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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

    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.

  3. 3

    Add one concrete 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.

  4. 4

    Avoid this incomplete answer

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

Definition

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

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.

Memory hook

Negative DNA runs to positive; small pieces run farther.

Examples and method

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 to apply

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.

Diagram support

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.

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.

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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.

Answer writing and exam use

1-mark answer

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

2-mark answer

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

3-mark answer

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