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AMINO ACID SEQUENCES AND EVOLUTIONARY RELATIONSHIPS BIO H LAB DR WEINER Background Homologous structures, those structures believed to have a common origin but not necessarily a common function, provide some of the most significant evidence supporting the theory of evolution. For example, the forelimbs of vertebrates often have different functions and outward appearances, yet the underlying similarity in the bone structure indicates a common origin. Although homologous structures can be used to demonstrate evolutionary relationships between similar organisms, they are of no use in determining evolutionary relationships among those structures that are dissimilar. Another technique used to determine evolutionary relationships is to study the biochemical similarity of organisms. Though bread molds, aardvarks, and humans appear to have little in common physically, a study of their protein sequences reveals certain similarities. By comparing the amino acid sequences in homologous proteins of similar and diverse animals, evolutionary relationships that might otherwise go undetected, can be determined. Biologists believe that the greater the similarity between the amino acid sequences of two organisms, the closer the evolutionary relationship. In other words, the more recently they diverged from their common ancestor. On the other hand, the more different the amino acid sequences, the more distant the relationship. The biochemical evidence often compares favorably with other lines of evidence concerning evolutionary relationships. Purpose To compare the amino acid sequences in proteins of several vertebrates. To study the differences and infer evolutionary relationships among the various vertebrates. Problem How do amino acid sequences provide evidence for evolution? Materials Lab sheet Procedure Part A. Comparing Amino Acid Sequences 1. Examine figure 1, which compares corresponding portions of the hemoglobin molecule in humans and five other vertebrates. Hemoglobin is the oxygen carrying protein in your blood. 2. In Data Table 1, notice that the abbreviated names of the amino acids in human hemoglobin are printed 3. In the appropriate spaces in Data Table 1, write the abbreviated name of each amino acid in chimpanzee hemoglobin THAT IS DIFFERENT FROM HUMAN. If there are no differences, leave the spaces blank. 4. Repeat step 3 for the remaining organisms. ALWAYS BE SURE TO COMPARE THE AMINO ACID SEQUENCES OF THE OTHER VERTEBRATES TO HUMAN. 5. Use Data Table 1 to complete Data Table 2. FIGURE 1: AMINO ACID SEQUENCES FOR HEMOGLOBIN IN VARIOUS VERTEBRATES 87 Thr Thr Thr Gln Ala lys 88 leu leu leu leu leu leu 89 ser ser ser ser ser ser 90 glu glu glu glu glu glu 91 leu leu leu leu leu leu 92 his his his his his his 93 cys cys cys cys cys cys 94 asp asp asp asp asp asp 95 lys lys lys lys lys lys 96 leu leu leu leu leu leu 103 Human phe Chimp phe Gorilla phe rhesus phe Horse phe kangaroo phe 104 arg arg lys lys arg lys 105 leu leu leu leu leu leu 106 leu leu leu leu leu leu 107 gly gly gly gly gly gly 108 asn asn asn asn asn asn 109 val val val val val ile 110 leu leu leu leu leu ile 111 val val val val ala val 112 cys cys cys cys leu ile Human Chimp Gorilla rhesus Horse kangaroo 97 his his his his his his 98 val val val val val val 99 asp asp asp asp asp asp 100 pro pro pro pro pro pro 101 glu glu glu glu glu glu 102 asn asn asn asn asn asn DATA TABLE 1 Human Chimp Gorilla Rhesus monkey Horse Kangaroo Human Chimp Gorilla Rhesus monkey Horse Kangaroo 87 Thr 88 Leu 89 Ser 90 Glu 91 Leu 92 His 93 Cys 94 Asp 95 Lys 96 Leu 97 His 98 Val 99 Asp 100 Pro 101 Glu 102 Asn 103 Phe 104 Arg 105 Leu 106 Leu 107 Gly 108 Asn 109 Val 110 Leu 111 Val 112 Cys DATA TABLE 2 ORGANISMS NUMBER OF AMINO ACID DIFFERENCES POSITIONS IN WHICH THEY VARY Human and Chimp Human and gorilla Human and rhesus monkey Human and horse Human and kangaroo Part B: Inferring Evolutionary Relationships from Differences in Amino Acid Sequences 1. Another commonly studied protein is cytochrome c. This protein functions during aerobic respiration in the mitochondria of cells. Examine Figure 2. Using human cytochrome c as a standard, the amino acid differences between humans and a number of other organisms in shown. 2. Using figure 2, construct a bar graph to show the amino acid differences between humans and other organisms. FIGURE 2 Species pairings Human-chimpanzee Human-fruit fly Human-horse Human-pigeon Human-rattlesnake Human-red bread mold Human-rhesus monkey Human-screwworm fly Human-snapping turtle Human-tuna Human-wheat Number of differences 0 29 12 12 14 48 1 27 15 21 43 3. Now examine figure 3. In this figure cytochrome c from a fruit fly is used as a standard. Compare the amino acid differences and construct a bar graph as you did in step 2. FIGURE 3 Species pairings Fruit fly-dogfish shark Fruit fly-pigeon Fruit fly-screwworm fly Fruit fly-silkworm moth Fruit fly-tobacco hornworm moth Fruit fly-wheat Number of differences 26 25 2 15 14 47 Conclusions Part A 1. On the basis of hemoglobin similarity, what organism is more closely related to humans? Explain why. 2. Among the organisms that you compared, which one appears to be least closely related to humans? Explain why. Part B 3. On the basis of differences in their cytochrome c, which organism appears to be more closely related to humans? Explain why. 4. Which organism appears to be least closely related to humans? Why? 5. Which pair of organisms appear to be more closely related to each other: Snapping turtle and tuna Snapping turtle and rattlesnake Snapping turtle and pigeon Give a reason for your answer. 6. Agree or disagree with the following statements and give reasons to support your answer. “Fruit flies appear to be more closely related to silkworm moths than they are to screwworm flies”. 7. Name the pair of organisms that appears to be equally related to humans on the basis of cytochrome c similarity. Explain why. 8. Is it possible that the organisms in question 7 can be equally related to humans but not equally related to each other? Explain. 9. Agree or disagree with the following statement “Fruit flies and humans have about the same evolutionary relationship to wheat”. Explain your answer.
