Kalvi.NEET

Gel Electrophoresis

ஜெல் மின்புல நகர்ச்சி

NCERT Class 12 · Biotechnology Principles and Processes · 9 NEET questions

Stop assuming DNA moves randomly. DNA is negatively charged due to its phosphate backbone, and it moves toward the anode. If you forget this polarity, you fail the entire Biotechnology unit.

The idea

Gel electrophoresis separates DNA fragments by size using an agarose gel matrix. An electric field forces DNA to migrate through the pores of the gel. Smaller fragments move faster and travel further than larger ones. The separated fragments are visualized by staining with ethidium bromide and exposure to UV radiation.

Why it matters

Since DNA is negatively charged, this method allows for the isolation and purification of specific DNA fragments required for cloning.

NEET lens

NEET focuses on the 'why' and 'how': DNA moves to the anode, smaller fragments move faster, and ethidium bromide is the staining agent. Expect questions on the direction of migration and the visualization process. Do not confuse agarose gel with polyacrylamide; NCERT specifies agarose for DNA.

Clinical significance

This technique is the backbone of DNA fingerprinting used in forensic medicine and paternity testing. It is also critical for diagnosing genetic disorders like Sickle Cell Anemia by analyzing the size of mutated hemoglobin gene fragments. Without precise separation, these diagnostic results are invalid.

Introduction

Gel electrophoresis separates DNA fragments by size. Since DNA is negatively charged, it moves towards the anode (positive electrode) through a gel matrix under an electric field.

Core concept

Method (NCERT): the most commonly used matrix is agarose, a natural polymer extracted from sea weeds. DNA fragments separate (resolve) according to their size through the sieving effect provided by the agarose gel — the smaller the fragment, the farther it moves. Separated DNA fragments can be visualised only after staining the DNA with ethidium bromide followed by exposure to UV radiation: you see bright orange coloured bands of DNA in a gel exposed to UV light. The separated bands are cut out from the agarose gel and extracted from the gel piece — this step is called elution. The purified DNA fragments are used in constructing recombinant DNA by joining them with cloning vectors.

Go deeper

Because all DNA has a uniform negative charge per unit length, gel electrophoresis separates fragments by size rather than by charge.

From Class 10

Gel electrophoresis is like a race through a sieve — small DNA pieces run faster and farther than big ones.

Previous year NEET questions (9)

  1. NEET 2026

    Which of the following statements are correct with respect to DNA separation, isolation and visualization? I. The cutting of DNA is done by molecular scissors. II. The DNA fragments separate according to their size in an agarose gel, upon electrophoresis. III. The separated DNA fragments can be seen without staining when exposed to UV light. IV. The separated DNA fragments, when stained with ethidium bromide, can be seen in visible light. Choose the correct answer from the options given below:

    1. I and IV only
    2. II and IV only
    3. II and III only
    4. I and II only
    Show answer

    Answer: (D) I and II only

  2. NEET 2023

    Upon exposure to UV radiation, DNA stained with ethidium bromide will show

    1. Bright orange colour
    2. Bright red colour
    3. Bright blue colour
    4. Bright yellow colour
    Show answer

    Answer: (A) Bright orange colour

  3. NEET 2023

    In gel electrophoresis, DNA fragments are visualized by staining with:

    1. Ethidium bromide
    2. Acetocarmine
    3. Safranin
    4. Methylene blue
    Show answer

    Answer: (A) Ethidium bromide

  4. NEET 2022

    Which one of the following statement is not true regarding gel electrophoresis technique?

    1. The presence of chromogenic substrate gives blue coloured DNA bands on the gel.
    2. Bright orange coloured bands of DNA can be observed in the gel when exposed to UV light.
    3. The process of extraction of separated DNA strands from gel is called elution.
    4. The separated DNA fragments are stained by using ethidium bromide.
    Show answer

    Answer: (A) The presence of chromogenic substrate gives blue coloured DNA bands on the gel.

  5. NEET 2021

    DNA strands on a gel stained with ethidium bromide when viewed under UV radiation, appear as:

    1. Bright blue bands
    2. Yellow bands
    3. Bright orange bands
    4. Dark red bands
    Show answer

    Answer: (C) Bright orange bands

  6. NEET 2020

    In gel electrophoresis, separated DNA fragments can be visualized with the help of:

    1. Ethidium bromide in UV radiation
    2. Acetocarmine in UV radiation
    3. Ethidium bromide in infrared radiation
    4. Acetocarmine in bright blue light
    Show answer

    Answer: (A) Ethidium bromide in UV radiation

  7. NEET 2017

    The DNA fragments separated on an agarose gel can be visualised after staining with

    1. bromophenol blue
    2. acetocarmine
    3. aniline blue
    4. ethidium bromide
    Show answer

    Answer: (D) ethidium bromide

  8. NEET 2017

    What is the criterion for DNA fragments movement on agarose gel during gel electrophoresis?

    1. The larger the fragment size, the farther it moves.
    2. The smaller the fragment size, the farther it moves.
    3. Positively charged fragments move to farther end.
    4. Negatively charged fragments do not move.
    Show answer

    Answer: (B) The smaller the fragment size, the farther it moves.

  9. NEET 2017

    DNA fragments are

    1. positively charged
    2. negatively charged
    3. neutral
    4. either positively or negatively charged depending on their size
    Show answer

    Answer: (B) negatively charged

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