Purpose

To identify DNA in onion root tip cells by staining it.

Materials and Methods

1. Place some tap water into a plastic treating dish and rinse a root tip in it.


2. The cells of the root tip must now be macerated so that they will separate and flatten when the smear is made. To do this, drain the water from the treating dish, holding the root tip with the brush. Cover the root tip with a few (3-4) drops of 1 M hydrochloric acid (CAUTION). After a few minutes, the root tip will become soft and ready for the next step.


3. Rinse the root tip with tap water thoroughly, washing away the acid and holding the root tip with the brush. Drain the dish, leaving the root tip behind.


4. Add enough of the leucofuchsin dye (5-7 drops) to cover the root tip. Allow it to stand for 20 minutes.


5. Rinse off the excess dye with tap water, holding the root tip behind with the brush. Drain the dish.


6. Cover the root tip with sodium bisulfite solution (5-7 drops) for 2-3 minutes. This step bleaches all parts of the cell which do not contain DNA.


7. Remove the root tip with the brush, place it on a clean slide, and add a drop or two of acetic acid.


8. Place the slide on a smooth, flat surface, add a cover slip to the slide and place the slide into the fold of a piece of paper toweling. Press firmly (BUT NOT BY TWISTING) through the toweling on the cover slip using the end of the middle finger. The acetic acid causes the cytoplasm to gradually swell. The gentle pressure on the cover slip will spread apart the chromosomes and the dye will stain the DNA in the cell purple.


9. Examine the slide with the microscope, first with low power and then high power.

Observations (Answer in your notes)

1. Make a drawing of what you observe.


2. Describe where the DNA is located in the cell.


3. Compare the colour of the DNA with that of the cytoplasm.


4. Draw some cells in the process of dividing.


5. Why is DNA considered to be such an important molecule?


6. Why must the quanitity of the DNA in cells remain the same from one generation to another?


7. If a DNA molecule contains 32% Adenine, what percent of the DNA molecule is Thymine? Cytosine? Guanine?