Question

Restriction enzymes that are used in the construction of recombinant DNA are endonucleases which cut the DNA at ‘specific-recognition sequence’. What would be the disadvantage if they do not cut the DNA at specific-recognition sequence?

Answer 1

Restriction endonucleases:

Restriction endonucleases are the enzymes which cleave the DNA by digesting phosphodiester bonds at the specific sequences within the DNA. Sticky ends of DNA are formed when restriction enzymes cut one strand of the DNA double helix at one end of the palindromic sequence (restriction site) and cut the other strand from the other end of the palindromic sequence.

Significance of specific recognition sites in restriction endonucleases:

Recombinant DNA can be formed by cutting both the foreign DNA and the vector DNA using the same restriction endonuclease to create complementary sticky ends in both the DNA fragments. Once the sticky ends anneal, DNA ligase seals the gaps between the DNA fragments by forming phosphodiester bonds between them resulting in the formation of the recombinant DNA molecule.

Disadvantage of restriction endonuclease if it loses its specificity:

$\bullet$ If the restriction enzymes cut DNA at random sites instead of at specific sites, then the DNA fragments obtained will not have ‘sticky ends’.

$\bullet$ In the absence of sticky ends, construction of recombinant DNA molecules would not be possible.