Observing Mitosis in Onion Root Tips

Materials needed:

(For growing roots in onion) Onion, small jar, water

(Experiment D-1) Acetic acid, ethanol, graduated cylinder, scissors, petri dish

(Experiment D-2) Distilled water, retort stand, dilute hydrochloric acid, cutter knife, acetocarmine solution, dissecting needle, filter paper, microscopic glass slides and cover glass, water bath, thermometer, test tube, microscope

Experiment date: 10/18/23 & 10/19/23

Written by: Sophia Han

Step 1

Leave an onion in a jar (a cup can also be used) filled with water for 5~7 days until roots grow from it.

Step 2

Cut around 1 cm of the tip of the root and keep it in a solution of ethanol and acetic acid (ratio of 3:1, with ethanol being 3 and acetic acid being 1) for a day.

Step 3

The next day, put the roots into a test tube with dilute hydrochloric acid of 60°C, and place it in a beaker of water. Warm/heat it in a water bath for 5~10 minutes.

Step 4

Put the root on top of a microscopic glass slide after washing it in distilled water and cut the end (1~2 mm) of it. Add a drop of acetocarmine solution on the cut-off part of the root.

Step 5

Use a dissecting needle to tear the (cut-off) root and cover it with a cover glass. Gently tap the cover glass with a rubber-tipped pencil.

Step 6

Wrap with a filter paper and press it with your thumb. Remove the filter paper, place the root under a microscope, and observe the mitosis occurring in the tip of the onion root.

Discussion

Results:

Explanation:

The tip of the root is used because it is a 'growing point' of a plant, in which the cells are dividing rapidly and multiply in numbers.
Leaving the root in the ethanol-acetic acid solution stops the cell division in progress (fix the cells in place, stopping mitosis).
By heating it in a water bath, the cells in the root tip are divided more easily.
Acetocarmine is used to stain the chromosome inside the cells red during different phases of mitosis. It makes the process easier to see.

What is mitosis?

Mitosis is a type of cell division by which the chromosome of a cell is replicated and get divided into two diploid (containing two complete sets of chromosomes, each parent contributing a chromosome to each pair) daughter cells.

The nucleus divides (nuclear division: karyokinesis) before the cytoplasm (cytoplasmic division: cytokinesis).

The number of chromosomes in a daughter cell (newly formed cell) is the same as the number of chromosomes in a parent cell (original cell' also called a mother cell).

Phases of mitosis:

1) Interphase: The chromosomes are replicated and prepares for mitosis.

2) Prophase: The chromosomes become condensed and coils, making them easier to pull apart. The mitotic spindle (ensures that the chromosomes are separated and positioned properly) begins to form. In the later phase of prophase (called prometaphase), the nuclear envelop is broken down and chromosomes are released.

3) Metaphase: The mitotic spindle lines the chromosomes at the middle of the cell (also known as the metaphase plate; is not a physical structure, but just a term for where the chromosomes are lined up) and gets ready to divide.

4) Anaphase: Two sister chromatids (two identical strands that make up a chromosome that are replicated) are separated from each other and pulled towards opposite ends of the cell.

5) Telophase: The final stage of mitosis; two daughter cells are formed from the parent cell. The nucleus forms around the chromosomes of the daughter cells. It is followed by cytokinesis, in which the cytoplasm divides physically. The chromosomes are decomposed, and the spindle is broken down.