Q.How are DNA fragments separated and isolated?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Gel Electrophoresis
Gel electrophoresis separates charged biomolecules - most commonly DNA - by applying a direct-current electric field, under which the negatively charged DNA backbone migrates through a porous gel matrix toward the positive electrode, with smaller fragments moving faster and travelling further than larger ones over a given time. Agarose gel electrophoresis is used for DNA fragments from a few hundred to about 20,000 base pairs (polyacrylamide is used for smaller fragments), with fragment sizes read off by comparison against a marker ladder of known sizes run in a parallel lane. DNA itself is invisible, so gels a …
After DNA has been cut into fragments by restriction enzymes, those fragments must be sorted by size and then physically recovered before they can be used further in cloning. …
DNA fragments are separated by gel electrophoresis according to size, then the required band is cut out of the gel and the DNA eluted, a step called elution.
After DNA is cut by restriction endonucleases, the resulting fragments are separated and isolated as follows:
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Separation by gel electrophoresis: The fragments are loaded into wells of an agarose gel and an electric field is applied. DNA is negatively charged (due to its phosphate groups), so all fragments move towards the positive electrode (anode).
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Size-based movement: The gel acts as a molecular sieve. Smaller fragments move faster and farther through the pores, while larger fragments move more slowly. Fragments of the same size collect together and form distinct bands.
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Visualisation: The gel is stained with ethidium bromide and viewed under UV light, where the DNA bands appear as bright orange fluorescent bands.
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Showing the 12 most recent of 20 on this concept.
- CBSE 2026Set A1 markMCQQ.During gel electrophoresis, fragments of DNA move towards which electrode?(a) Cathode(b) Anode(c) Towards any electrode depending upon the size of DNA fragment(d) None of these
›Reveal solutionSolution
Negatively charged DNA moves to the anode; the correct option is (b).
DNA carries a net negative charge because of its phosphate groups. In gel electrophoresis, DNA fragments are forced through an agarose gel under an electric field and therefore move towards the positively charged electrode, the anode. Smaller fragments move faster (farther) and lar …
- CBSE 2026Set ANNUAL1 markMCQQ.Gel electrophoresis is used for(a) Isolation of DNA molecule(b) Cutting of DNA into fragments(c) Separation of DNA fragments according to their size(d) Amplification of gene of interest
›Reveal solutionSolution
Gel electrophoresis separates a mixture of DNA fragments by forcing them to migrate through a gel matrix under an electric field, with smaller fragments moving faster.
In gel electrophoresis, DNA fragments (e.g., produced after cutting DNA with restriction enzymes) are loaded into wells of an agarose gel and an electric current is applied. Since DNA is negatively charged (due to its phosphate backbone), the fragments move towards the positive electrode (anode). Smaller fragments move faster and travel further through the gel's pores than larger fragments, resulting in fragments being separated according to their size, visible as distinct bands after staining (e.g., with ethi …
- CBSE 2026Set ANNUAL1 markQ.DNA fragments separated by gel electrophoresis can be visualized only after staining.(a) Name the compound used for staining DNA.(b) Name the light used for visualization. (21+21=1)
›Reveal solutionSolution
DNA fragments separated by gel electrophoresis are stained with ethidium bromide and made visible as glowing bands under UV light.
After DNA fragments have been separated by size through gel electrophoresis (in an agarose gel, under an electric field, with smaller fragments migrating faster/further towards the positive electrode), they are not visible to the naked eye on their own, since DNA is colourless. To visualise them, the gel is stained with a fluorescent dye — most commonly ethidium bromide, which intercalates (inserts itself) between the stacked bases of the DNA. When the stained gel is then exposed to ultraviolet (UV) light, the et …
- CBSE 2026Set ANNUAL1 markMCQQ.In which technique is Ethidium Bromide used ?(a) Polymerase Chain Reaction(b) Southern Blotting Technique(c) Agarose Gel Electrophoresis(d) Western Blotting Technique
›Reveal solutionSolution
Ethidium bromide is the intercalating dye used to stain and visualise DNA bands in agarose gel electrophoresis.
Working
In Agarose Gel Electrophoresis, DNA fragments (negatively charged, due to their phosphate backbone) are loaded into wells cut in an agarose gel, and an electric current is applied so that the fragments migrate through the gel matrix, with smaller fragments moving faster and travelling further than larger ones, achieving size-based separation. Because DNA itself is colourless, the gel is stained with Ethidium bromide, a planar dye molecule that inserts (intercalates) between the stacked base pairs of the double helix. When exposed to UV light, ethidium-bromide-bound DNA fluoresces bright orange, allowing the separated DNA bands to be seen and photographed directly on the gel. Ethidium bromide is not a reagent of the Polymerase Chai …
- CBSE 2025Set X11 markQ.The step in which bands of DNA are cutout and extracted from gel piece is termed as ____________.
›Reveal solutionSolution
Cutting out and extracting the separated DNA band from the gel is called elution.
After agarose gel electrophoresis, DNA fragments separate into bands. The band of the desired size is cut out from the gel and the DNA is extracted (recovered) from the gel piece. This step of extracting the separated D …
- CBSE 2025Set A1 markQ.Match the correct pair and write the match for 'Agarose'. Column I:(i) Apomixis(ii) Darwin(iii) Toddy(iv) Agarose(v) Detritivorous. Column II:(a) Galapagos Islands(b) Southern India(c) Sea weeds(d) Earthworm(e) Grass family.
›Reveal solutionSolution
Agarose matches with (c) Sea weeds, since agarose is a polysaccharide extracted from marine algae.
Agarose is a natural, chain-forming polysaccharide extracted from sea weeds (marine red algae). It is used to prepare agarose gels for gel electrophoresis, a key technique of recombinant DNA technology used to separate DNA fragments by size — smaller fragments move faster through the …
- CBSE 2024Set E1 markMCQQ.Reason for movement of DNA fragments towards anode in electrophoresis is DNA fragments being(a) Negatively charged(b) Positively charged(c) Charged(d) Charged with charges of both types
›Reveal solutionSolution
DNA fragments migrate to the anode because DNA is negatively charged.
In agarose gel electrophoresis, DNA fragments are separated according to size. DNA carries a net negative charge because of the phosphate groups in its sugar–phosphate backbone. When an electric field is applied, the negatively charged fragments move towards the positively charged electrode (anode). Smaller fragments move faster and farther through …
- CBSE 2024Set D1 markMCQQ.In agarose gel electrophoresis, DNA molecules are separated on the basis of their:(a) charge only(b) size only(c) volume only(d) charge and shape
›Reveal solutionSolution
Agarose gel electrophoresis separates DNA molecules purely by their size (molecular weight/length), not by charge or shape.
DNA molecules carry a uniform negative charge along their length because of the phosphate groups in the sugar-phosphate backbone. This means the charge-to-mass ratio of DNA is essentially constant regardless of fragment length — so, unlike protein electrophoresis (where net charge varies), charge cannot be the basis of separation for DNA.
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- CBSE 2024Set ANNUAL1 markMCQQ.Agarose gel is extracted from -(a) Fungus(b) Sea weeds(c) Hydrilla(d) Wheat
›Reveal solutionSolution
Agarose, the polysaccharide used to cast gels for electrophoresis, is a natural product extracted from marine red algae (seaweeds).
Agarose is a polysaccharide extracted from certain sea weeds (marine red algae). It is used to make gels for agarose gel electrophoresis, the standard technique use …
- CBSE 2024Set ANNUAL1 markQ.During gel electrophoresis DNA fragments move towards the anode instead of the cathode. Give reason.
›Reveal solutionSolution
The phosphate groups of the DNA backbone give it an overall negative charge, so DNA fragments move towards the anode during gel electrophoresis.
Gel electrophoresis separates DNA fragments by size by forcing them to migrate through the pores of an agarose gel matrix under an applied electric field. DNA is negatively charged because of the phosphate groups in its sugar-phosphate backbone (each phosphodiester linkage contributes a negative charge). Under the electric field, opposite charges attract, so the negatively charged DNA fragments move away from the cathode (negative electrode) and towards the anode (positive e …
- CBSE 2024Set ANNUAL1 markMCQQ.Assertion (A): During gel electrophoresis, the DNA fragments move towards the anode. Reason (R): DNA fragments are negatively charged molecules.(a) Both (A) and (R) are true and (R) is the correct explanation of (A)(b) Both (A) and (R) are true but (R) is not the correct explanation of (A)(c) (A) is true but (R) is false(d) Both (A) and (R) are false
›Reveal solutionSolution
DNA is negatively charged (phosphate backbone), so it moves towards the anode during gel electrophoresis — the reason correctly explains the assertion.
Gel electrophoresis is a technique used to separate DNA fragments according to their size, by forcing the DNA (mixed with a loading dye) to move through a gel matrix (usually agarose) under an electric field. Since the phosphate groups in the DNA's sugar-phosphate backbone carry a negative charge, DNA fragments are negatively charged molecules overall. When placed in an electric field, negatively charged molecules migrate towards the positive electrode (the anode); smaller fragments move faster/farther through the gel's …
- CBSE 2024Set ANNUAL1 markMCQQ.In which techniques Ethidium Bromide is used ?(a) Polymerase Chain Reaction(b) Southern Blotting Techniques(c) Agarose Gel Electrophoresis(d) Western Blotting Techniques
›Reveal solutionSolution
Ethidium bromide is the intercalating dye used to stain and visualise DNA bands during Agarose Gel Electrophoresis.
Working
In Agarose Gel Electrophoresis, DNA fragments (which are negatively charged due to their phosphate backbone) are loaded into wells of an agarose gel and separated by size when an electric current is applied, with smaller fragments migrating faster through the gel matrix than larger ones. Because DNA itself is colourless and cannot be seen directly, the gel is stained with Ethidium bromide, a dye that intercalates (inserts itself) between the stacked base pairs of the DNA double helix. When exposed to UV light, ethidium-bromide-bound DNA fluoresces bright orange, allowing the separated bands to be visualised and photographed. Ethidium bromide is no …
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