AMINO ACID SEQUENCES AND EVOLUTIONARY RELATIONSHIPS

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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.
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