(Original post by SimpleGirl)
PCR - Polymerase Chain Reaction
This is used in order to make copies of the DNA. It's helpful for evidence found at a crime scene that may only be present in very small amounts so they use PCR to make several copies which can be then be used for testing.
The sample of DNA, free nucleotides, primers and DNA polymerase are put together in a reaction tube.
Primers are short sections of DNA that are complementary to the bases adjacent to the fragment you want to amplify. The primers anneal (stick to) the start of the section so that DNA polymerase can begin complementary-base-pairing from there.
In simple terms, primers are like markers that attach to the beginning of the DNA so that the DNA polymerase knows where to start .
The mixture is heated at 3 different temperatures:
First at 95°C - In order to break the hydrogen bonds between the two strands of the DNA.
Then at 55°C - This is optimum temperature for primers to bind to the beginning of the desired fragment (STR).
Finally at 70°C - This is optimum temperature for the enzyme, DNA polymerase, to work at.
The DNA polymerase then attaches to where the primers have highlighted and it will move along the strand, lining up the free nulceotides and creating a new strand.
The one stand put into the PCR machine has now been copied into two. These two
strands are then put back into the machine and the whole process is repeated and four
strands will be made at the end. However many strands are put in at the beginning will be doubled
by the end of the PCR process.
The DNA can now be separated out according to length.
The DNA is placed on a slab of gel (usually agarose) which is covered in a buffer solution. The buffer solution is present as it conducts electricity; therefore, when the gel is connected to electrodes, a potential difference will be created across the gel.
The electric current then passes through the gel and causes the negatively charged DNA fragments to migrate (move towards) the positive electrode.
The smaller fragments move faster, allowing the bands to separate depending on size. This is what creates a series of bands on the profile.
Southern blotting (a staining technique) is used to highlight the bands. The DNA is transferred to a nylon membrane. The nylon membrane is then incubated with DNA probe .
Then finally, by using UV light, you can shine it over the membrane and the bands can be visualised.
Profiles are created and the bands are compared between them to identify the similiarity of the individuals.
(Used for matching DNA at crime scene with potential suspect's DNA or matching a child's DNA profile with the father's DNA profile for paternity testing)