Preparing a temporary mount of an onion root tip to study mitosis is an important Class 12 Biology practical that helps students observe cell division stages clearly under a microscope. Onion root tips are ideal because they contain actively dividing meristematic cells. In this practical, students soften the root tip, stain it using safranin, and gently squash it to create a thin, transparent slide. Under the microscope, stages like prophase, metaphase, anaphase, and telophase become visible. This experiment strengthens understanding of the cell cycle, mitosis, and chromosomal behaviour, making it a key part of Class 12 Biology practical exams and lab learning.

AIM:- Prepare a temporary mount of onion root tip to study mitosis

REQUIREMENTS

Onion bulbs, conical flasks/glass bottles, corked vial/tube, petri dishes, scissors, forceps, methyl alcohol, acetic acid, hydrochloric acid, acetocarmine, distilled water, spirit lamp, microscope. slides coverslips blotting paper etc.

PROCEDURE

1. Take medium sized bulb of onion and trim off the old roots from its base by means of a sharp blade,

Place the onion on a conical flask/ glass bottle full of water, with its base touching the water.

Keep it for a week to grow the roots.

Cut 5 mm off the tips of roots and put them into a vial containing a mixture of 1 : 3 acetic acid and methanol. Keep for one hour. This process is called fixation. (Cutting of root tips should be done in the morning between 7.00 a.m. to 8.00 a.m. during the summer and between 9.30 a.m.to 11.30 a.m. during the winter).

Remove 2 or 3 root tips and hydrolyse them by warming to 60⁰C in 1 N hydrochloric acid for 15 minutes

5. Remove the root tips and wash them thoroughly in water.

6. Place a drop of acetocarmine on a slide. Put one hydrolyse root tip in a drop and place a coverslip on the root.

7. Gently squash the root by tapping the coverslip with the blunt end of a pencil or needle until the cells separate and spread out into a very thin layer.

Make sure that there are no air bubbles under the coverslip.

8 Gently warm the slide over a flame for a few seconds.

9. Observe first under the low power of the microscope to locate the dividing cells. Examine the different stages of mitosis under the high power of the microscope.

OBSERVATIONS

Under low power of the microscope, rectangular cells with Pink nucleus are seen scattered. Under high power of the microscope following stages become distinct:

1. Interphase

(i)It is a non-dividing phase of the cell cycle between two successive cell divisions.

(ii) Chromatin fibres appear in the form of a network within the nucleus.

(iii) Nuclear envelop and nucleolus are distinct.

2. Prophase

(i) Chromatin material shortens and condenses into thread like structures called chromosomes

(ii) Each chromosome consists of two chromatids, jointed at a point called centromere.

(iii) Nuclear membrane and nucleolus start disintegration and disappear at the end of prophase.

3. Metaphase

(I) A bipolar spindle develops in the cell Chromosomes become thick and two chromatids of each chromosome become clear.

(ii) Chromosomes become arranged at the equator of the spindle

(i) Each chromosome get attached to the spindle fibres at its centromere.