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NCERT Solutions for Class 9th Science Chapter 2 Activity 2.5: Let us enhance our skills — How do Normal Cells Grow and Divide?

Book page 20 Updated on2026-09-08

Q1.
Leave the setup for 5–6 days and observe. Do you observe the roots growing?
Answer

Yes. Fresh white roots grow out from the base of the onion bulb into the water, usually a few centimetres in 5–6 days.

  • The base of the bulb bearing the roots must stay immersed; the rest of the bulb stays above the water.
  • Growth is fastest at the very tip. Cut 2–3 cm of the freshly grown roots for the slide, because it is the tip that contains dividing cells.
2–3 cm of root cut → aceto-alcohol (glacial acetic acid : ethanol :: 1 : 3) for 24 hours
→ 70 per cent ethanol for preservation
→ dilute HCl for 10–15 minutes to soften the tissue
→ 2–3 drops aceto-carmine stain, 5–10 minutes, warmed gently
→ squash under a coverslip and observe
Why it happens: a root grows by adding new cells at its tip, in a region called the meristem, so the tip is where cell division is actually going on. Each chemical in the list has a job: aceto-alcohol fixes the cells, killing them instantly so that the stage of division each was in is frozen exactly as it was; dilute HCl loosens the cement between cells so the tissue can be spread; aceto-carmine stains chromosomes deep red so they stand out; and the squash flattens the tip into a single layer, because overlapping cells cannot be focused on together.
Tip: warm the slide gently over a spirit lamp — do not boil it. Overheating makes the stain uneven and can destroy the chromosomes you are trying to see.
Q2.
What do you observe? Do you observe the cells of the onion root tip? Are they similar in structure? Do you find any structural differences in these cells? If yes, why is it so?
Answer

The root tip cells are visible, and they are not all alike. Different cells show different arrangements of stained material inside them.

  • Some cells show an ordinary round nucleus with a diffuse, thread-like chromatin network — these are not dividing at that moment.
  • Others show deeply stained, rod-shaped chromosomes, sometimes lined up along the middle of the cell, sometimes being pulled towards the two ends, and sometimes gathered into two separate groups.

The reason is that the cells of a growing onion root tip divide continuously. At the moment the tissue was fixed, each cell happened to be at a different point in the process, so the slide is a snapshot of many stages of cell division at once — which is exactly what Fig. 2.17 shows.

Why it happens: chromatin and chromosomes are the same DNA in two different states. In a non-dividing cell the DNA lies loose as chromatin, an entangled mass of thread-like structures — good for reading instructions but hopeless for sharing out. Whenever the cell is about to divide, the chromatin coils up tightly and organises itself into short rod-shaped chromosomes, which can be moved about without tangling or breaking. That is why chromosomes are visible only when a cell is about to divide, and it is why a single slide of a root tip can show so many different-looking cells even though they are all the same kind of cell.
Q3.
Fig. 2.17 shows various stages of cell division. Can you identify which stage comes first during cell division?
Answer

The first stage is the one in which the chromatin has just condensed into visible chromosomes but they are still scattered, with the nuclear membrane breaking down. It comes before any stage in which the chromosomes are neatly lined up or separated.

Use this order of events to place the cells in Fig. 2.17:

  1. Cell with a plain nucleus and diffuse chromatin — the cell before division begins.
  2. Chromosomes appear as distinct rod-shaped bodies, scattered in the cell; the nuclear membrane disappears.
  3. Chromosomes line up along the middle of the cell.
  4. Chromosomes are pulled apart towards the two opposite ends.
  5. Two groups of chromosomes gather at the ends, new nuclei form and a new wall grows between them — two daughter cells.
Why it happens: the sequence is forced by the logic of the job. Before DNA can be shared out it must first be packed into countable, movable units — so condensation must come first. It must then be gathered at one place so that each half goes to a different side — hence the lining up in the middle. Only then can the two sets be separated, and only after they are separated can two new nuclei be built. You will study the named stages of cell division in higher grades; at this level it is enough to read the sequence from what the chromosomes are doing.
Tip: when you see a squash slide, count how many cells show scattered chromosomes and how many show separated groups. Stages that take longer appear in more cells — the picture is a record of time as well as of shape.
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