Onion Root Tips for Mitosis by Squash Method

Chromosomes from Onion Root Tips for Mitosis by Squash Method

Material

Take a few onion bulbs. Place them in a tray containing moist soil with dried roots facing soil. After 4 or 5 days new roots would appear. Take out onions and wash them. Cut 2 to 3 cm root tips and fix them directly in specimen tubes having Carnoy's fluid.

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Method

Take one or two preserved root tips on a slide with acetocarmine stain. Place a coverslip over them and tap it gently with a needle or pencil. Warm it tightly over the flame of a spirit lamp. Now put a piece of blotting paper on coverslip and apply a uniform pressure by your thumb. Another method can also be applied. From Carnoy's fluid, transfer all the root tips in a small corning conical flask containing acetocarmine. Boil the material and apply squash technique as above. 

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Observation

Examine the slide under the microscope. The cells and their chromosomes become spread out and distinct. Search out for different early and late mitotic stages such as interphase prophase, metaphase, anaphase, telophase and cytokinesis. Study and draw such stages.

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1. Prophase

General characters

(1) The nucleus becomes enlarged. DNA synthesis is completed. Prophase comes after interphase.

(2) Chromosomes become filamentous, thin and distinct.

(3) Prophase may be early prophase or late prophase.

(4) In late prophase, each chromosome divides into sister chromatids attached at centromere or kinetochore.

(5) Nuclear membrane and nucleolus disappear and cell enters into metaphase.

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2. Metaphase

General characters 

 (1) The spindle tubules start appearing and get attached with centromeres of chromosomes in early metaphase.

(2) Chromosomes move actively and become arranged at the equatorial plate or centre in metaphase.

(3) Specially in animals, the centrosome helps in the formation of spindle apparatus. In plants, centrosomes are missing.

(4) Centrosome has two centrioles which separate and each occupies opposite sides of the nucleus,

(5) After metaphase next stage is anaphase. 

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3. Anaphase

General characters

(1) Each chromosome splits at the position of centromere, forming sister chromatids or daughter chromosomes showing early anaphase

(2) The sister chromatids move towards poles with centromeres facing periphery while arms towards each other showing late anaphase

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