Cell Division

• Chromosomes: Carry genetic information in a molecule called DNA

  The structure made of DNA that codes for all the characteristics of an organism
• Humans have 46 chromosomes arranged into 23 pairs
• In contrast, gametes do not have pairs of chromosomes in their nuclei and bacterial cells do not have a nucleus.
• The entire genetic material of an organism is known as the genome.
• A gene is a short length of DNA found on a chromosome that codes for a particular characteristic or protein
• Alleles are different forms of the same gene. For example, eye colour is the gene but blue, green, brown etc. are alleles
• DNA molecules are large and complex. They carry the genetic code that determines the characteristics of a living organism
• Each chromosome is made from a single molecule of DNA, but when a cell is ready to divide, the DNA copies itself, then coils and condenses to form the chromosomes that we see in micrographs.
• Each chromosome in a pair carries the same types of genes. The 23rd pair are the sex chromosomes:

  • In females, the two chromosomes are identical in shape. There are two X chromosomes. Females are referred to as XX.
  • In males, one of the chromosomes is a different in shape. There is an X and also a Y chromosome. Males are referred to as XY
• Mitosis: A type of cell division which produces daughter cells identical to the parent cell
• The cell cycle:

  1. Cell growth
  2. DNA synthesis -the chromosomes are now double stranded
  3. Further growth occurs and the DNA is checked for errors
  4. Mitosis
  5. The cytoplasm separates - two cells are formed
  6. Temporary cell resting period or the cell stops dividing
• Mitosis:

  1. The cell begins to divide
  2. The DNA replicates to form two copies of each chromosome
  3. The nuclear membrane breaks down. The chromosomes line up across the centre of the cell
  4. One set of chromosomes is pulled to each end of the cell and the nucleus divides
  5. The cytoplasm and cell membranes divide to form two identical cells
• When looking at cells with a microscope, the length of different stages of the cell cycle can be estimated using the formula: Length of time in phase = observed number of cells at that stage/total number of cells observed x total length of time of cell cycle
• A stem cell: A cell found in foetuses, embryos and some adult tissues that can give rise to a wide range of other cells
• Stem cells = cells that have not undergone differentiation. A cell which has not yet become specialised is called undifferentiated
• If cells are removed from the embryo – called embryonic stem cells - they will differentiate into any cell type
• Some stem cells remain in the bodies of adults – adult stem cells. Adult stem cells are found in limited numbers at certain locations in the body
• Adult stem cells can be found in several regions of the body, including the:

  • brain
  • eyes
  • blood
  • heart
  • liver
  • bone marrow
  • skin
  • muscle
• Adult stem cells can differentiate into related cell types only, for example, bone marrow cells can differentiate into blood cells and cells of the immune system but not other cell types
• Cell division in plants occurs in regions called meristems.
• Cells of the meristem can differentiate to produce all types of plant cells at any time during the life of the plant
• The main meristems are close to the tip of the shoot, and the tip of the root.