Notes to Instructors
Chapter 21 Genomes and Their Evolution
What is the focus of this activity?
While the Sanger method for sequencing DNA and the modifications that follow are
What is this particular activity designed to do?
Activity 21.1 How can we discover the sequence of an organism’s DNA?
This activity allows students to interpret the results of a classic Sanger method for
What misconceptions or difficulties can this activity reveal?
Because much of the methodology and information in this area is relatively new, students
Answers
Activity 21.1 How can we discover the sequence of an
organism’s DNA?
Bacterial genomes have between 1 million and 6 million base pairs (Mb). Most plants and
animals have about 100 Mb; humans have approximately 2,900 Mb. Individual
144 Notes to Instructors
Activity 21.1 145
By comparing the DNA sequences among organisms, scientists can determine
• what parts of the genomes are most similar among organisms and are therefore
likely to have evolved earliest,
The following activity has been designed to help you understand how genomes are
sequenced and how the sequence information may be used.
1. In 1980, Frederick Sanger was awarded the Nobel Prize for inventing the dideoxy
method (or Sanger method) of DNA sequencing. A double-stranded DNA segment
approximately 700 bp in length is heated (or treated chemically) to separate the
You set up each of four test tubes as noted below:
All tubes contain the same single-stranded DNA molecules and the same primers. All
other components required for DNA replication, such as enzymes, are present in each
Tube number Deoxynucleotides Dideoxynucleotide
1dATP, dTTP, dGTP, dCTP ddATP
2dATP, dTTP, dGTP, dCTP ddTTP
3dATP, dTTP, dGTP, dCTP ddGTP
4dATP, dTTP, dGTP, dCTP ddCTP
146 Activity 21.1
a. Which band in the gel contains the shortest DNA strand? What is the identity of
its terminal ddNTP?
The last band in the ddCTP column contains the shortest DNA strand. (Remember,
b. Which band contains the next shortest DNA strand? What is the identity of its
terminal ddNTP?
c. Continue reading the terminal ddNTP of each band from shortest to longest to
determine the linear sequence of nucleotides in the DNA strand complement.
What is the sequence?
2. The Sanger method has been modified so that each ddNTP used is now flagged with
an identifying fluorescent tag.
Assume that you run the same experiment that you did earlier. However, this time
ddATP tube ddTTP tube ddGTP tube ddCTP tube
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Activity 21.1 147
3. To help determine evolutionary relationships among different groups of organisms,
scientists compare gene sequences of highly conserved genes. What are “highly
conserved genes”? Give examples and indicate what is “highly conserved” and why.
Highly conserved genes are genes that have experienced little change throughout
evolution. Highly conserved genes are essential for the survival of organisms. For
4. What types of DNA do scientists use to determine individual identities of organisms
within the same species? Why do they use this type of DNA?
Repetitive D—for example, simple sequence DNA or short tandem repeat DNA—is
Band sequence
for combined
experiment
T red
T red
A green
C blue
T red
A green
A green