Mitosis

AS · 15 min

Mitosis is the division of a nucleus into two genetically identical nuclei. In a few minutes to an hour, a cell condenses its replicated chromosomes, lines them up on a spindle, pulls the sister chromatids apart and builds two new nuclei around them. You need to describe what the chromosomes, the nuclear envelope, the cell surface membrane and the spindle do at each stage, in both animal and plant cells, and to identify the stages in photomicrographs, diagrams and slides. The root tip squash and the mitotic index calculation are the standard practical and data skills.

The four stages

Definition

Mitosis is nuclear division that produces two genetically identical nuclei, each with the same number of chromosomes as the parent nucleus. It is divided into four stages: prophase, metaphase, anaphase and telophase. It is followed by cytokinesis, the division of the cytoplasm.

Mitosis is a continuous process; the four stages are names for recognisable points in it. Before mitosis begins, in interphase, the DNA has been replicated so that each chromosome consists of two identical sister chromatids joined at a centromere (see Chromosomes and the cell cycle).

The spindle

The spindle is a structure made of microtubules, hollow protein tubes built from the protein tubulin. Spindle microtubules radiate from the two poles of the cell. Some attach to the centromeres of the chromosomes (at protein structures called kinetochores); by getting shorter, they pull chromatids towards the poles. Others overlap at the equator and push the poles apart.

In animal cells each pole has a centrosome containing a pair of centrioles, from which the spindle microtubules grow. Plant cells have no centrioles: their spindle forms from microtubule-organising regions without them.

Prophase

The chromatin condenses (coils up tightly), so the chromosomes become shorter and thicker and visible with a light microscope. Each is seen to consist of two sister chromatids joined at the centromere.
The nucleolus disappears.
In animal cells, the two centrosomes (each with a pair of centrioles) move to opposite poles of the cell.
The spindle begins to form, with microtubules growing out from the poles.
At the end of prophase, the nuclear envelope breaks down into small vesicles, and spindle microtubules attach to the centromeres.

Metaphase

The chromosomes are moved by the spindle until they line up on the equator of the spindle (the metaphase plate), in the middle of the cell.
Each chromosome is attached by its centromere to spindle microtubules from both poles: one sister chromatid faces one pole, the other faces the opposite pole.
Chromosomes are at their most condensed, so metaphase is the best stage at which to count and examine them.

Anaphase

The centromeres divide (the proteins holding the sister chromatids together are broken down), so the sister chromatids separate.
The spindle microtubules attached to the centromeres shorten, pulling the chromatids to opposite poles, centromere first, so the chromatids appear V-shaped as their arms trail behind.
Each separated chromatid is now called a chromosome. One complete set of chromosomes moves to each pole.
This movement uses energy from ATP.

Telophase

The chromosomes (chromatids) arrive at the poles.
They uncoil (decondense) and become long and thin again, so they are no longer visible as separate threads.
A nuclear envelope reforms around each set of chromosomes, forming two nuclei.
The nucleolus reappears in each nucleus.
The spindle breaks down.

Cytokinesis

Cytokinesis usually begins during telophase.

In animal cells, the cell surface membrane is drawn inwards around the equator by a ring of protein filaments (actin), forming a cleavage furrow that deepens until the cell is pinched into two.
In plant cells, the rigid cell wall stops the cell from pinching in. Instead, vesicles from the Golgi body, carrying cell wall material, gather at the equator and fuse to form a cell plate. The cell plate grows outwards until it meets the existing walls; its membranes become the new cell surface membranes and the material between them becomes the new cell walls.

Key result

Stage	Chromosomes	Nuclear envelope	Spindle	Cell surface membrane
Prophase	condense; visible as two sister chromatids joined at centromere	breaks down (end of prophase); nucleolus disappears	forms from the poles; centrosomes (animal cells) move to opposite poles	unchanged
Metaphase	line up on the equator, attached by centromeres to spindle microtubules from both poles	absent	fully formed; microtubules attached to centromeres	unchanged
Anaphase	centromeres divide; sister chromatids separate and are pulled to opposite poles, centromere first	absent	microtubules attached to centromeres shorten	animal cells may begin to elongate
Telophase	reach poles; uncoil and become less visible	reforms around each set of chromosomes; nucleolus reappears	breaks down	cleavage furrow begins (animal); cell plate forms (plant)
Cytokinesis		two nuclei, each in its own cell		animal: membrane pinches in (cleavage furrow); plant: new membranes and walls form from cell plate

Plant and animal mitosis compared

Feature	Animal cell	Plant cell
Centrioles	present; centrosomes move to poles in prophase	absent
Spindle formation	from centrosomes (centrioles)	from microtubule-organising regions without centrioles
Cytokinesis	cleavage furrow: cell surface membrane pinches inwards	cell plate forms from Golgi vesicles at the equator; new cell wall laid down
Where it occurs	many tissues (e.g. skin, bone marrow, gut lining)	mainly in meristems (root and shoot tips, cambium)
Shape during division	becomes rounded; elongates in anaphase	shape fixed by cell wall

Identifying stages in a photomicrograph

Look for visible chromosomes. If none are visible and there is an intact nucleus with a nucleolus, the cell is in interphase.
Chromosomes visible as threads, scattered within the region of the nucleus, perhaps with the nuclear envelope still visible: prophase.
Chromosomes in a line across the middle of the cell: metaphase.
Two groups of chromosomes moving apart, often V-shaped with arms trailing: anaphase.
Two groups of chromosomes at opposite ends of the cell, clumped or becoming less distinct, perhaps with a new nuclear envelope or a cell plate/furrow forming between them: telophase.
Always justify your identification with a feature, e.g. "metaphase, because the chromosomes are lined up on the equator".

Naming stages from descriptions (routine)

Name the stage of mitosis in each case.

(a) Sister chromatids are pulled apart towards opposite poles. (b) A nuclear envelope forms around each group of chromosomes. (c) Chromosomes become visible and the nucleolus disappears. (d) Chromosomes are attached to spindle microtubules at the equator. (e) A cell plate forms across the middle of a plant cell.

Solution

(a) Anaphase. (b) Telophase. (c) Prophase. (d) Metaphase. (e) Cytokinesis (beginning in telophase).

Describing chromosome behaviour (moderate, 6 marks)

Describe the behaviour of chromosomes during mitosis in an animal cell.

Solution

Prophase: chromosomes condense / coil, becoming shorter and thicker and visible;
each consists of two sister chromatids joined at a centromere;
Metaphase: chromosomes line up on the equator of the spindle;
attached to spindle microtubules by their centromeres;
Anaphase: centromeres divide and sister chromatids separate;
chromatids are pulled to opposite poles, centromere first, by shortening of spindle microtubules;
Telophase: chromatids/chromosomes reach the poles and uncoil / decondense; nuclear envelope reforms around each set.

The question asks only about chromosomes, so marks are for chromosome behaviour. Details about the nucleolus or centrioles do not earn credit here unless linked to chromosomes.

The mitotic index

Definition

The mitotic index is the proportion of cells in a tissue that are undergoing mitosis (in prophase, metaphase, anaphase or telophase) at the time of observation.

Key result

\text{mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells observed}}

It may be given as a decimal or multiplied by 100 to give a percentage. A high mitotic index means a high proportion of cells are dividing: the tissue is growing or replacing cells rapidly (e.g. a root tip meristem, or a tumour).

Practical skills

Root tip squash: observing mitosis and calculating mitotic index

Root tips are used because the meristem, just behind the tip, contains many dividing cells. Garlic, onion or broad bean roots grown in water for a few days are commonly used.

Cut the terminal $5\ \text{mm}$ (or less) of a root tip with a scalpel; this is where the dividing cells are.
Hydrolyse (macerate) the tip by placing it in warm dilute hydrochloric acid (e.g. $1\ \text{mol dm}^{-3}$ at about $60\ ^\circ\text{C}$ ) for about 5 minutes. The acid breaks down the middle lamella (the pectin that holds plant cells together), so the cells can be separated and spread into a single layer; it also stops cell division, fixing the cells at their current stage.
Rinse in cold water to remove the acid (which would otherwise interfere with staining).
Place the tip on a slide, cut off and keep only the very end (about $1$ – $2\ \text{mm}$ ), and add a few drops of a stain that binds to DNA, such as toluidine blue, acetic orcein or Schiff's reagent (Feulgen stain), so the chromosomes show up darkly.
Break up the tissue with a mounted needle, lower a coverslip, and squash firmly but gently by pressing with a thumb (through filter paper) or tapping with the end of a pencil, without sliding the coverslip sideways. This spreads the cells into a single layer, so light passes through and individual cells and chromosomes can be seen.
Observe with a light microscope, first at low power to find the meristem region (small, square, closely packed cells with large nuclei), then at high power ( $\times 400$ ).
Count: in a field of view, count the total number of cells and the number with visible chromosomes (in any stage of mitosis). Repeat in several fields of view and calculate the mitotic index.
Counting rules: decide in advance how to treat cells cut by the edge of the field of view (e.g. count those on the top and left edges, not the bottom and right), so cells are not counted twice or missed.
Safety: hydrochloric acid is an irritant and stains can stain skin: wear eye protection and gloves; take care with scalpels and hot water.
Sources of error: difficulty identifying early prophase or late telophase (judgement); overlapping cells if not squashed enough; counting the same cell twice; field of view not representative (only part of the tip). Improvements: count many fields of view (at least 5) and calculate a mean; agree clear criteria for "in mitosis"; use a grid in the eyepiece; use a photomicrograph so counts can be checked.

Calculating mitotic index (moderate)

A student counted cells in fields of view at different distances from the end of an onion root.

Distance from root tip / mm	0–1	1–2	2–3	3–4
Total cells counted	200	200	200	200
Cells in mitosis	46	28	10	2

(a) Calculate the mitotic index at each distance, as a percentage. (b) Explain the trend.

Solution

(a) Mitotic index $= \dfrac{\text{cells in mitosis}}{\text{total cells}} \times 100$ :

0–1 mm: $46/200 \times 100 = 23\%$ ;
1–2 mm: $28/200 \times 100 = 14\%$ ;
2–3 mm: $10/200 \times 100 = 5\%$ ;
3–4 mm: $2/200 \times 100 = 1\%$ .

(b) The mitotic index decreases with distance from the root tip. Close to the tip is the meristem, where cells are dividing rapidly by mitosis to produce new cells for growth of the root. Further back, cells have stopped dividing and are elongating and differentiating into specialised cells (such as xylem and root hair cells), so few are in mitosis.

Identifying stages and counting chromosomes (moderate)

A photomicrograph of a root tip of a plant with $2n = 16$ shows a cell in which two groups of V-shaped chromosomes are moving apart.

(a) Name the stage and give one reason. (b) State the number of chromosomes moving to each pole. (c) State the number of chromatids in the cell at metaphase, and the number of chromosomes in each daughter cell. (d) Suggest why this cell would not show a cleavage furrow.

Solution

(a) Anaphase: the chromatids are separating and moving to opposite poles; they are V-shaped because they are pulled centromere first by the spindle microtubules.

(b) 16 to each pole (each separated chromatid is now a chromosome).

(d) It is a plant cell: it has a rigid cell wall, so cytokinesis is by formation of a cell plate from vesicles, not by the membrane pinching in.

Exam-hard: effect of a spindle inhibitor (6 marks)

Colchicine is a chemical that binds to tubulin and prevents the formation of spindle microtubules. A root tip was treated with colchicine for 4 hours, and the mitotic index was compared with an untreated root.

	Untreated	Treated with colchicine
Mitotic index / %	8	30
Percentage of dividing cells in metaphase (or appearing to be)	25	90

(a) Explain the effect of colchicine on the mitotic index and on the proportion of dividing cells in metaphase. (b) Suggest why colchicine has been tested as a treatment for cancer, and one problem with this.

Solution

(a)

Without spindle microtubules, chromosomes cannot be attached at their centromeres or pulled apart;
so cells that enter mitosis cannot complete it: sister chromatids are not separated and anaphase cannot occur;
cells continue to enter mitosis from interphase but do not leave it, so the number of cells in mitosis accumulates and the mitotic index rises (8% to 30%);
the cells are held with condensed chromosomes, not separated (a metaphase-like stage), so a much higher proportion of dividing cells appear to be in metaphase (90%);
the nuclear envelope has broken down but cannot reform around two separate sets of chromosomes.

(b) Cancer cells divide by mitosis rapidly and uncontrollably; a spindle inhibitor would stop them completing mitosis, so the tumour cannot grow. Problem: it also stops mitosis in normal rapidly dividing cells (e.g. bone marrow stem cells, gut lining, hair follicles), causing side effects such as anaemia, reduced immunity or hair loss; it may also be toxic at effective doses.

Watch out

Chromosomes do not replicate during prophase. Replication happens in S phase of interphase; in prophase the already-replicated chromosomes condense and become visible.
In metaphase, chromosomes line up on the equator, not "in the middle of the nucleus" (the nuclear envelope has gone).
In anaphase, it is the sister chromatids (not homologous chromosomes, which is meiosis) that separate.
Spindle fibres attach to the centromere, not to the ends of the chromosomes.
Mitosis is nuclear division. Do not include cytokinesis as a stage of mitosis.
In plant cells there are no centrioles: do not draw or label them.

Exam tip

Read whether the question asks about chromosomes, the nuclear envelope, the spindle or the cell surface membrane, and describe that structure at each stage in order.
Use the precise verbs: chromosomes condense, line up on the equator, chromatids separate and are pulled to opposite poles, chromosomes decondense/uncoil; nuclear envelope breaks down and reforms; spindle forms and breaks down.
When identifying stages in photomicrographs, justify each one with an observable feature. "Metaphase" alone may get no mark if the question says "explain".
Mitotic index: show the fraction or division, then multiply by 100 if a percentage is asked for. State what a high value means.
In Paper 3, you may be asked to draw cells in different stages: draw outlines with sharp, continuous lines, no shading, chromosomes as simple lines or V-shapes, and label at least the chromosomes, cell wall (plant) and spindle region if visible.

Summary

Mitosis produces two genetically identical nuclei; it has four stages: prophase, metaphase, anaphase, telophase.
Prophase: chromosomes condense (two sister chromatids joined at centromere); nucleolus disappears; spindle forms; centrosomes move to poles (animal); nuclear envelope breaks down.
Metaphase: chromosomes line up on the equator attached by centromeres to spindle microtubules from both poles.
Anaphase: centromeres divide; sister chromatids pulled to opposite poles (V-shapes) by shortening microtubules.
Telophase: chromosomes reach poles and uncoil; nuclear envelopes and nucleoli reform; spindle breaks down.
Cytokinesis: cleavage furrow in animal cells; cell plate in plant cells; plant cells have no centrioles.
Mitotic index = cells in mitosis ÷ total cells observed; high in meristems and tumours.
Root tip squash: cut tip, hydrochloric acid to macerate and fix, rinse, stain (toluidine blue/acetic orcein), squash into one layer, count several fields of view.

Practice questions

Question

State the four stages of mitosis in order.
Describe what happens to the nuclear envelope during mitosis.
State two differences between mitosis in an animal cell and in a plant cell.
Explain why chromosomes are visible during mitosis but not during interphase.
Explain why the chromatids appear V-shaped during anaphase. (2 marks)
In a root tip squash, a student counted 32 cells in mitosis out of a total of 640 cells. Calculate the mitotic index as a decimal and as a percentage.
Explain the purpose of each of the following steps in a root tip squash: (a) warming in hydrochloric acid; (b) adding toluidine blue; (c) pressing on the coverslip.
The mitotic index of cells from a tumour was 0.18 and that of the surrounding normal tissue was 0.03. Calculate how many times greater the tumour's mitotic index was, and explain the difference. (3 marks)
Describe the role of the spindle in mitosis. (4 marks)
A cell in an organism with $2n = 8$ is examined at four points in the cell cycle: G1, G2, metaphase and immediately after cytokinesis. (a) State the number of chromosomes and the number of DNA molecules in the cell at each point. (b) Explain how the events of mitosis ensure that each daughter cell receives a complete and identical set of genetic information. (6 marks)

Answers

Prophase, metaphase, anaphase, telophase.
It breaks down (into vesicles) at the end of prophase, is absent during metaphase and anaphase, and reforms around each group of chromosomes in telophase.
Any two: animal cells have centrioles (centrosomes) at the poles, plant cells do not; animal cytokinesis by cleavage furrow (membrane pinching in), plant cytokinesis by cell plate (vesicles fuse) and new cell wall; in plants mitosis mainly in meristems; animal cells change shape (round up, elongate), plant cells keep their shape due to the cell wall.
In interphase the DNA is uncoiled as chromatin, spread throughout the nucleus, too thin to see; in prophase the chromosomes condense (coil tightly around histones, supercoil), becoming shorter and thicker, so they can be seen (after staining) with a light microscope.
Spindle microtubules are attached at the centromere; they pull the chromatid by its centromere towards the pole, so the centromere leads and the arms trail behind.
$32/640 = 0.05$ ; $5\%$ .
(a) Breaks down the middle lamella (pectin) between cells so they can be separated and spread out; also kills/fixes cells, stopping division. (b) Stains the chromosomes (DNA) so they are visible/contrast with the cytoplasm. (c) Spreads the cells into a single layer so light can pass through and individual cells and chromosomes can be seen without overlapping.
$0.18 / 0.03 = 6$ times greater. Tumour cells have mutations in genes controlling the cell cycle, so they divide by mitosis uncontrollably/repeatedly; a higher proportion of cells are in mitosis at any time; normal cells divide only when needed for replacement or repair.
Spindle microtubules form from the poles (centrosomes/centrioles in animal cells) during prophase; they attach to chromosomes at their centromeres; they move/align chromosomes on the equator at metaphase; in anaphase they shorten, pulling sister chromatids apart to opposite poles; this ensures each pole receives one chromatid of each chromosome; the spindle breaks down in telophase.
(a) G1: 8 chromosomes, 8 DNA molecules. G2: 8 chromosomes, 16 DNA molecules. Metaphase: 8 chromosomes, 16 DNA molecules. After cytokinesis: 8 chromosomes, 8 DNA molecules (in each daughter cell). (b) DNA is replicated in S phase so each chromosome has two identical sister chromatids; in metaphase each chromosome is attached by its centromere to microtubules from both poles, lined up on the equator; in anaphase the centromeres divide and sister chromatids are separated and pulled to opposite poles, so each pole receives one chromatid of every chromosome; in telophase a nuclear envelope forms around each set; cytokinesis separates the two nuclei into two cells, each with 8 chromosomes identical to the parent.

The four stages

The spindle

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Plant and animal mitosis compared

The mitotic index

Practice questions

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