The Cell Cycle and Mitosis

A small cut on your finger usually closes on its own in roughly a week or a little longer. Near the tip of an onion root, cells divide, and the root pushes deeper into the soil. In both places the living thing is adding new cells. Why make more cells instead of letting each cell grow bigger? And when one cell becomes two, how does each new cell end up with a full set of instructions?

Explore: step a cell through division

In the mitosis stepper you step one model cell, with two color-coded pairs of chromosomes, through interphase and the four stages of mitosis. A second panel shows a field of root-tip cells to count and turns your counts into an estimate of how long each phase lasts.

Try this.

1. Before you start, predict how many chromosomes each new cell will have at the end. Then step through and count.
2. Stop in the middle of interphase and predict what will look different after S phase. Then step forward and compare.
3. Stop at metaphase. Predict where each half of a chromosome will go next, then step once.
4. In the counting panel, predict which phase will hold the most cells. Count, then see what that means for time.

Explain: the cell cycle

The cell cycle is the series of stages a cell goes through from the time it forms until it divides into two cells. In plants, animals, fungi and other eukaryotes it has four main phases: G1, S, G2 and M.

Interphase is the part of the cycle between divisions, made of G1, S and G2. It is not a rest.

• G1: the new cell grows, makes proteins and does its everyday work.
• S: the cell copies its DNA. “S” stands for synthesis, which means making.
• G2: the cell makes the proteins it will need to move its chromosomes, and gets ready to divide.

DNA is stored in chromosomes, long strands of DNA wrapped around proteins. Most of your body cells have 46 chromosomes. After S phase, each chromosome is two identical copies joined together.

Why copy first? Picture two groups that each need the whole recipe book. Tearing one book in half gives each group half the recipes; copying it first gives each a full book. The comparison breaks because a cell does not decide anything; proteins copy and share the DNA.

M phase is division itself, in two parts. Mitosis is the division of the nucleus into two nuclei, each with a full set of chromosomes. Cytokinesis is the division of the cytoplasm, which splits one cell into two. Most of the time cytokinesis follows mitosis, and the result is two new cells with the same DNA as each other and as the cell they came from.

When the controls on the cycle fail, cells can keep dividing when they should not. That uncontrolled division is a key feature of cancer.

Explain: the stages of mitosis

After S phase, a copied chromosome is made of two sister chromatids, the two identical copies of one chromosome. They are held together at the centromere, a narrow region of the chromosome. Spindle fibers are thin protein fibers that attach to the chromosomes and move them. You can picture them as ropes, but they do not reel anything in. They get shorter by losing pieces at their ends.

Cytokinesis often starts during late anaphase or telophase, and it looks different in plants and animals. In an animal cell, a ring at the middle pinches the membrane inward, making a furrow that deepens until the cell splits in two. A plant cell has a stiff wall and cannot pinch. Instead, a cell plate forms across the middle of the cell and grows outward until it becomes a new wall between the two cells.

Explain: why cells divide and stay small

Cells divide for several reasons:

• Growth. A tree or a teenager grows mostly by adding cells.
• Repair. Skin cells divide behind a wound to replace those that were lost.
• Replacement. The lining of your gut and the outer layer of your skin are replaced over and over.
• Asexual reproduction. A single-celled amoeba or yeast cell can make a new organism by dividing. Bacteria divide too, by a simpler process covered in How Microbes Multiply.

Why not grow bigger cells instead? Oxygen, food molecules and wastes all cross the cell membrane, which you study in Crossing the Membrane. The surface-area-to-volume ratio compares how much outside surface a cell has with how much inside it has. Model a cell as a cube:

These come from geometry (area = 6 × side × side, volume = side × side × side), not from measuring cells. Each time the side doubles, the volume grows eight times but the surface only four times. A large cell would have less membrane for each bit of its inside.

Real cells are not cubes, and long, thin or flat cells keep more surface. The model works best for comparing cells of similar shape.

The onion root shows both ideas together. Near the tip, cells divide. Just behind that zone, the new cells grow longer, and that lengthening pushes the root through the soil. So a root grows by making new cells and by letting them stretch.

Elaborate: count onion root-tip cells

A stained root tip is a snapshot: every cell is caught at one moment of its cycle. If cells are caught at random moments, a longer phase holds more of them.

Source: The Biology Project, University of Arizona, onion root-tip cell cycle activity (page dated 1997, revised August 2004). The counts are of the 36 photographed onion (Allium cepa) root-tip cells on its completed answer page, which does not say when or at what temperature the roots were grown. The activity assumes one cycle takes 24 hours. A 2018 study that gathered published estimates for roots grown from onion bulbs reported values from about 13.5 to 23 hours, so 24 hours is a round assumption, not a measurement.

1. Which phase holds the most cells? About what fraction of the cells is it?
2. One cycle of 24 hours is 1,440 minutes. Estimate the time in metaphase as its fraction times 1,440 minutes. Do the same for the other phases, then check that your five estimates add up to 1,440.
3. The activity does not say how these photographs were picked, and they may have been chosen to show every stage. Would a field picked at random probably show more or fewer cells in interphase? How would that change your estimates?

Design problem

Does the temperature an onion root grows at change how many of its tip cells are dividing? Plan an investigation. Name your variables, constants, control, sample size and repeated trials, and say how you will stay safe.

Evaluate: check your thinking

Experiments on mouse skin suggest that a cut fills in because skin cells near the edge crawl into the gap while cells behind them divide, and every new cell gets a full copy of the DNA through interphase and mitosis. An onion root grows deeper because cells at the tip divide and the cells behind them stretch. Living things add cells instead of growing giant ones partly because small cells have more membrane for each bit of their inside.

Science practices you used: making predictions and testing them with a model, reading counts and turning them into fractions and estimates, planning a fair test with a large enough sample, and naming the limits of a data set and a model.