Q.With the advent of sophisticated techniques of genetic engineering, we can now readily purify and isolate DNA. Name and explain the different steps involved in the separation and isolation of DNA fragments once cut by restriction endonucleases.

Once restriction endonucleases have cut DNA into fragments of different sizes, we need to separate and isolate them for further work. The technique used is gel electrophoresis, which exploits the fact that DNA fragments are negatively charged molecules.

The process unfolds in these steps:

• Preparation of the gel matrix: Agarose, a natural polymer extracted from seaweeds, is melted and poured into a mould to form a gel. Once set, it acts as a sieve with tiny pores.

• Loading the DNA samples: The digested DNA samples are loaded into wells cut into one end of the gel, and the gel is submerged in a conducting buffer solution.

• Application of electric field: An electric current is passed through the gel. Because DNA is negatively charged (due to its phosphate backbone), the fragments migrate toward the positive electrode (anode).

• Separation by size: Smaller DNA fragments move faster and travel farther through the gel pores, while larger fragments move more slowly. This differential migration separates the fragments into distinct bands based on size. …

When restriction endonucleases slice DNA at specific recognition sites, they produce a mixture of fragments of different lengths. The challenge now is to separate these fragments from one another and isolate the one we need for further work—cloning, sequencing, or any downstream application. The technique that makes this possible is gel electrophoresis, a cornerstone method in molecular biology that exploits a simple physical principle: DNA is negatively charged because of its phosphate backbone, so it will migrate toward a positive electrode when placed in an electric field.

The process unfolds in a logical sequence. First, you prepare an agarose gel—a porous matrix made by dissolving agarose powder in a buffer solution and pouring it into a mold with a comb that creates wells at one end. Once the gel solidifies, it resembles a firm jelly with tiny pores running through it. The size of these pores depends on the concentration of agarose: a higher concentration creates smaller pores that slow down even small fragments, while a lower concentration allows larger fragments to move more freely. You choose the concentration based on the size range of fragments you expect.

The DNA samples—your restriction digest mixture—are then loaded into the wells. A loading dye is mixed with the samples beforehand; this dye is denser than the buffer, so it helps the sample sink into the well, and it also contains colored tracking dyes that let you monitor how far the electrophoresis has progressed. The gel is submerged in a buffer solution that conducts electricity, and electrodes are connected: the negative cathode near the wells and the positive anode at the far end.

When you switch on the power supply, an electric field is established across the gel. DNA fragments, being negatively charged, begin migrating toward the positive electrode. Here's where separation happens: smaller fragments slip through the gel's pores easily and travel faster, while larger fragments get tangled and retarded, moving more slowly. Over time—typically 30 minutes to a few hours depending on gel length and voltage—the fragments sort themselves into distinct bands, each band representing fragments of a particular size. The relationship is inverse: the smaller the fragment, the farther it travels.

Once electrophoresis is complete, the DNA is invisible to the naked eye. To visualize the separated bands, the gel is stained with ethidium bromide, a fluorescent dye that intercalates between the stacked bases of DNA. When you place the stained gel on a UV transilluminator and shine ultraviolet light on it, the ethidium bromide fluoresces a bright orange, revealing the DNA bands as glowing stripes against a dark background. You can photograph this under UV light to create a permanent record. …