5) DNA Structure

• DNA is a polymer made up of repating units called nucleotides
• Each nucleotide consists of:

  • a sugar
  • a phosphate
  • one of four bases: A, C, G or T
• The nucleotides are joined together to form long stands
• Each molecule has two alternating sugar and phosphate strands, which are twisted to form a double helix
• Attached to each sugar is one of the 4 bases
• It is an attraction between the different bases the holds the two stands together:

  • a C on one strand always links with a G on the opposite strand
  • a T on one strand always links with an A on the opposite strand
• The order of bases on DNA controls the order in which amino acids are joined together to make a particular protein.
• A sequence of three bases is the code for one amino acid.
• T Proteins are synthesised on ribosomes using a template that has been taken from the DNA and carried out of the nucleus.
•  Carrier molecules then bring specific amino acids to add to the growing protein chain in the correct order.
•  When the protein chain is finished, it folds up to form a unique shape.
•  This unique shape allows the proteins to do their job as enzymes, hormones or structural proteins such as collagen.
•  A change in DNA structure is called a mutation.
• If any bases in the DNA are changed, then it may change the order of amino acids in the protein coded for by the gene.
• Mutations occur all the time. Most do not alter the protein, or only alter it slightly, so that it still works.
• A few mutations may cause the protein to have a different shape:

  If it is an enzyme, then the substrate may no longer fit into the active site.

  • If it is a structural protein, it may lose its strength.
• Not all parts of DNA code for proteins:

  • Non-coding parts of DNA can switch genes on / off so that they can / cannot make specific proteins.
  • Mutations in these areas of DNA may affect how genes are expressed.