CLASSIFYING LIFE BY USING DNA
OBJECTIVES
1. How is DNA used to classify life?
2. How is DNA used to show probable evolutionary relationships? (MCA-II science standard)
INTRODUCTION
Until the mid-1970s, taxonomists usually classified life by morphology (shape). For example, a
biologist might compare the structure of forelimbs of mammals. In recent years, biologists have
also been able to compare the DNA and thus proteins in different organisms.
A hypothesis known as the molecular clock hypothesis uses the comparison of DNA sequences to
make predictions about the relatedness of the organisms from which the DNA was taken.
According to the molecular clock hypothesis, any changes that occur in the genetic materials of
isolated populations are due to mutations in the DNA that are passed from one generation to the
next. The rate at which the mutations accumulate can be measured. On the basis of this
information, the approximate point at which two species diverged from a common ancestor can be
determined. This activity shows a model of this technology.
MATERIALS
PART A
4 containers of colored paper clips, masking tape
DNA Molecules
1. Construct a color key for the paper clips.
A
T
C
G
2. The nitrogenous bases listed in the table below represent a section of the gene that codes for the
protein hemoglobin. Note that “cDNA” stands for the “complementary DNA” that matches the
strand simply labeled “DNA.” Build each DNA sequence. Use tape to label the organism as
well as the left end of the sequence versus the right end.
organism
human DNA
human cDNA
chimpanzee cDNA
gorilla cDNA
common ancestor DNA
PART B
1
A
T
T
T
A
2
G
C
C
C
G
3
G
C
C
C
G
4
C
G
G
G
C
5
A
T
G
G
C
6
T
A
G
G
G
7
A
T
G
G
G
8
A
T
A
A
C
9
A
T
A
A
T
10
C
G
G
G
C
11
C
G
G
G
C
12
A
T
T
T
A
13
A
T
T
T
A
14
C
G
G
G
C
15
C
G
G
G
C
16
G
C
C
T
A
Relatedness
1. Compare the sequences of the human DNA and the chimpanzee cDNA.
a.
b.
c.
d.
e.
Match the sequences base by base (paper clip by paper clip)
Where the bases are complementary, allow the paper clips to touch
Where the bases are not complementary, separate the paper clips to form a loop
Count the number of loops (areas of difference)
Count the number of clips that do not touch (number of differences)
17
A
T
T
C
G
18
T
A
A
C
G
19
T
A
A
G
C
20
A
T
T
G
C
f. Record in the data table
2. In the same way, compare the sequences of the human DNA and the gorilla cDNA.
3.
human DNA compared to:
number of loops
number of differences
chimpanzee cDNA gorilla cDNA
4. Based on the data you have collected for this one protein (hemoglobin), is the gorilla or the
chimpanzee DNA more similar to the human DNA?
_______________________________
5. In addition to DNA, what other evidence might you look at in order to verify your work?
____________________________________________________________________________
PART C
An Evolutionary Puzzle
Biologists have determined that mutations in DNA occur at a regular rate. They use this rate to
predict how long ago in evolutionary history two organisms began to separate from a common
ancestor. In this part of the activity, you will use your paper clips to provide data in support of one
of two hypotheses about a common ancestor for humans, chimpanzees, and gorillas.
1. Read the following information about a current debate in evolution:
Most scientists agree that humans, gorillas, and chimpanzees shared a common
ancestor at one time in evolutionary history. However, one group thinks the
fossil record shows that gorillas, chimpanzees, and humans split from one
common ancestor at the same time.
A second group thinks the fossil record show there were two splits. In the first
split, gorillas split from the common ancestor. Humans and chimpanzees then
shared another common ancestor for perhaps 2 million years. They then split
again and evolved into their present states.
2. Match all three strands of cDNA (human, chimpanzee, and gorilla) with the hypothetical
common ancestor DNA. Use the same technique to compare sequences as in Part B.
3.
common ancestor DNA human chimpanzee gorilla
human compared to:
cDNA cDNA
cDNA
number of loops
number of differences
4. Which cDNA is most similar to the common ancestor DNA? ___________________
5. Which two cDNA give the most similar patterns of matching and looping when
compared to the common ancestor DNA? __________________________________
6. Which model in the evolutionary debate does your data support: 1 or 2 splits?
_______________
Explain your choice. _____________________________________
_________________________________________________________________________