How does gel electrophoresis work?

gel electrophoresis is based on the idea that DNA fragments are all negatively charged (due to the negative phosphate groups). this property causes different fragments of DNA to be separated on a gel traversed by a current, from the negative to the positive pole, depending only on the size and conformation, and some other factors such as gel properties. With respect to Restriction enzymes, if you want to check for the Molecular weight of a fragment, you digest this fragment using restriction enzymes, then clone it (amplify) and then perform electrophoresis. Depending on the position of the fragment (thickest usually), you can know the size (mol. weight ) of it, by comparing it to a molecular weight marker. note that the opposite is true, i mean if you know the mol. weight of the fragment, you can deduce its relative abundance (thickness).. hope this was helpful

Gel electrophoresis is a technique that is used to separate macromolecules on a gel. It consists of a flat, semisolid gel called a slab gel, with depressions at the top called wells where samples are added. An electric field is applied to the gel, causing charged molecules to migrate from the top of the gel toward the bottom—a process called electrophoresis. DNA is negatively charged and moves toward the positive end of the gel, which is at the bottom in this figure. Smaller DNA fragments move more quickly through the gel polymer and therefore are located closer to the bottom of the gel compared with larger ones. At the end of the electrophoretic run, fragments that differ in size have been separated from one another. The DNA fragments in each band can then be stained with a dye for identification.



Separation of molecules by gel electrophoresis. In this example, samples containing many fragments of DNA are loaded into wells at the top of the gel and then subjected to an electric field that causes the fragments to move toward the bottom of the gel. This separates the fragments according to their masses, with the smaller DNA fragments near the bottom of the gel.

DNA fragments must be cut and pasted to produce recombinant vectors. To cut DNA, researchers use enzymes known as restriction enzymes or restriction endonucleases. Type II restriction enzymes, most commonly used for DNA analysis and genetic engineering, each have a unique nucleotide sequence at which it cuts a DNA molecule. A particular restriction enzyme will cleave DNA at that recognition sequence and nowhere else. The recognition sequence is often a six base pair palindromic sequence (the top DNA strand from 5' to 3' is the same as the bottom DNA strand from 5' to 3'), but others recognize four or even eight base pair sequences.

Restriction enzymes can also differ in the way they cut the DNA molecule. Some enzymes cut in the middle of the recognition sequence, resulting in a flush or blunt end. Other enzymes cleave in a staggered fashion, resulting in DNA products that have short single-stranded overhangs (usually two or four nucleotides) at each end. These are often called cohesive ends, as these single-stranded overhangs could potentially come together again through complementary base-pairing.

I'll just answer the restriction enzyme part of the question.  Restriction enzymes are found in bacteria.  They cut DNA at specific sequences and they look for palindromic sequences (same sequence on other strand when you read it in the opposite direction).  There are over 200 different restriction enzymes known which cut at different sequences.  When DNA is exposed to them, they get cut and the result is DNA of different lengths.  The next step is to put the DNA in a gel and apply current (see posts above) and the fragments will be separated by size and charge.  DNA is negative and gets pulled to positive end of gel.  Smaller pieces travel farther than  longer pieces (easier to get through pores)
Hope that helps

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For extra information and animation visit:www.dnalc.org/resources/animations/gelelectrophoresis.html