CELL DIVISION AND GROWTH
Cell division is necessary for the growth of an organism, or for the repair of damaged tissues. Mitosis results in two identical cells being produced from the original cell. The chromosomes contain the alleles which must be passed on to each new cell. A copy of each chromosome is made before the cell divides and one of each chromosome goes into the new cell. In early development of plant and animal embryos, the cells are unspecialised and are called stem cells. Most animal cells differentiate early in development and cell division is mainly for repair and replacement. Plant cells can differentiate throughout the life of the plant as it continues to grow. Cells of offspring produced by asexual reproduction are produced by mitosis from the parent cell. They contain the same alleles as the parent.
CELL DIVISION IN SEXUAL REPRODUCTION
Cells in reproductive organs (testes and ovaries) divide by meiosis to form gametes. In humans, the gametes are the sperm and the ova. Each gamete has only one chromosome from the original pair. All of the cells are different from each other and the parent cell. Sexual reproduction results in variation as the gametes from each parent fuse. So half the genetic information comes from the father and half from the mother. When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual develops by this cell repeatedly dividing by mitosis.
Before division in meiosis, a copy of each chromosome is made. The cell divides twice to form four gametes. Each gamete has a single set of chromosomes, each with a different combination of genes.
Stem cells are unspecialised. They are found in the human embryo and adult bone marrow. Stem cells change into all the different types of body cell, eg nerve cells and muscle cells. We say that the cells differentiate. Layers of cells in the embryo differentiate into all the cells the body needs. Stem cells in the adult bone marrow can change into other types of cell, eg blood cells. It is hoped that human stem cells can be made to differentiate into many types of cell. The cells formed could then be used to treat conditions such as paralysis, eg differentiating into new nerve cells.
FROM MENDEL TO DNA
Gregor Mendel was a monk who worked out how characteristics were inherited. Mendel was the first person to suggest the idea of separately inherited 'factors'. It took a long time for Mendel's ideas to be accepted…