DNA and Identification

Discusses PCR, Gel Electrophoresis and Introns

  • Created by: Megz
  • Created on: 22-12-10 16:14


90% of the human genome is made up of introns while less than 2% actually codes for proteins!

An intron is a non-coding section of DNA that doesn't translate to a protein. Within this are short sequences of DNA that are repeated many times; micro-satellites and mini-satellites.

The likelihood of two individuals sharing the same pattern of DNA is extremely remote since there is a huge variation of repeats.

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DNA Profiling

To produce a profile strands of DNA are chopped up into fragments using restriction endonucleases (enzyme). They cut at particular points in the intron sequences. Fragments are then separated by gel electrophoresis:

- DNA samples are placed into an agarose gel medium in a buffering solution (maintains pH). Gel contains a dye called ethidium bromide that binds to the DNA and will fluoresce when under UV light- creates the banding effect.

- An electric current is passed through the apparatus and the DNA fragments move towards the positive anode due to the negative charge of phosphate group in the DNA. Rate of movement depends of mass and charge of each fragment.

- Once electrophoresis is complete, the plate is put under UV light and identification can be made.The pattern is transferred to a nylon membrane with a radioactive DNA probe that binds to specific sequences.

-Sheet of X-ray film placed on membrane to detect the pattern. This helps to keep the banding displayed longer.


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Polymerase Chain Reaction

Often there aren't enough human cells to work with if conducting a crime investigation so the amount needs to be amplified using the PCR.

  • Mixture is heated to 90-95°C which breaks the hydrogen bonding, between the nitrogenous bases.
  • Then it's cooled down to 50-60°C so the primers can anneal to the single DNA strands. Primers are small sequences of DNA.
  • The next step is to heat to 72°C, allowing DNA polymerase to extend the complementary strands of DNA, using artificially assembled units.

This process will be repeated lots of times, producing over a million copies ready for analysis.

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