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The Arizona Hedgehog
Cactus:
Endangered Species or not?
Lesson 2: Discussion
of the Problem and Introduction to Genetic Analysis
Purpose:
The teacher should lead a discussion with the
class to make list of the definitive characteristics of each of the
varieties of cactus in question. Through this process the students should
come to understand that there is so much variability of characteristics
within each population, and overlap between them, that defining each
variety based on morphological characteristics is very difficult. An
introduction to the idea that the genes contained on DNA (and their
sequence) are what determine characteristics and, consequently, comparing
DNA variability may help to define each population. Students should
then begin to think about where DNA is located within the cell and how
a scientist might go about isolating it from the rest of the cellular
contents.
Time:
One 50-60 minute class period
Materials Preparation:
- Student copies of "Chemical Detectives: How to
Find and Isolate DNA".
- Student copies of "DNA Isolation from Plant Tissue".
Student Objective(s):
- Formulate a hypothesis regarding the validity of
the current status of the Arizona Hedgehog cactus as a distinct
variety of cactus, taking into consideration the amount of diversity
found within each population and between the two populations in
question.
- Design a procedure to isolate DNA from the tissue
of plants, given a list of chemicals and equipment, and prior knowledge
of the structure and biochemistry of plant cells.
Lesson Plan:
- Have the students provide a list of characters
that they defined from the previous day's assignment to be compiled
on the board by the teacher. Discuss each character with the class
to come to a consensus about its validity.
- Compose a working hypothesis with class discussion
about whether or not the Cochise county cacti are the same as variety
arizonicus. Write the hypothesis on the board.
- Discuss the relationship between observed phenotypes
and genotypes. Introduce the concept of analyzing differences among
individuals and between populations by determining differences in
DNA sequence.
- Hand out the student activity sheet "Chemical Detectives:
How to Find and Isolate DNA". Review cellular structure and the
location of compounds such as lipids, proteins, polysaccharides
and nucleic acids with the class by drawing a "cell" on the board
and writing in the locations of each of the compounds above. Students
should then work in groups to complete the activity.
- Assign the reading "DNA Isolation From Plant Tissue"
for the next lesson.
Supplemental Materials:
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