1. National 5 Biology Course Notes Unit 1 : Cell Biology Part 3 : Producing new cells

2. Chromosomes Chromosomes are structures in the nucleus that contain DNA Since chromosomes are made of DNA, they contain the genes. Chromosomes can only be seen when the cell is dividing. The chromosomes contain two identical threads called chromatids since the DNA replicates (makes an exact copy of itself) just before the cell divides Single chromosome Two chromatids Diploid and haploid A diploid cell contains two sets of chromosomes (one set comes from each parent). So in diploid cells there are pairs of chromosomes (one chromosome in a pair comes from each parent) Body cells are diploid. Haploid cells have a single set of chromosomes. Sex cells, like sperm and egg cells, are haploid Chromosome complement Each species of plant or animal has it’s own particular number of chromosomes in the nucleus of it’s cells – this is the chromosome complement of that species. Human cells have 46 chromosomes – or 23 pairs of chromosomes. The diploid number of a human cell is 46, the haploid number is 23.

3. Mitosis Mitosis is division of the cell nucleus but the word is sometimes used to refer to cell division. Mitosis is a kind of cell division that maintains the chromosome number. When a cell divides by mitosis two new cells (daughter cells) are produced but they each have the same number of chromosomes as the original cell (the new cells in fact are genetically identical to each other and to the parent cell. So if a human cell with 46 chromosomes divides by mitosis, two new cells are produced but they each have the same 46 chromosomes as the original cell. Mitosis produces new body cells for growth and repair of the body.

4. Nuclear membrane disappears Stages of mitosis Diagram of stage Description of stage Photo showing stage in actual cell Chromosomes become visible in the nucleus Each has 2 chromatids Chromosomes line on the cell equator Spindle fibres form and join to the chromosomes Spindle fibres Chromatids are pulled apart (by the spindle fibres) to opposite poles of the cell Cytoplasm divides after a nuclear membrane forms around each set of chromosomes

5. Tissue Culture Tissue culture is an artificial way of producing new cells. Tissue culturing must be carried out under sterile conditions using aseptic techniques in order to prevent contamination (growth of unwanted microbes) in the growing medium (substance the cells are grown on) Conditions are controlled to make them ideal for the growing cells including the right: Temperature pH Oxygen concentration Growth Media 1. Nutrient agar jelly is a growth medium that cells or micro- organisms can be grown on. The medium contains nutrients (foodstuff for the cells) and agar jelly that solidifies the medium and allows the cells to grow on its surface. The agar medium being used must be sterile. This is prepared by placing the agar into an autoclave. An autoclave is heated to 121˚c for 20 minutes and to ensure that all microbes are dead. 2. Nutrient broth is also a growth medium that can support the growth of cells or micro-organisms. This growth medium is liquid and contains nutrients but no agar. Bacteria growing on nutrient agar Cells growing in nutrient broth

6. Aseptic Technique When working in the laboratory, whether growing micro-organisms or other types of cells, it is very important to take precautions. These precautions help to prevent contamination or avoid the escape of harmful micro-organisms. Some of the main precautions include: Washing hands before and after practical work Disinfecting surfaces Wearing protective clothing, e.g. lab coat or apron Sterilising equipment using an autoclave or flame sterilising