Why are those restriction enzymes which cut the DNA strands a little away from the centre of the recognition sequence more useful in the construction of recombinant DNA?
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There is two main ways to cut DNA, creating Blunt ends or Sticky ends.
Blunt end example of SmaI recognition site: ("|" is the cutting site)
5'---CCC | GGG---3'
3'---GGG | CCC---5'
Sticky end example of EcoRI recognition site:
5'---G | AATTC---3'
3'---CTTAA | G---5'
In blunt end example, the only way you can attach both pieces of DNA is one after the other, which is harder to do and not specific at all since any bases can go one after the other.
5'---CCC --> <-- GGG---3'
3'---GGG --> <-- CCC---5'
In the sticky example, the sequences will recognize its homologous sequence and attach to it (The TGGG will attach to the ACCC part, making a strong bond). When attaching one strand of DNA on the other, the bases must be homologous (A-T, G-C), this gives specificity since the strand can only be attached with specific sequences.
Here is some good references on restriction enzymes: