6.5.2Unit 6: Practical Skills in Biology II
Gel Electrophoresis in Forensics — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 18 of 48 Unit quiz
Section 1 of 5The Principle
Charge
- DNA is negatively charged because of its phosphate groups, so it moves towards the positive electrode (anode).
Gel
- Fragments travel through the pores of an agarose gel, which slow larger molecules more than smaller ones.
Separation
- Smaller, shorter fragments move faster and further than larger ones in the same time.
Used for
- The technique is used to analyse DNA, RNA and proteins.
Anode and cathode
- The wells are at the cathode end, and the DNA moves across the gel towards the anode.
Section 2 of 5Method
1
Cut the DNA into fragments with restriction endonucleases that cut at specific base sequences.
2
Make an agarose gel plate with wells at one end, and submerge it in a tank of electrolyte solution that conducts electricity.
3
Load the DNA samples into the wells with a micropipette, and add a DNA standard of known fragment sizes to one well.
4
Connect the electrodes so that the wells are at the negative end, and pass a current through the gel.
5
Switch off before the fragments reach the end of the gel, then make the bands visible.
6
Longer fragments stay closer to the wells and shorter ones travel further, which gives separate bands.
Section 3 of 5Making Bands Visible
Making Bands Visible
Probes
- Single-stranded DNA probes complementary to target sequences are added to bind the fragments of interest.
Radioactive labels
- A radioactive probe darkens X-ray film, so a pattern of dark bands appears.
Fluorescent labels
- A fluorescent dye glows under UV light and gives coloured bands.
Standard
- Comparing bands with the standard shows the size of the unknown fragments.
Section 4 of 5Why Patterns Differ Between People
The banding pattern is a record of the lengths of the repeated regions in a person’s DNA.
Repeats
- Regions of repeated bases, called variable number tandem repeats (VNTRs), lie between restriction sites.
Number varies
- Different people have different numbers of repeats, so their fragments differ in length.
Unique pattern
- Each person has a different banding pattern, apart from identical twins.
Comparison
- A sample from a crime scene can be compared with suspects’ patterns to show who was present.
Reliability
- Comparing several regions of DNA makes a match between samples more convincing.
Section 5 of 5Exam Checklist & Tips
6.5.2 – Key Points for the Exam
- Gel electrophoresis A technique that separates charged molecules by size in an electric field.
- Restriction endonuclease An enzyme that cuts DNA at a specific base sequence.
- VNTR A variable number tandem repeat, a region where a short sequence is repeated a different number of times in different people.
- Probe A labelled single-stranded DNA sequence that binds to a complementary target.
- Electrolyte A salt solution that conducts electricity through the gel tank.
Common Mistakes
- Saying that DNA moves towards the cathode, when its negative charge makes it move to the anode.
- Stating that large fragments move further, when smaller fragments move faster and further.
- Forgetting to load a standard, which gives sizes to compare with.
- Describing the bands as genes, when they are fragments of different lengths.
Last updated:
\n