Name ____________________ Period _______.
Evidence for Evolution. Part III…Molecular
Making Cladograms using Cytochrome C amino acid sequences.
BACKGROUND: You have just completed an activity in which you made a cladogram
showing the evolutionary relationships between seven organisms. The data used to
draw that cladogram was based on shared characteristics that were inherited from their
ancestors. This is anatomical evidence for evolution. Biochemical characteristics, like
similarities in DNA or protein structure, can also be used to produce cladograms. If
there is strong agreement between the patterns produced using anatomical similarities
and those produced by using biochemical structures, it provides what we call
"independent confirmation" of the cladogram. Independent confirmation is where two
or more sources of evidence that are not dependent on each other produce the same
pattern. The more independent confirmation that is available, the more confidence we
have that the evolutionary relationships shown in the cladogram are correct.
Cytochrome C is a protein involved in energy production in all eukaryotic cells.
Over time, random mutations in the DNA sequence occur. As a result, the amino acid
sequence of Cytochrome C also changes. Cells without usable Cytochrome C are
unlikely to survive. This mutation rate can be used much like a clock to infer how long
ago organisms shared a common ancestor. In this activity, we will examine and
compare the amino acid sequence of Cytochrome C. All seven of the organisms used in
the previous activity produce this protein (it is a homologous protein). The complete
sequence of amino acids for this protein has been determined for many organisms,
including six of those seven animals.
Our prediction is that that if we find fewer differences in the amino acid
sequences, those animals are more closely related.
METHOD:
A . Compare the amino acid sequence of Cytochrome C in human, rhesus
monkey, kangaroo, snapping turtle, bullfrog, and tuna.
1. Mark the amino acids (shown as single letter abbreviations) that are different
between Humans and the other organisms. The following example shows you how:
- - G V P
* * *
There
- G D V A K G K K
A G D V E K G K K
*
*
are 9 differences
T F V Q K C A Q C H T V E
I F V Q R C A Q C H T V E
*
*
in amino acid sequence in
N G G K H K V G P N L W G L F G R K T
A G G K H K V G P N L H G L F G R K T
*
*
this example.
Note that some of the amino acids are always the same in all species. These have
been shaded light gray.
2. Count and record the total number of differences. Write that information below:
Number of amino acid differences between human and
• Snapping turtle =
• Kangaroo =
• Bullfrog =
• Rhesus monkey =
• Tuna =
3. Note: that each protein sequence has 103-112 amino acids; the sequence extends
onto two (2) lines. So be sure to study the entire molecule.
B. Make a cladogram using the data above.
Complete the diagram shown below. Remember we are comparing relatedness to
Humans. The names of the most closely related species (those that have the fewest
differences in amino acid sequence) should be inserted into the empty rectangles (the
more related they are the closer they should be). Fill in the empty hexagons with the
number of amino acid changes in Cytochrome C (when compared to Humans).
Humans
C. Answer the following questions in complete sentences on a separate sheet of
paper.
1. Explain why more closely related organisms have more similar Cytochrome C
2. How does the cladogram drawn from the data using the amino acid sequence
compare with the cladogram drawn using anatomical data?
3. What does it mean when organisms with fewer shared anatomical traits also
have more amino acid differences?
4. If the molecular data, the structural similarities, and the fossil record all support
the same pattern of relationships, can we be fairly confident that the pattern is
accurate? Why or why not?
5. Chickens and turkeys are both birds and have the same sequence of amino
acids in their cytochrome-c protein. Explain how two species can have identical
cytochrome-c and still be different species.
6. Take a look at the sequences of Neurospora (bread mold) and Saccharomycetes
(bakers yeast). They are both fungi. What can you say about the inferred
evolutionary relationships between these two fungi? Explain your reasoning.