DNA profiling

• In non-coding DNA (introns, telomeres, centromeres) there are short sequences of DNA that are repeated many times. Variable number of tandem repeats (VNTR) and

  Short Tandem Repeats (STR) are just repeated sections of non-coding DNA.
• • The lengths of tandem repeats and how many times they repeat are unique to different individuals.
  • They are inherited so the more closely related you are to someone the more likely you are to have similar patterns.
• DNA profiling = Mapping these patterns and producing an image of them that can be analysed.
• DNA profiling procedure

  1. DNA extraction
  2. Digesting the sample
  3. Electrophoresis
  4. Hybridisation
  5. Imaging
• DNA extraction -

  With the invention of PCR we no longer need large samples of DNA to create a profile
• Digesting the sample -

  Restriction endonucleases are used to cut the DNA into fragments.

  Different restriction endonucleases have different recognition sites so enzymes that leave the VNTR and STR sections in tact are used.
• Gel electrophoresis –

  Separating the fragments
• Hybridisation -

  DNA probes = short sections of DNA that are complimentary to a known section e.g. a STR.They have either radioactive or fluorescent tags on them.
• Imaging -

  • If the probes were radioactive we use x-rays.
  • If the probes were fluorescent we use UV light.
  • Either way we end up with an image of the fragment pattern
• uses of DNA profiling -

  • paternity testing
  • food standards
  • phylogeny
  • identifying risk of genetic disease
  • forensic criminology
• PCR - polymerase chain reaction - a technique for the amplification of DNA
• • Taq polymerase (duplicates DNA, Taq is from Thermus aquaticus which is found in hot springs and is thus stable at high temperatures)
  • Primers x2  (piece of single stranded DNA which is complementary to the specific target sequence at the 3’ end of each DNA replicated strand) This specificity allows you to amplify any DNA sample – enabling you to find a needle in a haystack of DNA
  • Free nucleotides to make amplified DNA
  • Original DNA strand needed to be replicated.
• DNA is heated 94-96C -

  • Hydrogen bonds between chains break
  • Separate into 2 strands
• Mixture cooled to 50-65C -

  • Allows primers to anneal/attach to each 3’ end of each strand
• Heated to 72C for DNA polymerase to attach nucleotides -

  • Heat tolerant DNA polymerase then replicates the region of DNA
  • Takes longer for polymerisation of nucleotides