DNA Electrophoresis Quiz

Gel electrophoresis is a laboratory method utilized to separate DNA fragments based on:

• Charge (positive or negative)

• Size

• Colour

• Type of gene

The image shows fragments of DNA before they are run through electrophoresis.
Which one will stay closest to the beginning of the gel?

• A

• B

• C

• D

Answer explanation

In gel electrophoresis, the DNA fragments that stay closest to the beginning of the gel (near the wells) are the longest fragments.
As the electric current is applied, DNA fragments migrate towards the positive electrode due to their negative charge. However, longer DNA fragments have more difficulty moving through the gel matrix compared to shorter fragments. Consequently, while shorter fragments travel further down the gel, longer fragments remain closer to their starting point in the wells. Thus, the longest DNA fragments will be found nearest to the wells at the top of the gel after electrophoresis is complete.

The gel on the right shows fragments of DNA. Which fragment is the largest?

• A

• B

• C

• D

Answer explanation

Largest/longest fragments remain closer to their starting point in the wells. Thus, the largest/longest DNA fragments will be found nearest to the wells at the top of the gel after electrophoresis is complete.

The image shows a gel in which a set of DNA size reference standards (lane 1) were electrophoresed alongside a sample of experimental DNA fragments (lane 2). Which fragment is approximately 2.5 kb in size?

• A

• B

• C

• D

• E

Answer explanation

Sample D sits between 2 kb and 3 kb.

Based on the results of this analysis, did this sample likely come from any of the current suspects?

• Yes. The crime scene sample likely came from Suspect #1.

• Yes. The crime scene sample likely came from Suspect #2.

• Yes. The crime scene sample likely came from Suspect #3.

• No. None of the suspects are the likely source of the unknown sample.

Answer explanation

The banding sample from suspect #3 matches the sample found on the crime scene. Therefore, suspect #3 must have been present at the crime scene.