Exercise 13

Principle: Acetocarmine combines with nucleic acid present in the nuclei of cells to form a deep red conjugate.

Requirements: Onion bulb, onion root tips, 2 to 4% acetocarmine/acetoorcein stains, slide and coverslips, brush/needle, pair of fine scissors, filter paper and microscope.

Procedure:

(i) Peel off epidermis from the fleshy leaf of onion and put it on a slide.

Add a few drops of water over it to avoid desiccation.

(ii) Cut out a small piece (about 0.5 cm size) of the epidermal peel and discard the remaining portion.

(iii) Wipe out the water with a filter paper.

(iv) Put 2 drops of acetocarmine on the epidermal peel and heat gently on a spirit lamp.

(v) Apply a coverslip over the peel avoiding air bubbles and wrinkles of the material.

(vi) Wipe out the excess stain with help of blotting paper.

(vii) Examine the material under low magnification of a microscope.

Observation:

Record your observations with regard to shape of cell, the number of nuclei and their position in the cell. Draw a diagram based on your preparation and label its parts.

Discussion:

Nuclei in cells are extremely rich in nucleic acid which exist in a conjugated form with protein to form nucleoproteinous structures, called chromatin fibres/chromosomes.

Questions:

1. What are the building blocks of the nucleic acid?

2. What is DNA and how is it different from RNA?

3. Name different nitrogenous bases present in the nucleic acid.