1. Multicellular organisms

• Inside the nucleus we find long threadlike structures. These are called chromosomes.
• Every species of living organisms has a characteristic number of chromosomes called its chromosome complement.
• A cell has two matching sets of chromosomes is known as diploid.
• Normal body cells contain two matching sets of chromosomes so they are diploid. However, sex cells only contain a single set of chromosomes so therefore they are haploid.
• Sex cells must be haploid because the nuclei of two haploid cells fuse together during fertilisation in order to produce a diploid zygote.
• New cells always need to be produced in order to replace damaged cells and allow growth to take place. The process of producing new cells is Mitosis.
• Mitosis provides new cells for growth and repair of damaged cells while maintaining the diploid chromosome complement.
• Mitosis stage 1: At the beginning of mitosis, the cell prepares itself for division. The DNA of the cell is duplicated, creating two copies of each strand of DNA called a chromosome.
• Mitosis stage 2: The chromosomes shorten and thicken and are seen as a pair of chromatids.
• Mitosis stage 3: The nucleus breaks down and the chromosomes line up along the equator of the cell. Spindle fibres attach to the chromosomes.
• Mitosis stage 4: The pair of chromosomes are pulled apart and the chromatids move to the opposite ends of the cell. Duplicate copies of the cells DNA are now on each side of the cell and are ready to divide.
• Mitosis stage 5: The cytoplasm divides between the two new cells. The cell pinches in the middle and divides into two genetically identical daughter cells.
• Stem cells are unspecialised cells. This means they have the potential to become different types of cells,
• Stem cells divide in order to self-renew and are involved in growth and repair.
• Cells can be specialised to perform a certain function. This can lead to the formation of tissues and organs.
• Groups of organs work together to form systems.
• The hierarchy in cells

  Cells>Tissues>Organs>Systems