C
CraftExam
medium difficulty6 min

Gel Electrophoresis

Gel electrophoresis is a technique used to separate DNA fragments according to size by moving them through an agarose gel under an electric field.

In one line: Negative DNA runs to positive; small pieces run farther.

Concept Practice Coming Soon

Understand it

  • DNA fragments are negatively charged because of their phosphate groups, so they move towards the positive electrode during electrophoresis.
  • Agarose gel acts like a molecular sieve: smaller fragments move faster and travel farther than larger fragments.
  • After separation, DNA bands can be visualized using a staining method such as ethidium bromide under suitable illumination.
  • Desired DNA bands may be cut out and the DNA recovered by elution.
  • The important cause-effect idea is that charge causes movement, while gel pore size and fragment length affect distance travelled.
  • Diagram questions often test the direction of movement and band interpretation.

Example

After restriction digestion of a plasmid, gel electrophoresis can separate the resulting DNA fragments so that a desired band can be identified and eluted.

Rule to remember

Separation rule: DNA moves from negative to positive electrode; smaller fragments migrate farther through agarose gel than larger fragments.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If a lane shows one band far from the well and another near the well, infer that the farther band represents a smaller DNA fragment.

Method you can reuse

Prepare agarose gel with wells; load DNA samples into wells; apply electric field; allow fragments to migrate through gel; stain or visualize bands; cut out the required band; recover DNA by elution.

Sketch it in your answer

Label wells, agarose gel, negative electrode near wells, positive electrode at the far end, DNA bands, stain and direction of migration; notice that band distance indicates relative size.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why do DNA fragments move towards the positive electrode in gel electrophoresis?

Think first, then show answer
DNA fragments have negatively charged phosphate groups, so they are attracted to the positive electrode. The gel then separates them because smaller fragments pass through its pores more easily.

Common wrong answer

DNA moves to the negative electrode because it is a biological molecule.

Don't mix this up

Students often think larger DNA fragments move farther; actually, smaller fragments usually travel farther through agarose gel.

Exam tip

Mention both principles: DNA is negatively charged and agarose gel separates fragments mainly by size.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as diagram labelling, assertion-reason on DNA charge, short answer on agarose gel, or interpretation of DNA bands after restriction digestion.

Answer plan

Define gel electrophoresis, state DNA charge and direction, explain agarose size separation, mention staining of bands and elution of the desired DNA fragment.
  1. 1

    Enough for 1 mark

    Gel electrophoresis is a technique used to separate DNA fragments according to size by moving them through an agarose gel under an electric field.

  2. 2

    Add for 2 marks

    Example: After restriction digestion of a plasmid, gel electrophoresis can separate the resulting DNA fragments so that a desired band can be identified and eluted.

  3. 3

    Add for 3 marks

    DNA fragments are negatively charged because of their phosphate groups, so they move towards the positive electrode during electrophoresis.

Practice

Concept practice is coming soon

Login to access concept-level MCQs and weak-concept practice as they become available.

10 MCQs5 MinutesInstant Results
Login

Help improve this page

Found something confusing, incorrect, or missing?

Open Learning Map