B2 Cell division: gametes, body, mitosis, once, repair, asexual, copied, growth, two, testes, twice, differentiate, four, fertilisation, gamete, genetic, random, ovaries, identical Meiosis Gametes (sex cells) are made by meiosis. In humans sperm are made in the testes and eggs are made in ovaries. First each chromosome is copied (the DNA molecules replicate). The cell then divides twice to make four gametes, each with one set of chromosomes (23 in human gametes). Mitosis Body cells divide by mitosis to form new cells needed for growth and repair. First each chromosome is copied (the DNA molecules replicate). The cell then divides once to make two genetically identical cells. Fertilisation Gametes join at fertilisation forming a single body cell. This cell then divides by mitosis repeatedly. Most types of animal cells differentiate at an early stage and become specialised cells (eg nerve cells, muscle cells) Asexual vs sexual reproduction Some organisms can reproduce using a single parent and no gametes. Cells of offspring made by asexual reproduction are produced by mitosis, so they are genetically identical to their parents (no variation in alleles). Sexual reproduction results in genetic variation because… a) Gametes fuse at random. b) Every gamete is unique, with a different combination of alleles.

Treatment with stem cells may be able to help conditions B2 Chromosomes and stem cells: DNA, cancer, nucleus, 46, helix, 23, gene, alleles, embryos, animals, medical, long, plant Stem cells Stem cells from human embryos and adult bone marrow can differentiate into many different types of cells. Treatment with stem cells may be able to help conditions such as paralysis. Chromosomes Most body cells have a nucleus containing two sets of chromosomes. Human cells have 23 pairs, making a total of 46. Chromosomes contain genetic information and are made from DNA. Each DNA molecule has a double helix structure. A short section of chromosome DNA which codes for a characteristic is called a gene. Genes can exist as different forms called alleles, eg. the eye colour gene has a blue (b) allele and brown (B) allele. Say NO to stem cells: 1. Destroys embryos which have a right to life. 2. Unknown long term effects – stem cells may cause cancer. 3. Money could be spent on other medical research Say YES to stem cells: 1. May cure paralysis. 2. Makes use of spare IVF embryos. Differentiation. When cells become specialised, they are said to have differentiated. In animals, most cells differentiate at an early stage (in the embryo). Many plant cells can differentiate throughout life.