Why do DNA fragments move towards the positive electrode during electrophoresis?

DNA fragments are negatively charged due to their phosphate groups, so they are attracted to the positive electrode (anode) when an electric current is applied.

Explain why shorter DNA fragments travel further through the gel than longer fragments.

Shorter fragments move faster and further because they can pass more easily through the pores in the gel matrix.

After electrophoresis, where in the gel would you find the smallest DNA fragments?

Nearer the positive electrode, as smaller fragments travel further through the gel.

How can DNA fragments be visualised after gel electrophoresis?

Using stains or dyes that bind to DNA, such as ethidium bromide viewed under UV light or safer alternatives like SYBR Safe.

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Gel electrophoresis

3 things to know
Papers 1 and 3

Overview

Recall questions on why fragments move towards the positive electrode, why shorter fragments travel further, and how the results are read.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Name two techniques that can be used to separate biological compounds.

Key term

Define gel electrophoresis.

Key term

Describe how DNA samples are set up for gel electrophoresis.

Questions from this subtopic

Each one with the model answer underneath.

1
Define gel electrophoresis.
Model answer
A technique used to separate DNA fragments according to their length.
2
Describe how DNA samples are set up for gel electrophoresis.
Model answer
DNA samples are loaded into wells in an agarose gel and an electric current is applied.
3
Why do DNA fragments move towards the positive electrode during electrophoresis?
Model answer
DNA fragments are negatively charged due to their phosphate groups, so they are attracted to the positive electrode (anode) when an electric current is applied.
4
Explain why shorter DNA fragments travel further through the gel than longer fragments.
Model answer
Shorter fragments move faster and further because they can pass more easily through the pores in the gel matrix.
5
After electrophoresis, where in the gel would you find the smallest DNA fragments?
Model answer
Nearer the positive electrode, as smaller fragments travel further through the gel.
6
How can DNA fragments be visualised after gel electrophoresis?
Model answer
Using stains or dyes that bind to DNA, such as ethidium bromide viewed under UV light or safer alternatives like SYBR Safe.

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