• Microscopes have improved greatly over the past 350 years. The best modern light microscopes can magnify specimens more than 1500 times.
  • As a result, we can observe very small structures inside cells. For example, we can see inside bacterial cells.
  • In the 1930s, the electron microscope was invented. This casts out a beam of electrons to magnify specimens up to about 2,000,000 times! As well as this, electron microscopes produce very clear images and show us more about the detailed structure of cells.
  • Electron microscopes have showed us about the chromosomal and plasmid DNA in a bacterium cell.


  • A DNA molecule has two strands coiled together in the shape of a double helix.
  • Chemicals called bases hold the strands together. There are four different kinds of bases: Adenine, cytosine, guanine, thymine.
  • Adenine and Thymine always join together and Cytosine and Guanine always join together. These are called base pairings.
  • The base pairs are joined together by weak hydrogen bonds.
  • A gene is a section of DNA and the sequence of bases in a gene code for a specific protein.


  • A gene is a section of DNA. It contains the instructions to make a specific protein.
  • Cells make proteins by stringing amino acids together in a particular order.
  • There are only 20 amino acids, but they can make thousands of different proteins.
  • The order of the bases in a gene simply tells cells in what order to put the amino acids together.
    • A each of triplet bases codes for a particular amino acid. For example, TAT codes for tyrosine and GCA for alanine. So TAT-GCA is tyrosine-alanine.
  • DNA also determines which genes are switched on or off – and so which proteins the cell produces e.g. haemoglobin or keratin. That in turn determines what type of cell it is such as a red blood cell or a skin cell.
  • Some proteins help to make other things that aren’t made of proteins from substances that come from your diet.