why does dna move toward a positive charge

The samples can contain DNA fragments of known or unknown length. Standards (or DNA ladders) are run on the gel in order to get a better estimate of the lengths of the DNA fragments in the samples. These standards can be prepared in the lab ahead of time or purchased pre-made. The loading dye present in both the samples and standards helps make sure each well loads properly and makes it easy to keep track of which wells already contain DNA. Once the wells are loaded, the power is turned on. The current creates the electrical field across the gel needed to force the DNA towards the positive end of the circuit. At the beginning of the run, DNA of all lengths are relatively close together. As time goes on the diference in the rate of migration of fragments of different length causes them to separate. Longer fragments take more time to move through the pores in the gel so they move more slowly.


Each individual strand of DNA in a sample is too small to be seen. Gel electrophoresis works because the samples and standards contain billions of copies of the DNA fragments being analyzed. The movement of all of these billions of fragments of the same lengths moving together forms the visible bands. DNA at the same vertical position in two different lanes are fragments of the same length. By comparing the position of each band to bands in the standard or ladder the lengths of bands in the samples can be estimated. position of a band may vary from run to run even though the relationship between bands doesnвt change: shorter fragments move faster and end up lower in the gel when compared to longer fragments. Even with billions of copies of a DNA fragment at a position in the gel, the DNA is not visible until it is stained or marked in some other way.


Visualizing the DNA is done after the power is turned off using one of a number of different DNA stains available for this purpose. Test your understanding of these concepts with the Video Overview Related Content
Gel electrophoresis is a laboratory method used to separate mixtures of DNA, RNA, or proteins according to molecular size. In gel electrophoresis, the molecules to be separated are pushed by an electrical field through a gel that contains small pores. The molecules travel through the pores in the gel at a speed that is inversely related to their lengths. This means that a small DNA molecule will travel a greater distance through the gel than will a larger DNA molecule. As previously mentioned, gel electrophoresis involves an electrical field; in particular, this field is applied such that one end of the gel has a positive charge and the other end has a negative charge.


Because DNA and RNA are negatively charged molecules, they will be pulled toward the positively charged end of the gel. Proteins, however, are not negatively charged; thus, when researchers want to separate proteins using gel electrophoresis, they must first mix the proteins with a detergent called sodium dodecyl sulfate. This treatment makes the proteins unfold into a linear shape and coats them with a negative charge, which allows them to migrate toward the positive end of the gel and be separated. Finally, after the DNA, RNA, or protein molecules have been separated using gel electrophoresis, bands representing molecules of different sizes can be detected.

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