Mitosis in a Root Tip

Four Zones at the Tip

A root’s tip wears a cap. It shields the dividing cells behind it; starch-⁠filled amyloplasts settle to the lower side of its central columella cells and let the root sense gravity; its outermost cells keep coming away and new ones are added from inside.

Behind the cap is the meristem: small, nearly square cells with large central nuclei, where cells divide. Further back is the elongation zone, where cells lengthen fast and push the tip forwards; then the maturation zone, where the epidermis grows root hairs. It starts at the first root-⁠hair bulges.

The zones don’t meet at neat lines: different files of cells stop dividing at different distances from the tip.

Root cap · meristem · elongation zone · maturation zone (root hairs)

Where Cells Divide

In the meristem the cells stand in files along the root. When a cell divides, a cross wall grows through its middle and the file gains a cell; the cells at the back are pushed into the elongation zone. In each cycle about half the meristem’s cells pass to the elongation zone and as many new ones are made.

At the meristem’s far tip is the quiescent centre, ringed by stem cells: they make new cap cells forwards and the files of cells backwards.

Dividing cells crowd the first short stretch of the root, so a school squash keeps only the last two or three millimetres.

Files of cells · the quiescent centre and stem cells · a squash takes the last 2⁠–⁠3 mm

Copy First, Then Divide

From one division to the next a cell goes through one cell cycle. Most of it is interphase: in G1 the cell grows, in S it copies its DNA, in G2 it gets ready to divide. Then comes mitosis, and last the cytoplasm splits in two.

In onion root tips at 21 °C one cycle was measured at about 23 hours, the S phase the longest at 12 hours and mitosis about 4 hours. Other studies found about 17⁠–⁠24 hours; temperature and method change the answer.

After S, each chromosome is made of two identical sister chromatids, held tightly together at the centromere.

Onion root tips at 21 °C: a cycle of about 23 hours · S about half of it · mitosis about 4 hours

The Chromosomes Condense

In prophase each chromosome coils up, thicker and shorter, until a light microscope can see it; the nucleolus disperses.

A plant cell makes one preparation an animal cell doesn’t: from G2 into prophase it builds a ring of microtubules and actin filaments against its membrane, the preprophase band. The band marks where the new wall will meet the old one; it comes apart as the nuclear envelope breaks down, but the place stays marked.

Onion chromosomes are large and few: 16 in a body cell (2n = 16), with about 16.4 billion base pairs in one set of eight, about five times a human set. That is why onion root tips are the usual material for watching division.

Chromosomes condense · the nucleolus disperses · the preprophase band (schematic)

Sixteen on One Plane

In prometaphase the nuclear envelope goes and the spindle’s microtubules reach in. Each chromatid has a kinetochore at its centromere; microtubules catch the kinetochores until the two sisters of each pair are tied to opposite poles.

In metaphase all 16 chromosomes lie on one plane midway between the poles, the metaphase plate. It is a place, not a structure.

Seed-⁠plant cells such as the onion’s have no centrosomes: in prophase microtubules gather round the nuclear envelope, and when it breaks down they feed the spindle. The metaphase spindle is barrel-⁠shaped, its poles not focused to a point. The cell waits until every kinetochore is attached before anaphase begins.

16 chromosomes · two chromatids each · the spindle (schematic)

The Copies Part

As anaphase begins, an enzyme cuts the cohesin that held each pair of sister chromatids together, and the two come apart, each now a chromosome of its own.

The microtubules tied to the kinetochores shorten and the chromosomes move towards the poles; the microtubules between the poles push them apart and the spindle lengthens. Each pole receives 16 chromosomes, the same as the cell had.

Cohesin cut · microtubules shorten · 16 to each pole

A Wall Grows Down the Middle

In telophase the two sets reach the poles and loosen, and a nuclear envelope forms round each: two nuclei.

A plant cell has a stiff wall and can’t pinch in from outside as an animal cell does; it builds a cell plate between the nuclei instead. The phragmoplast is an array of microtubules and actin filaments; vesicles from the Golgi carrying wall material travel along it to the midline and fuse into the plate. The phragmoplast turns from a cylinder into a ring and spreads outwards, and the plate grows from the centre out until it meets the old wall, where the preprophase band marked.

The plate becomes a new wall. One cell has become two, each with the same 16 chromosomes.

Nuclear envelopes return · the phragmoplast (schematic) · the cell plate grows outwards

Squashed and Stained

In a school practical the root tips are cut and fixed in a mixture of ethanol and acetic acid, which stops every cell where it is; warm dilute hydrochloric acid then loosens the middle lamella that cements the walls together; only the last two or three millimetres are kept, stained, covered with a coverslip and squashed to one layer of cells, and looked at at 400 times.

The usual stains are acetic orcein and toluidine blue; they darken the chromosomes, so dividing cells stand out at a glance.

A squash is a picture of the instant it was fixed: each cell is held at the stage it had reached. The order of the stages is pieced together from many cells, not watched in one.

Fix · soften in acid · stain · squash to one layer · look at ×400

Counting Out the Time

The share of cells caught dividing is the mitotic index. In control onion root tips it was measured at about 6⁠–⁠11 %.

When cells divide on their own, out of step with each other, the longer a stage lasts, the more cells a squash catches in it. So a stage’s length is about its share of the cells times the length of the cycle.

This page’s model takes the 23-⁠hour cycle with 4 hours of mitosis at 21 °C and shares the 4 hours among the stages as measured at 15 °C: prophase about 99 minutes, metaphase (with prometaphase) about 33, anaphase about 25, telophase about 83.

It is only an approximation. On this model about 17 % of cells would be dividing; real counts find fewer, and with a 23-⁠hour cycle that puts mitosis at only about 1.3⁠–⁠2.6 hours. Root tips also hold cells that have stopped dividing, and laboratories draw the line between stages differently: some count most cells in prophase, others in metaphase.

Control mitotic index about 6⁠–⁠11 % · the page’s model: prophase about 99 min, anaphase about 25 min

When the Split Goes Wrong

Now and then a pair of sister chromatids fails to separate and both go to the same pole: non-⁠disjunction. One daughter cell gets an extra chromosome, the other is one short.

The spindle assembly checkpoint holds anaphase back until every kinetochore is attached, but some wrong attachments, such as one kinetochore tied to both poles, slip past it.

Colchicine binds the protein that microtubules are made of, so they cannot grow and the cell stops at metaphase; with no spindle, the chromosomes don’t line up but lie scattered through the cell. Since they never reach the poles, the cell’s chromosome number can double: in 1937 colchicine was first used to double the chromosomes of plants and make polyploids. Colchicine is highly poisonous.

Non-⁠disjunction · the checkpoint · colchicine stops the spindle

The Cell Taken Apart

The dividing cell comes apart into five: the cells around it, its own wall and cytoplasm, its nuclear envelope breaking up, its 16 chromosomes and its spindle.

Taken apart, it is held in prometaphase: the envelope breaking up, the spindle’s microtubules reaching in. The cells around it fall back and it stands out alone. The part under the pointer lifts out alone and is named.

Cells around · wall and cytoplasm · nuclear envelope · chromosomes · spindle