FIGURE 8-4 Comparison of gene density in higher and lower eukaryotes. (a) In this diagram of the β-globin gene cluster on human chromosome 11, the green boxes represent exons of β-globin–related genes. Exons spliced together to form one mRNA are connected by caret-like spikes. The human β-globin gene cluster contains two pseudogenes (white); these regions are related to the functional β-globin genes but are not transcribed. Each red arrow indicates the location of an Alu sequence, a roughly 300-bp noncoding repeated sequence that is abundant in the human genome. See F. S. Collins and S. M. Weissman, 1984, Prog. Nucl. Acid Res. Mol. Biol. 31:315. (b) In this diagram of yeast DNA from chromosome III, the green boxes indicate open reading frames. Most of these potential protein-coding sequences are functional genes without introns. Note the much higher proportion of noncoding to coding sequences in the human DNA than in the yeast DNA. See S. G. Oliver et al., 1992, Nature 357:28.