surface rendering viewed down a threefold axis of the icosahedral particle. One of 20 identical facets is circumscribed by the white triangle, which has three of the 12 viral penton proteins at its vertices and 12 of the 240 viral hexon proteins uniquely within its boundaries. The icosahedral asymmetric unit thus comprises one-fifth of a vertex penton (pink) and four hexons (one cyan, one orange, one yellow and one green) plus accessory cement proteins (deep blue, between orange and yellow hexons; magenta, between hexons at four places) and a trimeric fiber (yellow–orange) at each penton base. Other accessory proteins are at the capsid interior and out of view.