Assertion : Plasmids are extrachromosomal $DNA$.
Reason : Plasmids are found in bacteria and are useful in genetic engineering.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: When a particular restriction enzyme cuts a strand of $DNA$,overhanging stretches or sticky ends are formed.
Reason $(R)$: Some restriction enzymes cut the strand of $DNA$ a little away from the centre of the palindromic site.
In the light of the above statements,choose the correct answer from the options given below:

$A$ specific recognition sequence identified by endonucleases to make cuts at specific positions within the $DNA$ is:

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?

In which year were the two enzymes responsible for restricting the growth of bacteriophage in $E. coli$ isolated?

Do eukaryotic cells have restriction endonucleases? Justify your answer.