Lesson 2

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:

(1) 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.

(2) 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.

(3) 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.

(4) 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.

(5) Assign the reading "DNA Isolation From Plant Tissue" for the next lesson.

Supplemental Materials:

"Genetic Analysis of a Rare, Endangered Cactus"- Discussion Guide
"Chemical Detectives: How to Find and Isolate DNA" - Activity Sheet
"Chemical Detectives: How to Find and Isolate DNA" - Answer Key
"DNA Isolation from Plant Tissue" - Lab Procedure