1. Cell Division in the Onion Root Tip
RS: Ms. Alvarez
CT: Mrs. Rojas
Adapted from M. Arias

2. The Cell Cycle Cells grow and divide.
As one cell enters Mitosis, two cells exit which are exact replicas or clones of the original “parent” cell.
Cells that no longer need to divide exit the cell cycle in G1(ex. neurons, muscle cells, fat cells do this).
Purpose: to replenish dead or dying cells, to allow an organism to grow and develop

3. Interphase A cell spends the majority of its lifetime in interphase.
Cell grows and carries out normal cell processes
DNA replicates
Prepares for cell division
Plant cell
Onion root tip

4. 4 Stages of Mitosis: Plants & Animal Cells
Onion root tip
whitefish
Telophase:
Nuclear envelope reappears.
Cytokinesis
The cytoplasm and all its contents divide
Prophase:
Chromosomes condense and form visible bodies.
Nuclear
envelope breaks
down.
Metaphase:
Chromosomes line up in the middle of the cell.
Spindle fibers attach to the centromeres.
Anaphase:
Centromeres split.
Sister chromatids separate and are pulled to opposite sides of the cell.

5. What happens when you cut your finger?1. 2. 4. 3. 5.
The cells bordering the injury will begin dividing to fill in the gap in the tissues that have been torn or broken. This is the process known as wound healing.

6. Mitosis in the Onion Root Tip
Today’s lab activity:
examine the tip of an onion root
Identify and draw the different stages of mitosis.
Root tips are useful to observe mitosis because the cells are frequently dividing as the root grows.

7. Examining root tip cells
When you examine a cross section of a root
tip, you will see lots of cells at different growth
stages.
Goals: Today’s activity will help you understand the phases of the cell cycle and determine what happens when Mitosis is disrupted.
Hypothesis: If the cells located in the onion root tip are frequently dividing, then we should be able to _________________.

8. Material • Onions root tips •Microscope
Microscope slides and cover slips
• Toluidine blue or aceto-orcein
• 1M hydrochloric acid
• Razor blades
• Safety goggles
• paper towel
Water

10. Procedure
Submerge the base of an onion in water at a constant temperature to grown roots for 48 hours.
Obtain an onion root and cut off the bottom 1 or 2 mm of the root tip and place it on a Slide.
**Wear proper safety equipment, including goggles and gloves, for the next steps.
Add a very small drop of 1M HCL acid to the root tip on the slide and let it sit for 4 min.
Using a paper towel soak the HCL away from the root tip.
Cover the root tip with a drop staining solution and let stand for 2 minutes.

11. Procedure
Blot excess stain and rinse with water until it runs clear. Add one drop of water and gently lay a coverslip over the root tip.
Using a pencil eraser, carefully apply pressure to the coverslip area to squash and spread the root tip tissue.
Use the low power objective on your microscope to look for thin layers of cells and then use the 40X power objective to observe mitotic stages in individual cells.
Identify chromosomes at the various stages of mitosis in the water treated root tips. Make sketches of the stages observed.

12. Conclusion Questions Summarize the procedure for this lab.
The cells in the root of an onion are actively dividing. How might the numbers of cells found in this region differ from a different part of the plant?
What stage were the majority of the cells in the water treated root tips?
What happens when cell division is interrupted?
1. What process must take place before mitosis can begin?
What process must take place before mitosis can begin?

13. Conclusion Questions
Why is mitosis important? Where does mitosis take place?
Give an example of cells in your body that divide slowly or do not divide at all.
What is cancer? How does cancer affect the rate of mitosis?
How would a slide of cancerous cells look different from this one?
What type of cells in your body do not divide by mitosis?

14. Acknowledgements: M. Arias Images taken from the following sites: