The Cell Cycle, Mitosis and Cancer

How cell division allows an organism to grow

A multicellular organism grows by increasing its number of cells. New cells come from existing cells: a parent cell grows and then divides to produce two daughter cells. Repeated divisions allow a single fertilised egg cell to develop into an organism containing many cells.

This process also continues after an organism has stopped growing. For example, damaged or worn-out skin cells need replacing. Cell division therefore supports both growth and the repair and replacement of tissues.

The cell cycle is the sequence of growth, copying genetic information and division that produces new cells. Its key parts are interphase, mitosis and division of the cytoplasm.

Interphase prepares the cell for division

Before a cell divides, it needs enough cell material and genetic information for both daughter cells. During interphase, the cell grows and the number of sub-cellular structures, such as mitochondria and ribosomes, increases.

The cell also replicates its DNA. DNA carries genetic information and is organised into structures called chromosomes in the nucleus. Replication makes an identical copy of the DNA in each chromosome.

After replication, each chromosome consists of two identical copies called sister chromatids, joined together. When these copied chromosomes condense before separation, they can appear X-shaped. The two sides of the X are copies of the same chromosome, ready to be shared between the daughter cells.

Mitosis shares the genetic information equally

Mitosis is division of the nucleus. Its role is to separate the chromosome copies so that each new nucleus receives a complete, identical set.

The chromosomes become condensed and visible, and the membrane around the nucleus breaks down. The copied chromosomes then line up across the centre of the cell. Spindle fibres pull the sister chromatids apart towards opposite ends, placing one copy of each chromosome at each end. New nuclear membranes form around the two sets.

The cytoplasm then divides in a process called cytokinesis, producing two separate daughter cells. Each daughter cell has the same genetic information and chromosome number as the parent cell. For example, a human body cell with 46 chromosomes produces two daughter cells, each with 46 chromosomes—not two cells with 23 each.

DNA replication and equal separation work together: replication supplies two copies, and mitosis distributes one complete set to each new nucleus.

Using a light microscope to observe mitosis

Growing onion root tips are useful specimens because they contain many dividing cells. A prepared root-tip slide allows you to observe cells at different points in the cell cycle. You need a light microscope with low- and higher-power objective lenses, a light source and a prepared slide.

The eyepiece and objective lenses magnify the specimen. The stage holds the slide, while the coarse and fine focus controls bring the image into focus.

1. Select the lowest-power objective lens. This gives a wider view, making the root-tip tissue easier to find.
2. Place the prepared slide on the stage and secure it with the clips. Position the specimen over the opening through which light passes.
3. Looking from the side, use the coarse focus control to bring the stage close to the objective lens. Do not let the lens touch the slide.
4. Switch on the light source. Look through the eyepiece and slowly lower the stage with the coarse focus control until the cells come into rough focus.
5. Use the fine focus control until the cell boundaries and nuclear material appear sharp. Move the slide to centre a suitable group of cells.
6. Change to a higher-power objective to see chromosome arrangements more clearly. Use fine focus to sharpen the image, keeping the lens clear of the slide.

Handle the glass slide carefully, keep it away from the edge of the bench and report any broken glass rather than picking it up. Looking from the side when bringing the stage towards the lens helps prevent damage to both the slide and the lens.

Recognising changes in dividing cells

Examine several cells rather than expecting every cell to look the same. In interphase, a cell has a visible nucleus, but individual chromosomes are not clearly distinguishable. DNA replication itself cannot be identified simply by seeing a nucleus.

In cells undergoing mitosis, look for condensed chromosomes. Their arrangement helps you interpret what is happening: chromosomes gathered across the middle indicate alignment before separation; two groups moving towards opposite ends indicate that the copies are being separated. Two newly forming nuclei show that nuclear division is reaching completion.

Record what you can actually see in labelled drawings, including the chromosome positions. A prepared slide provides a snapshot of many different cells, not a live sequence of one cell dividing.

When control of cell division is lost

Normally, cell growth and division are controlled. Cancer results from changes in a cell’s DNA that disrupt this control, leading to uncontrolled growth and division.

The important distinction is not that cancer cells divide—healthy cells also divide for growth and repair—but that their division is no longer properly controlled. Cancer is a non-communicable disease: it is not passed from one person to another by ordinary contact.

Cell cycle and growth

• Growth of multicellular organisms involves an increase in cell number.
• Interphase: cell growth, more sub-cellular structures and DNA replication.
• Mitosis: chromosome copies separate and the nucleus divides.