NAME: ________________________________________ Period: ______________
Biochemical Comparisons and Molecular Clocks
Introduction: Mutation provides all initial change. A mutation occurs when the DNA
does not replicate perfectly. When a mutation occurs, a new allele is created. As a first
approximation, these mutations are random (can occur at any location along the DNA).
The rate of these mutations is relatively constant within a given species. If the mutation
occurs in a critical location the result is usually disastrous. Other areas will accept
change with no immediate consequence. Once made, the mutation is perpetuated and
variability within the gene pool of the species is increased. Mutations add variability to
the gene pool.
Examine the amino acid sequences below which compare corresponding portions of
hemoglobin molecules in humans and five other vertebrate animals. The sequence shown
is only a small portion of the chain of 146 amino acids. The numbered columns indicate
the position of each amino acid within the whole chain.
Procedure: 1. Look at each numbered column and circle the amino acids in each column
that are different from the human amino acid for that column.
2. Put an X in the blank under each numbered column in which all of the organisms have
the same amino acid listed in that column.
3. In the blanks on the right hand side of each row, identify how many amino acids were
circles (different from the human amino acids).
Table A: Comparing hemoglobin of a human to other vertebrates
Human
Chimp
Gorilla
Rhesus
Monkey
Horse
Kangaroo
Human
Chimp
Gorilla
Rhesus
Monkey
Horse
Kangaroo
87
THR
THR
THR
GLU
88
LEU
LEU
LEU
LEU
89
SER
SER
SER
SER
90
GLU
GLU
GLU
GLU
91
LEU
LEU
LEU
LEU
92
HIS
HIS
HIS
HIS
93
CYS
CYS
CYS
CYS
94
ASP
ASP
ASP
ASP
95
LYS
LYS
LYS
LYS
96
LEU
LEU
LEU
LEU
97
HIS
HIS
HIS
HIS
98
VAL
VAL
VAL
VAL
99
ASP
ASP
ASP
ASP
100
PRO
PRO
PRO
PRO
101
GLU
GLU
GLU
GLU
ALA
LYS
LEU
LEU
X
SER
SER
X
GLU
GLU
X
LEU
LEU
X
HIS
HIS
X
CYS
CYS
X
ASP
ASP
X
LYS
LYS
X
LEU
LEU
X
HIS
HIS
X
VAL
VAL
X
ASP
ASP
X
PRO
PRO
X
GLU
GLU
X
0
0
0
1
1
1
102
ASN
ASN
ASN
ASN
103
PHE
PHE
PHE
PHE
104
ARG
ARG
LYS
LYS
105
LEU
LEU
LEU
LEU
106
LEU
LEU
LEU
LEU
107
GLY
GLY
GLY
GLY
108
ASN
ASN
ASN
ASN
109
VAL
VAL
VAL
VAL
110
LEU
LEU
LEU
LEU
111
VAL
VAL
VAL
VAL
112
CYS
CYS
CYS
CYS
113
VAL
VAL
VAL
VAL
114
LEU
LEU
LEU
LEU
115
ALA
ALA
ALA
ALA
116
HIS
HIS
HIS
HIS
0
0
1
1
ASN
ASN
X
PHE
PHE
X
ARG
LYS
LEU
LEU
X
LEU
LEU
X
GLY
GLY
X
ASN
ASN
X
VAL
ILE
LEU
ILE
ALA
VAL
LEU
ILE
VAL
CYS
VAL
LEU
ALA
ALA
X
ARG
GLU
4
6
Questions:
1. Identify in the chart below how many total differences of amino acids each
organism had when compared to the human amino acid sequence.
Table B
Organism
# of differences from human
Chimpanzee
0
Gorilla
1
Rhesus monkey
2
Horse
5
Kangaroo
7
2. How many columns from Table A were the amino acids that same for each
organism? 21 COLUMNS
3. Based on the information in Table B above, which organism is the most closely
related to humans? Explain how you know this.
CHIMPANZEE, THERE ARE ZERO DIFFERENCES BETWEEN THE
HEMOGLOBIN PROTEIN BETWEEN HUMANS AND CHIMPS.
4. Based on the information in Table B above, which organism is the least closely
related to humans? Explain how you know this.
KANGAROO, THERE ARE SEVEN DIFFERENCES BETWEEN THE
HEMOGLOBIN PROTEIN BETWEEN HUMANS AND KANGROOS.
5. There is a difference of only one amino acid in one chain of hemoglobin between
humans and gorillas. What might have caused this difference?
A MUTATION
6. If the amino acid sequences in the proteins of two organisms are similar, why will
their DNA also be similar? (NOTE: What is the relationship between amino acid
sequences and DNA?)
DNA DETERMINES THE AMINO ACID SEQUENCE OF A PROTEIN (REMEMBER
THAT DNA CODONS CODE FOR A PARTICULAR AMINO ACID)
Many biologists believe that the number of differences between the amino acid
sequences (proteins) of different species indicate how long ago the species diverged
from a common ancestor.
Molecular Clock
Geologic Time Scale (million years ago)
450
350
270
70
0
_______________________________________________
79
70
60
50
40
30
20
10
0
Hemoglobin Amino Acid Differences
7. Why do these biologists believe that humans and gorillas diverged from a
common ancestor only a few million years ago?
THERE IS ONLY ENOUGH TIME FOR ONE DIFFERENCE BETWEEN THEIR
HEMOGLOBIN PROTEIN TO OCCUR SO HUMANS AND GORILLAS DIVERGED
FROM A COMMON ANCESTOR ONLY A FEW MILLION YEARS AGO WHICH IS
NOT VERY LONG AGO IN THE EVOLUTIONARY TIME SCALE.
8. There are 79 differences in the amino acid sequence of hemoglobin when you
compare humans to sharks. How long ago do you think humans and sharks
diverged from a common ancestor?
450 MILLION YEARS AGO
9. There are 35 differences in the amino acid sequence of hemoglobin when you
compare humans to birds. How long ago do you think humans and birds diverged
from a common ancestor? (just estimate)
ABOUT 200 MILLION YEARS AGO
Table C: Comparing cytochrome C of a human to other organisms
Organism
Chimpanzee
Fruit Fly
Horse
Gorilla
Rattlesnake
Red Bread Mold
Rhesus Monkey
Screwworm Fly
Snapping Turtle
Tuna Fish
Wheat
Pigeon
# of Differences
0
29
6
1
14
28
1
27
15
21
43
12
10. Based on the information in Table C above, which organism is the most closely
related to humans? Explain how you know this.
CHIMPANZEE, THERE ARE ZERO DIFFERENCES BETWEEN THE
CYTOCHROME C PROTEIN BETWEEN HUMANS AND CHIMPS.
11. Based on the information in Table C above, which organism is the least closely
related to humans? Explain how you know this.
WHEAT, THERE ARE 43 DIFFERENCES BETWEEN THE CYTOCHROME C
PROTEIN BETWEEN HUMANS AND WHEAT.
12. Other proteins can be used to establish degrees of evolutionary relatedness
between organisms. Would you expect to find roughly the same number of
differences in the amino acid sequence when comparing organisms? Explain your
answer.
YES, THE # OF MUTATIONS SHOULD REMAIN SOMEWHAT CONSTANT
BECAUSE TWO ORGANISMS DIVERGED FROM THE COMMON ANCESTOR
THE SAME LENGTH OF TIME AGO REGARDLESS OF WHICH PROTEIN YOU
COMPARE.