Application of PCR

• Polymerase Chain Reaction (PCR)


  Amplifies DNA by making multiple identical copies of the DNA
• Purpose of Polymerase Chain Reaction (PCR)

  • Useful if only small amount of DNA found at a crime scene or obtained from a sample
  • Scientists can use PCR to make enough copies of that DNA to work with
• 4 Steps in Polymerase chain reaction

  1. Denaturation: Double-stranded DNA is heated to 90-95°C to break bonds between complementary bases, causing the 2 strands to separate, forming single-stranded DNA

  2. Annealing: Temperature is reduced to 50-55°C to allow primers to anneal / join to complementary sequences on opposite ends of each strand

  3. Extension/elongation: Temperature is raised to 72°C which is optimum temperature for the enzyme DNA polymerase to function. DNA polymerase binds to the primer and begins adding nucleotides to build a complementary strand of DNA

  4. Repeat: Steps 1-3 are repeated multiple times until there enough copies of DNA. Every time a repeat/cycle occurs, number of DNA strands double
• Lucky for the detectives, the hair follicle at the base of the hair is coated in cells that contain DNA. This means scientists can amplify the DNA using PCR to analyse it and find the perpetrator.
• Polymerase Chain Reaction


  Polymerase-Kettenreaktion
• Gel Electrophoresis


  Uses an electrical current to separate DNA fragments based on their size/length
• Steps in Gel Electrophoresis

  1. DNA is placed in wells on end of negative electrode

  2. Electric current repels negative DNA fragments and causes it to move towards positive electrode

  3. Smaller strands can wiggle through gel faster than larger strands

  4. Smallest strands will move the furthest along the gel

  5. DNA is difficult to see with the naked eye, so the gel is stained with a fluorescent dye, allowing bands of DNA to be visualised under an ultraviolet (UV) lamp
• Sorting DNA fragments: Gel Electrophoresis

  • Can have samples of known sizes of DNA fragments (called standard ladder) to compare to determine the size of the DNA fragment
  • Larger fragments travel less distance
  • Smaller fragments travel further
  • If fragments are same size, they will travel same distance
• Applications of PCR and Gel Electrophoresis

  • Genetic Testing: To diagnose cystic fibrosis, a blood sample is taken, the DNA is amplified using PCR and it is run through gel electrophoresis
  • DNA Profiling: A method of obtaining a unique pattern of DNA for an individual that can be used for identification purposes
• STR


  Short string of bases that repeat again and again in tandem
• DNA profile: obtained when STR regions of an individual's genome are analysed. Gel electrophoresis used to sort out different sized STRs – pattern unique to an individual.
• Remember that 1 allele of each gene is inherited from the mother and the other allele is inherited from the father. So, half profile will match mother and half will match father.