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Destination: Galapagos Islands Cyber Field Trip
ON THE ISLANDS

Cyber Field Trip Student Activity
DNA Adventure: The Web of Life


Introduction
Vocabulary
Materials
Procedure
Critical Thinking Questions




INTRODUCTION

On December 27, 1831, Charles Darwin set sail aboard the H.M.S. Beagle. He spent the next five years aboard the ship observing and collecting organisms from around the world, including the Galapagos Islands. The voyage of the Beagle was one of the most important events in science. It was on this voyage that Darwin laid the basis for his theories on natural selection. Darwin observed that species were constantly changing, and that they could evolve over time to produce new species.



VOCABULARY

Use the glossary to define any unfamiliar terms:



MATERIALS

  • 8 1/2 x 11 paper cut into five long strips.
  • 50 black jelly beans
  • 50 red jelly beans
  • 50 white jelly beans
  • 50 green jelly beans
  • needle and thread
  • paper and pencil (to create chart)


PROCEDURE

You will "synthesize" and "hybridize" five strands of DNA, which represent the genetic codes for hemoglobin, the oxygen-carrying pigment in blood, for Human, Hominoid (common ancestor), Gorilla and Chimpanzee.

Use the following color codes for the bases that make up DNA:

adenine (A) = black
cytostine (C) = red
thymine (T) = white
guanine (G) = green
  1. Label the five strips of paper as Human DNA, Human cDNA, Hominoid DNA (common ancestor), Gorilla cDNA and Chimpanzee cDNA.

  2. Using the needle and thread, string the following sequences of jelly beans (each on a different thread):

Hominoid (common ancestor) DNA
A-G-G-C-C-G-G-C-T-C-C-A-A-C-C-A-G-G-C-C

Human DNA
A-G-G-C-A-T-A-A-A-C-C-A-A-C-C-G-A-T-T-A

Human cDNA
T-C-C-G-T-A-T-T-T-G-G-T-T-G-G-C-T-A-A-T

Gorilla cDNA
T-C-C-G-G-G-G-A-A-G-G-T-T-G-G-T-C-C-G-G

Chimpanzee cDNA
T-C-C-G-G-G-G-A-A-G-G-T-T-G-G-C-T-A-A-T

Note: cDNA, or complementary DNA, is a single strand of DNA produced by an individual member of a species -- in this case, a human, a gorilla and a chimpanzee.
  1. Place each "DNA strand" on its labled piece of paper.

  2. Keeping in mind that complementary nitrogen bases always pair in DNA as A-T (adenine to thymine) and G-C (guanine to cytosine), match the human DNA with the chimpanzee cDNA, base by base (complementary bases should touch, while those that do not match should form "loops," as shown in the graphic below). By doing this, you are creating a representation of a hybridization of human and chimpanzee DNA.

    illustration of loops


  3. Make a copy of the chart below. Count the number of loops, which represent bases that do not match, and record your findings in the chart.

Human DNA Hybridized to: Chimp cDNA Gorilla cDNA
Number of Loops    
Number of Base Differences    
  1. Now match all three cDNA strands (human, chimp, gorilla) with the hominoid or common ancestor strand by repeating steps 4 and 5, using the chart below.

  2. Make a copy of the chart below. Record your results.
Hominoid DNA Hybridized to: Human cDNA Chimp cDNA Gorilla cDNA
Number of Loops      
Number of Base Differences      




CRITICAL THINKING QUESTIONS

  • Based on your findings, which hemoglobin gene do you believe is most closely related to humans -- that of gorillas or chimpanzees?

  • Based on your hemoglobin comparison, which species do you conclude is the most closely related to humans?

  • Which cDNA is most similar to the common ancestor hominoid DNA?

  • What other methods of determining species relatedness might scientists use?

  • Scientists use the rate of mutations in DNA, which occur at regular rates, to predict when two organisms branched off from a common ancestor. Most scientists agree that the great apes -- chimpanzees, gorillas and orangutans -- share a common ancestor with humans, but there is fierce disagreement as to when the branching-off occurred. Some scientists believe that the split was simultaneous, while others believe that the first to branch away were the orangutans, then gorillas, then chimps and, finally, humans. In the branching debate, which is more plausible according to your findings?

  • Explain the difference between convergent and divergent evolution. What forces drive convergent evolution? Divergent evolution?

  • In terms of natural selection and adaptation, why are the Galapagos Islands unique?

  • Based on your findings, explain Darwin's discovery of 13 distinct finch species derived from one ancestor. What factors were involved in their speciation?




 

Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.