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 and are called neutral mutations. 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 the models of the section of the protein hemoglobin between amino acid 87 and amino acid 116 and complete the empty cells on Table A below for the chimp, gorilla, Rhesus monkey, horse and kangaroo.
  2. Look at each numbered column and circle the amino acids in each column that are different from the human amino acid for that column.
  3.  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.
  4.  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

 

 

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

 

Human

THR

 

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

Chimp

THR

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

 

Gorilla

THR

 

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

Rhesus

Monkey

GLU

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

Horse

ALA

 

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

Kangaroo

LYS

 

LEU

SER

GLU

LEU

HIS

CYS

ASP

LYS

LEU

HIS

VAL

ASP

PRO

GLU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

 

Human

ASN

 

PHE

ARG

LEU

LEU

GLY

ASN

VAL

LEU

VAL

CYS

VAL

LEU

ALA

HIS

 

Chimp

ASN

 

PHE

ARG

LEU

LEU

GLY

ASN

VAL

LEU

VAL

CYS

VAL

LEU

ALA

HIS

 

Gorilla

ASN

 

PHE

LYS

LEU

LEU

GLY

ASN

VAL

LEU

VAL

CYS

VAL

LEU

ALA

HIS

 

Rhesus Monkey

ASN

PHE

LYS

LEU

LEU

GLY

ASN

VAL

LEU

VAL

CYS

VAL

LEU

ALA

HIS

 

Horse

ASN

 

PHE

ARG

LEU

LEU

GLY

ASN

VAL

LEU

ALA

LEU

VAL

VAL

ALA

ARG

 

Kangaroo

ASN

 

PHE

LYS

LEU

LEU

GLY

ASN

ILE

ILE

VAL

ILE

CYS

LEU

ALA

GLU

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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 amino acid differences from human

Chimpanzee

 

Gorilla

 

Rhesus monkey

 

Horse

 

Kangaroo

 

 

  1. How many columns from Table A were the amino acids that same for each organism?

 

 

  1. Based on the information in Table B above, which organism is the most closely related to humans? Explain how you know this.

 

 

 

  1. Based on the information in Table B above, which organism is the least closely related to humans? Explain how you know this.

 

 

 

  1. There is a difference of only one amino acid in one chain of hemoglobin between humans and gorillas. What might have caused this difference?

 

 

 

  1. If the amino acid sequences in the proteins of two organisms are similar, why will their DNA also be similar? (NOTE: Use the picture to help you. What is the relationship between amino acid sequences and DNA?)

 

Text Box: Protein

 

 

 

  1. Compare the DNA models for the Rhesus monkey and the horse. What has changing the amino acid 111 from VAL to ALA and the amino acid 114 from LEU to VAL done to the protein?

 

 

 

  1. What holds a protein in a particular shape?

 

 

 

  1. When a protein changes shape, how does that affect its function?

 

 

 

  1.  Using the picture below, if there are 4 differences in the DNA of species A and species B, how long ago did they diverge from a common ancestor?

Text Box: Species B

 

Text Box: Species A

 

 

 

 

  1. Using the picture in questions10, estimate how long ago species A and species B diverged if these are 16 differences in their DNA?

 

 

 

 

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. Use the molecular clock below to answer questions 12-15.

 

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

 

 

 

  1. Why do these biologists believe that humans and gorillas diverged from a common ancestor only a few million years ago?

 

 

 

 

  1. 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?

 

 

 

 

  1. There are 20 differences in the amino acid sequence of hemoglobin when you compare humans to mice. How long ago do you think humans and mice diverged from a common ancestor?

 

 

 

 

  1. 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? (make a good estimate)

 

 

Table C: Comparing cytochrome C of a human to other organisms

 

Organism

# of Differences

Chimpanzee

0

Fruit Fly

29

Horse

6

Gorilla

1

Rattlesnake

16

Red Bread Mold

43

Rhesus Monkey

1

Screwworm Fly

28

Snapping Turtle

15

Tuna Fish

21

Wheat

38

Pigeon

12

 

 

  1. Based on the information in Table C above, which organism is the most closely related to humans? Explain how you know this.

 

 

 

 

  1. Based on the information in Table C above, which organism is the least closely related to humans? Explain how you know this.

 

 

 

 

  1. Why is it that only neutral mutations can be used as a molecular clock?

 

 

 

 

  1.  Explain how comparing the number of neutral mutations between amino acid sequences of two different organisms can be used as a molecular clock.

 

 

 

 

 

  1. Other proteins besides hemoglobin and cytochrome C 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 organismsí other proteins? Explain your answer.