Cell division and growth
New cells are needed for an organism, or part of an organism, to grow. They are also needed to replace cells which become worn out and to repair damaged tissue. However, the new cells must have the same genetic information as the originals. Then they can do the same job.
Each of your cells has a nucleus containing the instructions for making both new cells and all the tissues and organs needed to make an entire new you. These instructions are carried in the form of genes.
A gene is a small packet of information that controls a characteristic or part of a characteristic, of your body. It is a section of DNA. Different forms of the same gene are known as alleles. The genes are grouped together on chromosomes. A chromosome may carry several hundred or even thousands of genes.
You have 46 chromosomes in the nucleus of your body cells. They are arranged in 23 pairs. One of each pair is inherited from your father and one from your mother. Your sex cells (gametes) have only one of each pair of chromosomes.
The cell division in normal body cells produces two identical cells and is called mitosis. As a result of mitosis all your body cells have the same chromosomes. This means they have the same genetic information.
In asexual reproduction, the cells of the offspring are produced by mitosis from the cells of their parent. This is why they contain exactly the same alleles as their parent with no genetic variation.
How does mitosis work? Before a cell divides it produces new copies of the chromosomes in the nucleus. The the cell divides once to form two genetically identical cells.
In some parts of an animal or plant, cell division like this carries on rapidly all the time. Your skin is a good example. You constantly lose cells from the skin's surface, and make new cells to replace them. In fact about 300 million body cells die every minute so mitosis is very important.
In the early development of animal and plant embryos the cells are unspecialised. Each one of them (known as a stem cell) can become any type of cell that is needed.
In many animals, the cells become specialised very early in life. By the time a human baby is born most of its cells are specialised. They will all do a particular job, such as liver cells, skin cells or muscle cells. They have differentiated. Some of their genes have been switched on and other have been switched off.
This means that when, for example, a muscle cell divides by mitosis it can only form more muscle cells. So in a mature (adult) animal, cell division is mainly restricted. It is needed for the repair of damaged tissue and to replace worn out cells. This is because in most adult cells differentiation has already occurred. Specialised cells can divide by mitosis, but they only form the same sort of cell. Therefore growth stops once the animal is mature.
In contrast, most plant cells are able to differentiate all through their life. Undifferentiated cells are formed at active regions of the stems and roots. In these areas mitosis takes place almost continuously.
Plants keep growing all through their lives at these 'growing points'. The plant cells produced don't differentiate until they are in their final position in the plant. Even then the differentiation isn't permanent. You can move a plant cell from one part of a plant to another. There it can differentiate and become a completely different type of cell.