Explain gel electrophoresis
Explain how a gel separates DNA fragments of different lengths.
Section 19.1 · Principles of genetic technology
A Level; Cambridge International Biology 9700, 2028–2030. This is curriculum teaching, not medical advice or a claim that a treatment is suitable for an individual. Written responses are self-checked
Before you begin
- DNA stores genetic information; genes can be expressed to produce RNA and protein.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why does DNA move toward the positive electrode?
What you will learn
- Explain the charge-driven movement of DNA in an electric field
- Relate fragment length to movement through a gel
Build the explanation
DNA has negatively charged phosphate groups, so DNA fragments move through a gel toward the positive electrode when voltage is applied. The gel matrix impedes larger fragments more than smaller fragments; shorter fragments generally travel farther in the same time. Bands can be compared with a size marker.
Use the evidence carefully
Use the direction of travel relative to the electrodes and wells. Band distance depends on conditions and fragment length; a band pattern does not by itself identify a person or gene without a reference and validated procedure.
Pause and explain
A DNA band appears faint but travels the same distance as a reference band. What can distance and intensity indicate separately?
Worked example
A short and a long DNA fragment enter the same gel lane. Which usually travels farther?
Show the worked solution
- The gel impedes larger fragments more.
- Shorter fragments move farther.
Answer The shorter fragment generally travels farther through the gel under the same conditions.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Which DNA fragment usually moves farther in the same time: 300 bases or 1200 bases?
Give me a hint
Identify the relevant material or technology and explain each step.
Compare my reasoning
- Compare fragment lengths.
- The shorter fragment travels farther.
The 300-base fragment usually travels farther through the gel.
Look for these in your work
- Method identified
- Steps linked to function
Why should a size marker be run beside samples?
Give me a hint
Use the stated example to justify a conclusion.
Compare my reasoning
- Known lengths provide a reference.
- Compare band positions.
A size marker provides bands of known fragment lengths to help estimate sample-fragment sizes.
Look for these in your work
- Evidence interpreted
- Limits stated
A DNA band appears faint but travels the same distance as a reference band. What can distance and intensity indicate separately?
Give me a hint
Apply the idea to a new application or trade-off.
Compare my reasoning
- Distance relates to fragment length under the method.
- Intensity relates to amount of DNA.
Band position helps estimate fragment length; band intensity can relate to DNA amount, subject to staining and imaging conditions.
Look for these in your work
- Application explained
- Claim appropriately bounded
Common mistakes
- Saying DNA moves toward the negative electrode
- Claiming distance depends only on DNA amount
What can you explain now?
Why does DNA move toward the positive electrode?
Compare with the explanation
Its phosphate backbone is negatively charged
Opposite charges attract in the electric field.
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Make it stick
How do charge and fragment length determine DNA movement in a gel?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.