In this activity, you will observe cell division happening
in real time. Onion root tips are actively growing regions where cells divide rapidly, making them perfect for studying
the different stages of cell division.
Aim
To observe different stages of cell division (mitosis) in onion root tip cells under a microscope.
Materials Required
Fresh onion bulb
Glass jar or beaker with water
Aceto-alcohol (1:3 ratio — glacial acetic acid : ethanol)
70% ethanol
Dilute hydrochloric acid (HCl)
Aceto-carmine stain
Glass slides and coverslips
Spirit lamp or burner
Scissors or blade
Forceps
Compound microscope
Dropper or pipette
Procedure
Step-by-Step Instructions
Part A: Growing the Roots (5-6 days before experiment)
Prepare onion bulb: Take a fresh onion bulb and remove old dried roots from the bottom.
Set up for rooting: Place the onion bulb over a jar filled with water so that only the base (root region)
touches water. You can use toothpicks to support the bulb.
Wait for roots to grow: Keep in a warm place for 5-6 days. Change water daily. Fresh white roots will
grow 2-3 cm long.
Part B: Fixing and Preserving (1 day before observation)
Cut root tips: Using scissors, cut 2-3 cm of the freshly grown roots (the white part).
Fix in aceto-alcohol: Immediately transfer root pieces to a test tube containing aceto-alcohol
(1 part glacial acetic acid + 3 parts ethanol). Leave for 24 hours. This kills and fixes the cells.
Transfer to ethanol: After 24 hours, transfer roots to 70% ethanol for preservation. Roots can be
stored here for several days.
Part C: Staining and Slide Preparation (Day of observation)
Wash the roots: Take preserved roots from ethanol and wash with water.
Soften with HCl: Place roots in dilute HCl (1N) for 10-15 minutes. This softens the tissue and separates cells.
Rinse again: Wash roots thoroughly with water to remove acid.
Stain with aceto-carmine: Transfer to aceto-carmine stain. Leave for 5-10 minutes until roots turn dark red/pink.
Warm gently: Warm the stain gently over a spirit lamp for a few seconds (do NOT boil). This helps stain
penetrate better.
Prepare slide: Cut the tip portion (2-3 mm from the very end) of a stained root. Place on a clean glass slide.
Add coverslip and squash: Add a drop of aceto-carmine, place coverslip. Gently press with your thumb to
squash the root tip into a single layer of cells. Do not press too hard.
Observe under microscope: Start with low power (10×), then switch to high power (40×) to see individual cells clearly.
Why Use Root Tips?
Root tips are actively growing regions called meristematic zones. Cells here divide rapidly by
mitosis to help roots grow longer. This makes them perfect
for observing cell division. Other parts of the plant (like leaves) have mature cells that rarely divide.
Observations Under Microscope
What you will see:
Many cells visible: Hundreds of rectangular cells packed together
Different appearances: Cells look different from each other — some have visible thread-like structures
(chromosomes), others don't
Stained chromosomes: Some cells show dark red/pink thread-like or rod-like structures — these are chromosomes
during division
Clear nuclei: Some cells show round, darkly stained nucleus (cells not dividing)
Various stages: Cells at different stages of division are visible in the same slide
Why Do Cells Look Different?
Cells look different because cell division is a continuous process. At any given moment:
Mitosis is a type of cell division where
one parent cell divides to produce two identical daughter cells with the same number of chromosomes. It is essential for:
Growth: Making more cells to increase body size
Repair: Replacing damaged or dead cells
Maintenance: Replacing old cells with new ones
The Cell Cycle
The cell cycle is the series of events that happen from one cell division to the next. It has two main phases:
Phase
What Happens
What You See
Interphase (Resting phase)
• Cell grows
• DNA replicates (makes copies)
• Prepares for division
• Takes 90% of cell cycle time
Round, darkly stained nucleus
No visible chromosomes
Normal cell appearance
Mitosis (Division phase)
• Chromosomes become visible
• Nuclear membrane breaks
• Chromosomes separate
• Two nuclei form
• Cell divides into two
Thread-like chromosomes visible
Chromosomes at different positions
Cells look different from each other
Why This Experiment Shows Continuous Division
In your slide, you saw many cells at different stages. This is proof that:
Cell division doesn't happen simultaneously in all cells
Some cells are dividing while others are resting
The process is continuous and ongoing in growing tissues
This ensures steady growth of the root
Connection to Cell Theory
This experiment proves the third postulate of Cell Theory:
"All cells arise from pre-existing cells." You directly observed how new cells are formed through division
of existing cells, validating what Rudolf Virchow stated in 1855.
Precautions and Pro Tips
Precautions:
Use fresh, actively growing roots (not old brown roots)
Don't overheat while warming the slide — gentle warmth only
Fix roots in aceto-alcohol for full 24 hours
Don't press coverslip too hard (cells will rupture)
Cut only the very tip (2-3 mm) where division is most active
Handle HCl carefully — it's corrosive
Pro Tips:
Change water daily while growing roots for best results
Roots should be white and 2-3 cm long before cutting
If stain is too dark, rinse slide slightly and re-observe
Look for the whitest, freshest root tips for best division
Squash gently but firmly to spread cells in single layer
Scan different areas of slide to find dividing cells
5 Important Viva Questions
Q1. Why is the root tip used for this experiment and not a leaf or stem?
Root tips are used because they contain meristematic tissue — a region of actively
dividing cells.
Why root tips are ideal:
Actively dividing cells: Root tip cells divide rapidly by mitosis
to help roots grow longer
High mitotic index: At any time, many cells are undergoing division (10-20% of cells)
Easy to observe: Large number of dividing cells visible in small area
All stages visible: Different stages of mitosis can be seen in the same slide
Why NOT leaves or stems:
Most leaf and stem cells are mature and differentiated
They rarely undergo division (unless injured)
Very few dividing cells would be visible
Would be difficult to observe mitosis
Other examples of meristematic tissue: Shoot tips (growing ends of stems), lateral buds, cambium
(in woody plants) — all these also have actively dividing cells.
Q2. What does aceto-carmine stain? Why is staining necessary?
Aceto-carmine stains chromosomes and the nucleus because it binds to DNA.
What it stains:
Chromosomes: Appear dark red/pink during cell division
Nucleus: Also stains red in non-dividing cells (contains DNA)
Makes DNA-containing structures clearly visible
Why staining is necessary:
Chromosomes are transparent:
Without stain, chromosomes are nearly invisible under microscope
They're made of DNA and proteins which are transparent
Stain makes them dark and visible
Provides contrast:
Separates chromosomes from cytoplasm
Makes different stages of division easy to identify
Helps distinguish dividing cells from non-dividing cells
Alternative stains: Other DNA stains like acetocarmine, safranin, or methylene blue can also be used,
but aceto-carmine gives the best results for observing chromosomes during mitosis.
Q3. Why are different stages of cell division visible in the same slide?
Different stages are visible because cell division is a continuous, asynchronous process — not all
cells divide at the same time.
Why this happens:
Cells divide independently:
Each cell has its own internal clock (cell cycle)
Cells don't wait for their neighbors to divide
Division happens continuously but independently
Division takes time:
Mitosis is not instant — it takes time to complete
Different cells are at different points in the process
When you observe, you're seeing a "snapshot" of ongoing division
Constant cell cycle:
Some cells are in interphase (resting/preparing)
Some are in early mitosis (prophase)
Some are in middle stages (metaphase, anaphase)
Some are finishing division (telophase)
Think of it like traffic:
Imagine cars at a traffic signal
At any moment, some cars are waiting (interphase)
Some are starting to move (early mitosis)
Some are crossing (middle stages)
Some have just crossed (late mitosis)
Similarly, root tip cells are constantly "moving through" the cell cycle
This is evidence that cell division is a continuous process in growing tissues,
not something that happens all at once. Learn more about how
cells divide.
Q4. What is the cell cycle? What are its main phases?
The cell cycle is the series of events that take place in a cell
from one division to the next division.
Main phases of the cell cycle:
1. Interphase (I-phase) — "Resting" Phase
Takes up about 90-95% of cell cycle time
Cell is NOT actually resting — very active!
What happens:
Cell grows in size
DNA replicates (makes copies) — now cell has double DNA
Appearance: Chromosomes visible at various positions
After mitosis:
Two new daughter cells enter interphase
They grow and prepare for their own division
The cycle continues — this is how organisms grow
Important: The cell cycle is a controlled process. Cells have checkpoints to ensure
division happens correctly. If control is lost, uncontrolled division occurs, leading to tumors. Learn about
mitosis in detail.
Q5. What type of cell division occurs in root tip cells — mitosis or meiosis? Why?
• For growth and repair
• Produces 2 daughter cells
• Chromosome number same (16→16)
• Occurs in body cells
• For reproduction only
• Produces 4 daughter cells
• Chromosome number halved (16→8)
• Occurs only in reproductive organs
Where meiosis occurs in plants: Only in anthers (male) and ovaries (female) to produce pollen and egg
cells (gametes) with half the chromosomes. Compare
mitosis and
meiosis in detail.
Key Terms to Remember
Mitosis
Type of cell division producing two identical daughter cells; used for growth, repair, and maintenance.
Cell Cycle
Series of events from one cell division to the next; includes interphase and mitosis.
Meristematic Tissue
Plant tissue with actively dividing cells; found in root tips, shoot tips, and cambium.
Aceto-carmine
Red/pink stain that binds to DNA; makes chromosomes and nuclei visible under microscope.
Chromosome
Thread-like structure containing DNA; becomes visible during cell division when DNA condenses.
Interphase
Phase between divisions when cell grows, duplicates DNA, and prepares for division; 90% of cell cycle.
Fixing
Process of preserving cells by killing them while maintaining structure; done using aceto-alcohol.
Squash Preparation
Technique of gently pressing coverslip to spread cells into single layer for better observation.
