Topic 2: PCR, Gel Electrophoresis & DNA Profiling
By: Vicky Phan, Hannah Arnaldo and Gurleen Kaur
*syllabus statements
4.4.5 DNA Profiling
In forensic sciences, DNA profiling is important to find out who was at the crime scene.
Some samples that can be taken for DNA testing includes: fingerprints, hair, skin, blood
etc.
Also can be used for paternity tests.
Matching DNA from sample to an individual is called DNA profiling.
*know how to read DNA profiles
4.4.1 Polymerase Chain Reaction (PCR)
*In order to create a DNA profile, there needs to be enough DNA. The polymerase chain
reaction takes the tiny amounts of DNA available to produce a profile.
This is done at high temperatures using a special DNA polymerase enzyme. After this, it
moves onto gel electrophoresis.
How it works:
1) Denaturation: DNA sample is heated an separated into two strands
2) Annealing: DNA primes attach to opposite ends of target sequence
3) Elongation: Heat-tolerant DNA polymerase copies strand
4.4.2-4.4.3 Gel Electrophoresis
Gel Electrophoresis is a method of separating mixtures of protein, DNA or other charged
molecules.
How it works:
1) On a thin sheet of gel (acts as a sieve), a mixture is placed on top
2) Two electrodes are applied to the gel, creating an electric field
3) Particles move towards electrodes depending on size/charge of molecules
4) Stained to produce a patterned band
*In gel electrophoresis, fragments of DNA can move in an electric field and separated
according to size.
*Gel electrophoresis of DNA is used in DNA profiling.
4.4.4 Applications of DNA Profiling—Forensic Investigations and Investigating Paternity
Organisms have short sequences of bases, which are repeated many times, called satellite
DNA. These repeated sequences vary in length from person to person. The DNA is
copied using PCR and then cut up into small fragments using restriction enzymes. Gel
electrophoresis separates fragmented pieces of DNA according to their size and charge.
This gives a pattern of bands on a gel, which is unlikely to be the same for two
individuals. This is called DNA profiling.
*how it works
FUN FACTS/QUESTIONS!!!
DNA profiles do not show individual base sequences but only identify
repeated sequences. How much confidence should be placed on DNA evidence?
How secure is DNA profiling? What are the implications for society if the authorities were to hold a
DNA profile for every person? Is it right to convict a person on DNA evidence alone?
Practice Questions:
1. The following is a DNA gel. The results are from a single probe showing a DNA
profile for a man, a woman and their four children.
mother
child 2 father child 4
child 1
child 3
origin
I.
II.
III.
IV.
1 2 3 4 5 6
[Source: The Biology Project, University of Arizona]
Which fragment of DNA is the smallest?
A. I
B. II
C. III
D. IV
2.
3.
4. Outline the process of DNA profiling (genetic fingerprinting), including ways in which it can
be used.