Biotechnology
...endonucleases scanning and binding to double-stranded DNA at specific base-pair sequences, the recognition sites, in a predictable manner. The restriction sites are usually 4 to 8 base pairs long and are characterized by the palindromic sequences, with both strands having the same sequence when read in opposite direction. After the restriction endonuclease binds, it starts to disrupt, using hydrolysis, the phosphodiester bonds between neighbor nucleotides, causing the H-bonds between base pairs in the cutting region to be broken. This cuts the original double-stranded DNA strand, producing two DNA fragments, which may differ for different restriction endonucleases, depending on where the phosphodiester bond is broken when cut by the endonuclease. This process can produce either blunt ends (where ends of the DNA fragment are fully paired with no overhangs), or sticky ends (where both DNA fragments have nucleotides lacking complementary bases and overhangs are produced). However, sticky ends are more useful for genetic engineering. The next step, gel electrophoresis, separates the gene that has been excised, from the unwanted fragments taking advantage of chemical and physical properties of DNA. The DNA fragments travel through gel as a result of charge passed through it causing the longer fragments to separate from shorter ones, which helps in identifying gene and makes it easier to cut it out from the gel. The DNA fragment with the desired gene is, therefore, excised and purified. The same restriction endonuclease, that is used to cut the original DNA strand, then splices this gene into a plasmid (small, circular DNA molecules found in bacteria). Because the plasmid and the foreign gene are cut by the same restriction endonuclease, the sticky ends formed, are complementary and anneal to each other forming H-bonds. The DNA ligase reforms the phosphodiester...
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