Lesson 2-3

Chemical Detectives: How to Find and Isolate DNA
Answer Key

To analyze organisms at the molecular genetic level, DNA must be extracted and isolated from other compounds in the cells of the organism. How can this be done?

Directions: You are a molecular biologist who needs to isolate DNA from the tissue of a plant, without allowing it to break down in the process. Using your knowledge of plant cells and biochemistry, you must develop a technique to extract DNA in your laboratory. You review your knowledge of plant cells and summarize it below.

Plant cell structure and biochemistry:
Where DNA is located in the cell: in the nucleus

DNA is a (charged or uncharged) molecule: DNA is negatively charged

Other types of molecules found within the cell:

Compound	Charged or Uncharged?	Location in the Cell
Lipids	uncharged (nonpolar)	membranes
Proteins	both charged and uncharged	throughout cytoplasm, attached to DNA
Carbohydrates	uncharged	cytoplasm
RNAs	charged	nucleus, cytoplasm, ribosomes

Students may list more specific types of compounds as well.

Physical barriers in the cell to be overcome to find and isolate the DNA:
The cell wall, plasma membrane and the nuclear membranes must all be broken and disrupted, to allow the DNA to be released into solution for further chemical isolation.

Warning!: There are enzymes (proteins) in the cell which break down DNA. Where are they? How can they be disabled?

Enzymes are present throughout the cytoplasm. They can be broken down using other enzymes or heat.
Chemicals and Equipment Available:

Blender - breaks cell walls and plasma membranes but won't break nucleus or organelle membranes

Coffee grinder or mortar and pestle - breaks cell wall and all cell membranes including nuclear membrane

Liquid nitrogen or dry ice - freezes tissue so enzymes are inactivated and tissue shatters

Centrifuge - separates components according to density

Hot water bath - heat helps break up lipids and denature proteins

Detergent (CTAB) - breaks up lipid complexes

Salt solution - polar water molecules and charged ions dissolve other charged molecules

Chloroform - a nonpolar (noncharged) liquid which denatures proteins and dissolves other nonpolar substances

Isopropyl alcohol - a weakly polar compound, DNA is not soluble in it

Using the equipment and chemicals above, design a procedure to isolate and extract DNA from the cells of plant tissue. Write out your procedure, step-by-step, below.

Answers will vary.