An early graphical method for comparing nucleotide sequences—the so-called diagon plot—still yields one of the best visual comparisons of sequence relatedness. An example is illustrated in Figure Q9–11, in which the human β-globin gene is compared with the human cDNA for β globin (which contains only the coding portion of the gene; Figure Q9–11A) and with the mouse β-globin gene (Figure Q9–11B). Diagon plots are generated by comparing blocks of sequence, in this case blocks of 11 nucleotides at a time. If 9 or more of the nucleotides match, a dot is placed on the diagram at the coordinates corresponding to the blocks being compared. A comparison of all possible blocks generates diagrams such as the ones shown in Figure Q9–11, in which sequence similarities show up as diagonallines.

A. From the comparison of the human β-globin gene with the human β-globin cDNA (Figure Q9–11A), can you deduce the positions of exons and introns in the β-globin gene?

B. Are the exons of the human β-globin gene (indicated by shading in Figure Q9–11B) similar to those of the mouse β-globin gene? Identify and explain any key differences.

C. Is there any sequence similarity between the human and mouse β-globin genes that lies outside the exons? If so, identify its location and offer an explanation for its preservation during evolution.

D. Did the mouse or human gene undergo a change of intron length during their evolutionary divergence? How can you tell?

5' human -globin cDNA 3' (A) HUMAN B-GLOBIN CDNA COMPARED WITH HUMAN B-GLOBIN GENE 5' human B-globin gene 3' 3 mouse B-globin gene 5 (B) MOUSE B-GLOBIN GENE COMPARED WITH HUMAN B-GLOBIN GENE 5' human B-globin gene 3'