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Cell Reproduction

The Cell Cycle

Home | 6-1 Chromosomes | 6-2 Mitosis and Cell Division | The Cell Cycle | 6-3 How Gametes Form: Meiosis | Vocabulary

Most of a cell's lifetime is spent growing. This cycle of growth and division is called the cell cycle. The cell cycle is divided into five phases. Mitosis, Cytokinesis, G1, S phase, and G2 phase. 

The Cell Cycle

A new cell begins its life by undergoing a period of growth and activity called interpahse. During interphase. the new cell is hard at work carrying out tasks of life. Like taking in food, converting energy, and growing. It is also making copies of its genetic material and duplicating its organelles.

Interpahse has three distinct stages. The first stage is a gap in the cell cycle. This is called G1 phase. In this stage the cell goes through a period of intense growth. Most cell spend most of their lives in this stage.

Next the cell may enter the S phase of the cycle. The S stands for synthesis. In this stage the cell will make a capy of the DNA molecule in each of the chromosomes. When they are copied, the two identical copies remain together, joined by the centromere.

The S phase is followed by another gap in the cycle. This is called G2 phase. Which is the final stage of interphase. Here, the cell keeps growing and also starts to make preparations for cell division. The cell is also duplicating its organelles.

Once the G2 phase is finished, interphase is finished. Now it will go into mitosis or M phase. During this phase the two copies of each chromosome are sperated. When the cell reaches the final stage, Cytokinesis, or C phase, it divides in half nad becomes two new, identical cells.

The length of the cell cycle varies greatly in organisms. In plants it lasts between 10 and 30 hours. In animals it last between 18 and 24 hours but it could be much shorter. Fruit flies hold the record for the fastest animal cell cycle. Which is 8 minutes. Some cells may never complete the cycle, such as nerve cells. No cell lives forever. Cells appear to be programmed to only go through division so many times and then die. Its like the cells in our body have a hidden hourglass, and the grains of sand are cell divisions.  

What controls the cell cycle? The answer to that question has been unknown, until recently. Using the techniques to identify and trace movements of cell proteins, researchers have discovered that animal cells use a class of proteins called cyclins to help control differnt phases of the cell cycle. Cyclins are influenced by another class of proteins, called growth factors. Growth factors supervise and monitor a cell's progress through the phases.

The mechanisms controlling the cell cycle occasionally become damaged and fail. The effects can be disastrous. Without such regulations, a cell will grow and divide without restraint. The result of this is a tumor. A tumor is a clump of cells. The tumor may stay in the region in wihich it originally formed. When this happens the tumor is usually harmless, which you can call benign.
It may also continue to grow and invade other tissues. When this happens the tumor is described as cancerous. Cancer is a term used to decribe a disease characterized by uncontrolled cell division and growth. Cancer is a result of a malfunction of the cell cycle.
A faulty cell cycle can produce many changes in a cell, making it way differnt from a normal one. Unlike a normal cell, which can only grow on a solid surface, a cancer cell can also grow in liquids. A cancer cell aslo has abnormal proteins and lipids in its outer membrane, which may prevent it from reaching to signals in its environment.

Brain Tumor

A normal healthy cell has to go through a lot of indepentent changes to become life-threatening. Some of the changes are loss of regulated cell division and it must be able to maneuver around and break through protective membranes in tissues. It also has to mantain a supply of oxygen and nutrients.
To become life-threatening the genes regulating cell division must have defects. The cell cycle is regulated in several ways, and all must be damaged for cancer to occur. A change in a gene is called a mutation. Cancer-causing mutations are usually not hereditary.
Occasionally the cell-cycle controls genes that become mutated in eggs and sperm, leading to an increased chance of developing cancer later in life.
If you limit your exposure to things that can cuase cancer, like radiation and cancer-cuasing chemicals present in cigarette smoke, you would be less likely to obtain cancer.

"Biology is the only science in which multiplication means the same thing as division."