Unit VI: Evolution
Lab: Using Amino Acid Sequences To Show Evolutionary
Relationships
Introduction: In different organisms the amino acid sequences of proteins with the same function
are similar, but not identical. These differences can be used to indicate how closely organisms
are related. This activity uses the amino acid sequences from certain globin proteins.
Goal: The students will determine the number of differences in fragments of several globin
molecules and will construct a phylogenetic tree showing which polypeptides are most similar and
which are most different. The students will be able to see the chronological order in which the
proteins diverged from each other.
Directions:
1. Count the number of differences in amino acids between protein fragment #1 and protein
fragment #2 (a blank will be scored as a difference). The box below shows you how to
score the differences.
The circled letters indicate differences in each protein fragment; thus, there are 3
differences between the amino acid sequences in polypeptides #1 and #2.
These are different globin molecule fragments. The letters are symbols for the amino
acids in the sequence of the molecule. (See Table 1.)
#1 VNFKLLSHCLLVTLAAHLPAEFTPAVHASLDKFLASVSTVLTSKYR-#2 VNFKLLSHCLLVTLACHHPTEFTPAVHASLDKFFLAAVTVLTSKYR-#3 ENFRLLGNVLVCVLAHHFGKFFTPPVQAAYQKVVAGVANALAHKYH-#4 ENFKLLGNVLVCVLAHHFGKFFTPOVQAAYQKVGVAGVANALAKYH-#5 KYLEFISECIIQVLQSKHPGDFGADAQGAMNKALELFRKDMASNYKELGFQG
AP Biology/ Zeiher
Unit VI: Evolution
Table 1. Amino Acid Symbols
A = alanine
G = glycine
C = cysteine
H = histidine N = asparagine T = threonine
M = methionine S = serine
D = aspartic acid I = isoleucine P = proline
V = valine
E = glutamic acid K = lysine
Q = glutamine
W = tryptophan
F = phenylalanine L = leucine
R = arginine
Y = tyrosine
2. In the same way as step one, count the number of differences in the fragments making
comparisons between each pair (1<-->2, 3, 4, 5; 2<-->3, 4, 5; 3<-->4, 5; 4<-->5) and write
the number of differences in the following chart.
We can assume that these five proteins diverged from a common ancestor-protein
sometime in the past; they diverged‹changed but did not replace this ancestor-protein
over time. It is most likely that the genes which held the recipes for these proteins have
been duplicated several times to allow for these changes. Using this concept, place the
protein fragments in the "tree" below.
AP Biology/ Zeiher
Unit VI: Evolution
Analysis Questions
1. Describe the reason you put each fragment in its particular location on the "tree."
2. 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) Which organism (X, Y, or B) diverged from the common ancestor first? ____________
Explain your reasoning.
b) Which pair of organisms--X & Y, X & B, or Y & B--are likely to share more
characteristics than the other two pairs. ______________ Again, explain your reasoning.
AP Biology/ Zeiher