Amino Acid Sequences and Evolutionary Relationships
Introduction:
_______________ structures are believed to have a common origin but not necessarily a common function. They
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 of the bones indicates
a common origin. Although these structures can be used to demonstrate relationships between similar organisms, they
are of little value 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 molds, aardvarks, and humans appear to have little in common physically, a study of their proteins reveals
certain similarities. Biologists have perfected the techniques for determining the sequences of amino acids that make
up _________________. By comparing these sequences, some evolutionary relationships that might otherwise go
undetected can be determined (the close relationship of the elephant and hyrax for example). Further, biologists have
found that such biochemical evidence compares favorably with the other lines of evidence for evolutionary
relationships.
Problem:
How do amino acid sequences provide evidence for evolution?
Hypothesis:
Describe in a formal hypothesis the general relationships you will expect to find among the organisms
listed in Figure 1 based on the amino acid sequences. In other words, “what will be closer related to what” and how will
the amino acid differences tell us that?
Remember, a formal hypothesis will be in the format – “If ______ is related to _____, then ________ because.” It can be
explained in more than one sentence if you have to.
Methods/Procedure:
Part A. Comparing Amino Acid Sequences
1.
Examine Figure 1, which compares corresponding portions of hemoglobin molecules in humans and 5 other
vertebrate animals. Hemoglobin, a protein composed of several long chains of amino acids, carries
________________ in ________ blood cells. The sequence shown is only a portion of a chain made up of 146
amino acids. The numbers in Figure 1 indicate the position of a particular amino acid in the long chain.
2. Use Figure 1 to complete Data Table 1. You will be indicating the amino acids that are different only. Note:
Always be sure that you compare the amino acid sequence of each organism with that of the human and not the
organism on the line above!
Figure 1
87
THR
THR
THR
Human
chimpanzee
Gorilla
Rhesus
GLN
monkey
Horse ALA
Kangaroo LYS
102
Human ASN
chimpanzee ASN
Gorilla ASN
Rhesus
ASN
monkey
Horse ASN
Kangaroo ASN
88
LEU
LEU
LEU
89
SER
SER
SER
90
GLU
GLU
GLU
91
LEU
LEU
LEU
92
HIS
HIS
HIS
93
CYS
CYS
CYS
94
ASP
ASP
ASP
95
LYS
LYS
LYS
96
LEU
LEU
LEU
97
HIS
HIS
HIS
98
VAL
VAL
VAL
99
ASP
ASP
ASP
100
PRO
PRO
PRO
101
GLU
GLU
GLU
LEU
SER
GLU
LEU
HIS
CYS
ASP
LYS
LEU
HIS
VAL
ASP
PRO
GLU
LEU
LEU
SER
SER
GLU
GLU
LEU
LEU
HIS
HIS
CYS
CYS
ASP
ASP
LYS
LYS
LEU
LEU
HIS
HIS
VAL
VAL
ASP
ASP
PRO
PRO
GLU
GLU
103
PHE
PHE
PHE
104
ARG
ARG
LYS
105
LEU
LEU
LEU
106
LEU
LEU
LEU
107
GLY
GLY
GLY
108
ASN
ASN
ASN
109
VAL
VAL
VAL
110
LEU
LEU
LEU
111
VAL
VAL
VAL
112
CYS
CYS
CYS
113
VAL
VAL
VAL
114
LEU
LEU
LEU
115
ALA
ALA
ALA
116
HIS
HIS
HIS
PHE
LYS
LEU
LEU
GLY
ASN
VAL
LEU
VAL
CYS
VAL
LEU
ALA
HIS
PHE
PHE
ARG
LYS
LEU
LEU
LEU
LEU
GLY
GLY
ASN
ASN
VAL
ILE
LEU
ILE
ALA
VAL
LEU
ILE
VAL
CYS
VAL
LEU
ALA
ALA
ARG
GLU
*Human hemoglobin is being used as the standard for comparison.
Data/Results
Data Table 1 – give it a title and put into your notebook.
Organisms
Human &
Chimpanzee
Human & Gorilla
Human & Rhesus
Monkey
Human & Horse
Human & Kangaroo
Number of
Amino Acid
Differences
Positions in which They Vary
Part B. Inferring Evolutionary Relationships from Differences in amino Acid Sequences
1.
Another commonly studied protein is cytochrome c. This protein, consisting of 104 amino acids, is located in
the mitochondria of cells. There it functions as a respiratory enzyme. Examine figure 2. Using human
cytochrome c as a standard, the amino acid differences between humans and a number of other organisms are
shown.
Figure 2
Species Pairings
Human - chimpanzee
Human - fruit fly
Human - horse
Human - pigeon
Human - rattlesnake
Human - red bread mold
Human - rhesus monkey
Human - screwworm
Human - snapping turtle
Human - tuna
Human - wheat
2.
Number of
Differences
0
29
12
12
14
48
1
27
15
21
43
Using Figure 2, construct a bar graph in your notebooks to show the amino acid differences between humans
and other organisms. Be sure that your graph is properly labeled and easy to read. Spend some time organizing
it before you start.
3. Now examine Figure 3. In this figure the cytochrome c of a fruit fly is used as the standard in comparing amino
acid differences among several organisms. Which animal is the fruit fly’s closest relative? _________________
Construct a pie chart to show these differences.
Figure 3
Species Pairings
Number of Differences
Fruit fly-dogfish shark
Fruit fly-pigeon
Fruit fly-screwworm fly
Fruit fly-silkworm moth
26
25
2
15
Fruit fly-tobacco
hornworm moth
Fruit fly-wheat
14
47
Analysis of Results – write a 1 paragraph summary of what the data tables and
graphs show.
Discussion – put the answers in compete sentences in your notebooks
1.
According to Figure 1 and the bar graph, which organism appears to be the least closely related to humans?
Explain why. (Include a comparison of the type of animal it is.)
2.
According to cytochrome c differences, which 2 organisms are the most similar to humans?
3. What are some physical characteristics that we have in common with those 2 organisms?
4. Which 2 are the least similar to humans?
5. There is a difference of only one amino acid in one chain of the hemoglobin of humans and gorillas. What might
have caused this difference?
6. If the amino acid sequences in the proteins of 2 organisms are similar, why will their DNA also be similar?
7. Many biologists believe that the number of differences between the proteins of different species indicates how
long ago the species diverged from common ancestors. Why do these biologists believe that humans,
chimpanzees, and gorillas diverged from a common ancestor only a few million years ago?
8.
Species X and Y have 25 amino acid differences. Species X and B have 10 amino acid differences in the same
protein. Species Y and B have 27 amino acid differences. A phylogenetic tree/cladogram would look like this:
Y
X
B
Y and B diverged first because they have the most differences. X and B diverged more recently because they have
fewer differences.
---Construct a phylogenetic tree based on the species and information in Figure 2.