RP2 Root tip squash and mitosis Questions

Specification route: RP2
Question bank: 15 questions
Course stage: AS / Year 12

Sample questions

QUESTION 1 · RP2 · LEVEL 1

When preparing a root tip squash to observe mitosis, why is the tissue sample taken specifically from the very tip of the root?

This is the meristem region, where cells are actively undergoing mitosis and cell division.
The cells at the tip lack a cell wall, making them easier to squash.
This region contains the largest, most easily visible chromosomes.
The cells here are dead and therefore take up the stain much more readily.

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Plant growth primarily occurs in specific zones, not uniformly throughout the plant.

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Answer: This is the meristem region, where cells are actively undergoing mitosis and cell division.

Required practical 2. The root tip contains meristematic tissue. This is the region of active cell division, providing a high mitotic index for observation.

QUESTION 2 · RP2 · LEVEL 2

During the preparation of a root tip squash, the student presses down firmly on the coverslip, being careful not to slide it sideways. What is the purpose of this action?

To spread the cells out into a single layer so that light can pass through them, without rolling cells over each other and breaking the chromosomes.
To physically burst the cell-surface membranes to release the chromosomes into the stain.
To squeeze out excess stain so the background isn't too dark.
To permanently adhere the coverslip to the glass slide.

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An optical microscope needs light to penetrate the sample to form an image.

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Answer: To spread the cells out into a single layer so that light can pass through them, without rolling cells over each other and breaking the chromosomes.

Required practical 2. Pressing firmly spreads the tissue into a layer one cell thick, allowing light to pass through. Avoiding sideways movement prevents the chromosomes from breaking or the cells from rolling over one another.

QUESTION 3 · RP2 · LEVEL 3

During the preparation of a root tip squash, the sample is heated gently with dilute hydrochloric acid. What is the biological purpose of this step?

To break down the pectin in the middle lamella, softening the tissue so the cells can be separated into a single layer.
To dissolve the nuclear envelope so the chromosomes can be released into the cytoplasm.
To act as a stain, turning the DNA an intense pink colour under the microscope.
To permanently stop the process of mitosis by denaturing the spindle fibres.

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Plant cells are glued together by a substance between their cell walls.

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Answer: To break down the pectin in the middle lamella, softening the tissue so the cells can be separated into a single layer.

Required practical 2. Acid hydrolysis macerates (softens) the plant tissue by breaking down the middle lamella, allowing the cells to be easily squashed into a thin layer for light to pass through.

QUESTION 4 · RP2 · LEVEL 1

Why are stains such as toluidine blue or acetic orcein used when preparing cells for viewing mitosis under an optical microscope?

They bind specifically to DNA, making the chromosomes highly visible against the paler cytoplasm.
They bind specifically to the cell wall, highlighting the boundary of the cell.
They react with the spindle fibres to temporarily freeze them in place.
They act as a buffer to prevent the rapid degradation of the dividing cells.

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Chromosomes are naturally transparent and almost invisible under standard light.

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Answer: They bind specifically to DNA, making the chromosomes highly visible against the paler cytoplasm.

Required practical 2. Stains like toluidine blue or acetic orcein are highly attracted to chromatin/DNA, providing contrast so the chromosomes and stages of mitosis can be identified.

QUESTION 5 · RP2 · LEVEL 2

During the root tip squash procedure, why must the very tip of the root (the apical meristem) be used, rather than a section $2\ cm$ further up the root?

Cell division (mitosis) is highly concentrated in the meristem at the tip; cells further up the root are elongating and differentiating, not actively dividing.
The cells further up the root have developed a thick bark layer that cannot be squashed.
The DNA in older cells further up the root has naturally degraded and won't take up the stain.
The cells at the tip contain twice as many chromosomes as normal body cells.

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Plants grow from their tips (roots and shoots).

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Answer: Cell division (mitosis) is highly concentrated in the meristem at the tip; cells further up the root are elongating and differentiating, not actively dividing.

Required practical 2. The apical meristem is the zone of active cell division. Observing tissue from further back will yield a mitotic index of zero, as those cells are in interphase or have permanently differentiated.

QUESTION 6 · RP2 · LEVEL 2

When selecting a garlic root to prepare a squash for viewing mitosis, why must the root be actively growing?

Because mitosis only occurs in the actively dividing cells of the meristem tissue, which is only present at the tip of a growing root.
Because dead roots contain massive amounts of starch that obscure the chromosomes.
Because actively growing roots have much larger cells that are easier to squash.
Because the stain only binds to DNA that is undergoing semi-conservative replication in the S phase.

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If the plant isn't growing, the cells aren't dividing.

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Answer: Because mitosis only occurs in the actively dividing cells of the meristem tissue, which is only present at the tip of a growing root.

Required practical 2. To observe the stages of mitosis, you must look at tissue where cells are actively dividing. In plants, this is restricted to specific regions called meristems, found at the tips of growing roots and shoots.

QUESTION 7 · RP2 · LEVEL 3

During the root tip squash procedure, what is the specific biological purpose of placing the root tip in dilute hydrochloric acid for $5$ minutes?

The acid hydrolyses and breaks down the pectin in the middle lamella, allowing the cells to be separated from one another.
The acid dissolves the tough cellulose cell wall entirely, leaving only a fragile protoplast.
The acid acts as a permanent stain for the histone proteins.
The acid forces all the cells to instantly halt in the metaphase stage of mitosis.

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Plant cells are 'glued' together. The acid dissolves the glue so you can squash them flat.

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Answer: The acid hydrolyses and breaks down the pectin in the middle lamella, allowing the cells to be separated from one another.

Required practical 2. Maceration with acid breaks down the middle lamella (the pectin layer cementing plant cells together). This softens the tissue so it can be squashed into a single layer of cells.

QUESTION 8 · RP2 · LEVEL 2

After adding the stain, the student places a coverslip over the root tip and presses down firmly and vertically with their thumb. Why is it critical NOT to let the coverslip slide sideways?

Sliding the coverslip sideways will roll the cells over each other, damaging them and breaking the chromosomes, making them impossible to view clearly.
Sliding the coverslip will push all the stain out from underneath it, leaving the cells colourless.
Sliding the coverslip will physically pull the DNA strands out of the nucleus into the cytoplasm.
Sliding the coverslip generates friction and heat, which denatures the chromosomes.

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You want to squash them flat like a pancake, not smear them like butter.

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Answer: Sliding the coverslip sideways will roll the cells over each other, damaging them and breaking the chromosomes, making them impossible to view clearly.

Required practical 2. Pressing vertically spreads the tissue into a layer one cell thick. Sliding the coverslip will shear and roll the cells, damaging the fragile chromosomes and ruining the slide.

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