Control of cell division and Stem Cells Haploid and Diploid cells Each cell contains genes that help to control when it divides. Cells divide by mitosis only when required. When receives signals from neighbouring cells, it responds by dividing or not dividing. If this control goes wrong, cells may not divide when they should (growth does not take place, or wounds do not heal) or they may divide when they should not (a tumour may form). 1. Cancer and uncontrolled cell division In cancer: genes that control cell division mutate cell divide over and over again, forming an irregular mass of cells. In malignant tumour: some of cancer cells may break off and start to form new tumours elsewhere in the body. Several genes must mutate before a cell becomes cancerous. This can happen just by chance. The risk is increased by factors that cause mutation (carcinogenic factors): - Ionising radiation (from X ray and radioactive sources emitting α, β or γ radiation) - Ultraviolet radiation (in sunlight) - Chemicals (e.g. asbestos, some component in tar from tobacco smoke) - Viruses (e.g. human papilloma virus - HPV, causing cervical cancer). 2. Significance of mitosis in cell replacement and tissue repair by stem cells Stem cells are undifferentiated biological cells that can differentiate into specialized cells and can divide to produce more stem cells. They are present both during embryonic development (embryonic stem cells) and in the adult body (adult stem cells). In adult organisms, stem cells act as a repair system for the body, replenishing adult tissues. They divide by mitosis to form either two stem cells, thus increasing the size of the stem cell "pool", or one daughter that goes on to differentiate, and one daughter that retains its stem-cell properties. Niche cells (green) underlying a basement membrane signal to stem cells (red)to block differentiation and regulate division. The stem cell divides such that one daughter retains its connections to the niche, while the other (yellow) becomes untethered (released) and begins to differentiate. 3. Haploid and Diploid cells Haploid cells Haploid cells are cells that contain only one complete set of chromosomes. The most common type of haploid cells is gametes, or sex cells. Haploid cells are produced from diploid cells by meiosis (each daughter cell gets only half of the original number of chromosomes). In human, when the sperm and egg (haploid cells with one set of 23 chromosomes) fused together, this produced a diploid zygote with two sets of chromosomes (46 chromosomes). As this cell divided by mitosis, each daughter cell obtained a complete copy of each set. Diploid cells Most of the cells in the body are diploid cells, they contain two complete sets of chromosomes, one from mother and one from father. Each cell has 46 chromosomes. Diploid cells reproduce using mitosis, which creates a completely identical copy of the cell. Meiosis helps to produce haploid cells from diploid cells (it is a reduction division, because it reduces the number of chromosomes in a cell by half). Meiosis must take place at some point in the life cycle before fertilisation. In humans, it only happens in the testes and ovaries.