![]() ![]() Of course, the distribution of nucleotides in DNA is not random, so the actual sizes of DNA fragments produced by EcoRI range from hundreds to many thousands of base pairs, but the mean size is close to 4000 bp. Assuming a random distribution of A, C, G and Ts in DNA, probability predicts that a recognition site for a 6-cutter should occur about once for every 4096 bp (4 6) in DNA. Restriction enzymes like EcoRI are frequently called 6-cutters, because they recognize a 6-nucleotide sequence. ![]() Restriction enzymes cleave specific sites in DNA coli contains multiple REs, but only EcoRI and EcoRV, are widely used in molecular biology. coli), followed by a strain identifier (R for RY13), and ends with a Roman numeral that distinguishes the different REs found in the strain. Its name begins with the genus and species (Eco for E. EcoRI is found naturally in the RY13 strain of Escherichia coli. Although the names of individual REs may sound a bit like baby talk, the nomenclature is actually very systematic and is based on its biological source. To understand how REs work, we will use EcoRI, one of the best-studied REs, as an example. These recombinant REs have often been engineered by molecular biologists to include amino acid changes that increase the catalytic activity or stability of the RE. Instead, REs are usually isolated from bacteria that overexpress large quantities of REs from plasmids. Today, most commercially available REs are not purified from their natural sources. The combined activities of the endonuclease and methyltransferase are referred to as a restriction/ modification system. The RE sites in the bacterium’s own DNA are protected from cleavage because they have been modified by a methyltransferase that specifically modifies the RE sites. ![]() REs are part of a bacterial defense system against foreign DNA, such as an infectious bacteriophage. You might wonder why bacteria harbor these potentially destructive enzymes. To date, thousands of REs with distinct specificities have been described. The recognition site (also known as a restriction site). REs occur naturally in bacteria, where they specifically recognize short stretches of nucleotides in DNA and catalyze double-strand breaks at or near The discovery of restriction enzymes, or restriction endonucleases (REs), was pivotal to the development of molecular cloning. \)īacterial restriction/modification systems protect against invaders ![]()
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