Study of mitosis in onion root tip

Different stages of mitosis in onion root

Fixation of onion root tips

Onion root tip is the most suitable material for studying stages of mitosis. Precisely fixed onion root tips may be procured from the authentic supplier. Method of root tip fixation is described here.

Procedure

Take a healthy medium-sized onion and put it upright on a bottle or conical flask full of tap water. Ensure, the stem of onion bulb is dipped in water. Keep this setup in dim light condition for few days.

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With in a week, some fine roots grow in the water.

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Prepare fixative solution (acetoalcohol) by mixing 95 % ethyl alcohol and glacial acetic acid in 3:1 ratio.

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Tip cells of onion root divide once in 24 hours, Hence, tips must be cut and fixed between 12:00 pm to 12:15 pm, which is the mean time of cells to divide. Root tips must be immediately fixed in the acetoalcohol to arrest the stages of cell division.

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After 24 hours, fixed root tips should be transferred to 70 % ethyl alcohol for preservation.

Preparation of mitosis slide

Requirements

Fixed onion root tips, slide, coverslip, watchglass, blotting paper, glass rod, spirit lamp, acetocarmine, 1N HCl, compound microscope.

Procedure

Take few fixed onion root tips in a watch glass containing 1N HCl. and heat on sprit lamp at 60 °C for 2 minutes (or until roots become soft.)

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With the help of sharp blade, cut the transparent part of root and remove it. Retain only few mm bright white colored tip.

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Transfer the cleared root tips into another watchglass containing acetocarmine and gently heat for 2 minutes to increase the reactivity of stain with chromosomes. Allow it to cool down.

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With the help of fine forceps transfer the stained root tip onto a glass slide. Pour one drop of acetocarmine and cover with the help of coverslip.

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Wrap the coverslip into blotting paper and tap with help of a gloss rod to smash the tip. This may also be done by applying pressure with the thumb.

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Remove the blotting paper and observe under compound microscope; first under low power and then under high power.

Observation

All the stages of cell division will be visible in the slide.Four stages of mitosis (as shown in the figure) will be visible which are as follows:

Four stages of mitosis in onion root tip

1. Prophase: This stage (figure a) is marked by condensation of chromatin into visible chromosme. Nuclear envelop is degraded.
2. Metaphase: At this stage (figure b), chromosomes are aligned by their centromere on metaphasic plate (equatorial plane). Chromosomes are studied at this stage only and karyotype is prepared.
3. Anaphase: Chromosomes are dragged in two part at opposite poles. Centromere is leading and arms are trailing (figure c).
4. Telophase: At this stage (figure d), equal halve of the chromosomes reach two opposite poles. This stage marks end of karyokinesis.

Precautions

• During rooting bottom of onion bulb must be dipped in water.
• Fixation of root should be very precise and in time.
• Clearing in 1N HCl should be very proper to allow the stain to penetrate cell.
• Avoid the direct contact of acetocarmine ot skin.
• Tapping of coverslip with glass rod should be gentle to avoid any breakage.

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First published on 23-04-2021
Last updated on 24-04-2021