Recombinant DNA techniques often involve the cutting and sticking together of DNA strands. E.g. a useful gene may need to be cut out of the chromosome on which it has been found, the sealed into a plasmid vector.
Enzymes known as restriction enzymes-restriction endonucleases-are used to cut through DNA at specific points. These enzymes were first extracted from bacterial cells, where they perform a natural defence function against inection by viruses. There are now more than 50 different commonly used restriction enzymes.
A particular restiction enzyme will cut DNA wherever a specific base sequence occurs and only where that sequence occurs. This sequence is called the restriction site, and is usually less than 1- base pairs long. In most of the restiction enzymes in use, the enzyme catalyses a hydrolysis reaction which breaks the phosphate-sugar backbones of exposed bases known as a sticky end.
When separate fragments of DNA need to be stuck together, an ezyme known as DNA ligase is used to catalyse a condensation reaction which koins the phosphate-sugar backbones of the DNA double helix together. This enzyme is the same as that used in natural DNA replication to seal DNA nucleotides together to form new DNA strands
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