Cell Growth
Doubling Up

In 1891, when Katsusaburo Yamagiwa intentionally produced cancer cells by applying coal tar extract to the skin of rabbits, he demonstrated for the first time that a specific substance can cause cancer. In doing so he had applied the idea omnis cellula e cellule ("all cells arise from cells") advanced by his former teacher Rudolph Virchow years earlier. Yamagiwa had, like the sorcerer's apprentice in the Goethe work, initiated a process that simply ran amok. In Yamagiwa's case it was unchecked cancer cell growth. This student activity is about growth rate and an idea called "doubling time."

One of the defining characteristics of cancer cells is their uncontrolled division. In humans, groups of cells normally grow and divide on a regular schedule. This process is usually referred to as the "cell cycle." In embryos and young children the cell cycle is shorter than it is in adults. That is, cells multiply faster leading to an overall quicker rate of growth. In adults, only a small fraction of the cells are in a growth state at any one time—except for cells in organs like the skin and bone marrow that are continuously producing new cells.

The entire cycle can be thought of as having four or five distinct stages. Actual cell division is called mitosis and is called the "M stage." When a mother cell divides into two daughter cells, the daughter cells often enter what is termed the "G1" or "gap" stage. This is a resting period prior to the next active stage of division. If the cell remains in an active growth cycle, it then embarks on the "S" stage in which DNA is synthesized and chromosomes are replicated. Another resting stage (S2) follows, and the cell then undergoes mitiosis (M). Often cells in humans enter a G0 (G-zero) stage, essentially a temporary period of nongrowth. At any given time most human cells are in the G0 stage.

How long does this cycle usually take? The length of the cycle varies greatly depending upon the type of organism. Bacteria, for example, may divide as often as every 20-30 minutes. In mammals the cycle is from 12 to 24 hours. Remember that in humans most cells are not in a growth phase at all.

Now suppose that for some reason there is a mutation in a cell and that leads it to begin dividing. Further, let's suppose that cell divides only every 100 days. In this case, after 100 days there would be 2 cells exactly like the original cell. After 200 days there would be 4 cells. After 300 days, 8 cells and so on.

The length of time it takes cells to double in number is called the doubling time. In our example, the doubling time is 100 days.

The role of doubling time in detecting cancer is related to the length of time that elapses between the event that originally caused the cell to begin uncontrolled division and the appearance of a tumor. How many years elapsed before the cell cluster in our example would be visible, even if it were on the surface of the skin? Size is also important as doctors and patients begin to use the word "cured" after cancer treatment. Are all the cancer cells really gone? In many cases, even after a physician indicates that the cancer is cured, cancer cells may remain undetected in the body.

Cell Growth Worksheet

To carry out the activity you will need the Cell Growth worksheet. For a printable version of the worksheet, click here.

For more information, at other Web sites...

Cell Form and Function — a collection of activities and demonstrations from Biology - Resources for the Classroom, created by Jenni Gottschalk.

Doubling Time — this interactive simulation calculates how fast a tumor will grow when you tell the program how much it has grown since the last time you measured it, from ChestX-ray.com.

Doubling Time and Population Growth — the math behind growth rates in human populations, from About.com.

Inside the Cell — learn the fundamental science of cells, from the National Institute of General Medical Sciences, National Institutes of Health.