Cell Division
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Chromosomes
The nucleus of a cell contains chromosomes, which are made of DNA molecules. Each chromosome carries a large number of genes. In body cells, chromosomes are normally found in pairs — humans have 23 pairs, so 46 chromosomes in total.
Mitosis and the cell cycle
Cells divide by mitosis in a series of stages known as the cell cycle. Before a cell can divide it has to grow and copy its contents.
- Growth and DNA replication — the cell grows, sub-cellular structures such as ribosomes and mitochondria increase in number, and the DNA replicates so each chromosome forms an identical copy of itself.
- Mitosis — one set of chromosomes is pulled to each end of the cell and the nucleus divides.
- Division — the cytoplasm and cell membrane divide, forming two genetically identical daughter cells.
Mitosis matters because it produces genetically identical cells. It is used for growth, for development, and for repairing damaged tissue.
Stem cells
A stem cell is an undifferentiated cell that can give rise to more cells of the same type, and to other cell types by differentiating.
- Embryonic stem cells can differentiate into most types of human cell.
- Adult stem cells, such as those in bone marrow, form a more limited range — bone marrow stem cells form blood cells.
- In plants, the meristem tissue found at root and shoot tips can differentiate into any plant cell throughout the plant's life.
Stem cells in medicine
Stem cells from human embryos can be cloned and made to differentiate into specialised cells for treating conditions such as diabetes and paralysis.
Therapeutic cloning produces an embryo with the same genes as the patient, so stem cells from it are not rejected by the patient's immune system.
Balanced arguments you should be able to give:
- In favour — the potential to cure currently untreatable conditions; embryos used are often unwanted from fertility clinics.
- Against — some people object on religious or ethical grounds because an embryo is a potential human life; there are risks of transferring viral infection; procedures are expensive and the technology is still developing.
Plant meristem stem cells can be used to produce clones of rare species to protect them from extinction, or to produce large numbers of identical crop plants with desirable characteristics such as disease resistance.
