Theory & Procedure, Study Mitosis in Onion Root Tip | Additional Study Material for NEET PDF Download

Aim

Our aim is to prepare a temporary mount of onion root tip to study mitosis.

Theory

All organisms are made of cells. For an organism to grow, mature and maintain tissue, new cells must be made.  All cells are produced by division of pre-existing cells. Continuity of life depends on cell division.  There are two main methods of cell division: mitosis and meiosis. In this tutorial we will learn about mitosis.

What is Mitosis?

Mitosis is very important to life because it provides new cells for growth and replaces dead cells. Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. Each cell division consists of two events: cytokinesis and karyokinesis.  Karyokinesis is the process of division of the nucleus and cytokinesis is the process of division of cytoplasm.

Events during Mitosis

1. Prophase:

   1. Mitosis begins at prophase with the thickening and coiling of the chromosomes.

   2. The nuclear membrane and nucleolus shrinks and disappears.

   3. The end of prophase is marked by the beginning of the organization of a group of fibres to form a spindle.

2. Metaphase

   1. The chromosome become thick and two chromatids of each chromosome become clear. 

   2. Each chromosome attaches to spindle fibres at its centromere.

   3. The chromosomes are arranged at the midline of the cell.

3. Anaphase

   1. In anaphase each chromatid pair separates from the centromere and move towards the opposite ends of the cell by the spindle fibres.

   2. The cell membrane begins to pinch at the centre.

4. Telophase

   1. Chromatids arrive at opposite poles of cell.

   2. The spindle disappears and the daughter chromosome uncoils to form chromatin fibres.

   3. The nuclear membranes and nucleolus re-form and two daughter nuclei appear at opposite poles.

   4. Cytokinesis or the partitioning of the cell may also begin during this stage.

The stage, or phase, after the completion of mitosis is called interphase.  It is the non dividing phase of the cell cycle between two successive cell divisions.  Mitosis is only one part of the cell cycle. Most of the life of a cell is spent in interphase.  Interphase consist of three stages call G1, S and G2.

Mitosis in Onion Root Tip

The meristamatic cells located in the root tips provide the most suitable material for the study of mitosis. The chromosome of monocotyledonous plants is large and more visible, therefore, onion root tips are used to study mitosis. Based on the kind of cells and species of organism, the time taken for mitosis may vary. Mitosis is influenced by factors like temperature and time

Learning Outcomes:

• Students understand the term mitosis.

• Students understand the different events during mitosis.

• Students do the experiment better in the real lab having gone through the animation and simulation.

Materials Required:

1. Real Lab Procedure:

• Take an onion and place it on the tile.
• Carefully remove the dry roots present using a sharp blade.
• Grow root tips by placing the bulbs in a beaker filled with water.
• New roots may take 3–6 days to grow.
• Cut off 2–3 cm of freshly grown roots and let them drop into a watch glass.
• Using a forceps, transfer them to the vial containing freshly prepared fixative of aceto-alcohol (1:3: glacial acetic acid: ethanol).
• Keep the root tips in the fixative for 24 hours.
• Using a forceps, take one root and place it on a clean glass slide.
• Using a dropper, place one drop of N/10 HCl on the root tip followed by 2–3 drops of acetocarmine stain.
• Warm it slightly on burner. Care should be taken that the stain is not dried up.
• Carefully blot the excess stain using filter paper.
• Using a blade, cut the comparatively more stained tip portion of the root, retain it on the slide and discard the remaining portion.
• After that, put one drop of water on the root tip.
• Mount a cover slip on it using a needle.
• Now, slowly tap the cover slip using the blunt end of a needle so that the meristematic tissue of the root tip below the cover slip is properly squashed and spread as a thin layer of cells.
• This preparation of onion root tip cells is now ready for the study of mitosis.
• Place the slide under the compound microscope and observe the different stages of mitosis.
• Various stages of mitosis are prophase, metaphase, anaphase and telophase.

Simulator Procedure (as performed through the Online Labs)

• Drag and drop the slide onto the stage of the compound microscope.
• Click on the eye piece of the compound microscope to view the cells.
• To change the power of the lens, you can choose a lens from the ‘Select objective lens’ drop down list.
• For coarse adjustments, you can either click on the up and down arrow of ‘Coarse adjustment’ knob, or click on the left and right arrows of 'Course Adjustment' seen on the left control panel.
• For fine adjustments, you can click on the left and right arrows of ‘Fine adjustment’ seen on the left control panel.
• To observe each part, you can click on the four way directional arrow of ‘Slide adjustment’ seen on the left control panel.
• You can redo the experiment by clicking on the ‘Reset’ button.