Question

Which of the following restriction enzymes/endonuclease is most widely used genetic in engineering?
A) RE type Ⅰ
B) RE type Ⅱ
C) RE type Ⅲ
D) Both A&C;

Answer 1

The restriction enzymes belong to a larger class of enzymes called nucleases.These are of two kinds: exonucleases and endonucleases.Exonucleases enzymes remove the nucleotides from the ends of DNA whereas endonucleases make cuts at specific positions within the DNA.The restriction endonucleases are those enzymes which are used as molecular scissors or scalpels.

Complete answer:

There are three types of restriction endonucleases such as Type I, Type II and Type III differing in their mode of action.Out of these three, Type II is used in gene cloning.
Type I: The Type I restriction endonuclease recognises a specific sequence of DNA molecules but cleavage does not take place at the site of recognition, instead it takes place elsewhere.It is clear that the enzyme shows specificity for recognition site but not for cleavage site.
Type II : This restriction endonuclease makes cuts only within restriction sites and produces two single strand breaks, one break in each strand.
These enzymes recognise the specific nucleotide sequences and these are GC rich.They recognise short sequences ranging from 4 to 8 base pairs of double stranded DNA as targets of cleavage.
Recognition sites are the palindromes or palindromic sequences where most Type II restriction enzymes bind and cut DNA molecules.
These palindromic sequences show similar nucleotide base pair sequences when read in 5' 3 direction in forward and backward manner.
Type III : This restriction enzyme has two sub-units.one for site recognition and modification, and another site for cleavage.
It also requires ATP as source of energy and Mg as cofactor, ATPase activity is lacking in these enzymes.Moreover, these enzymes cleave DNA at specific non-palindromic sequences.
For cleaving, it is essential to have two sites in opposite directions in double stranded DNA.The cleavage products of these enzymes are homologous populations of DNA fragments which are not suitable for gene cloning.

Hence the correct answer is option ‘D’.

Note: Three basic steps in the genetical alternation of an organism:-
(i) Identification of DNA with desirable genes.
(ii) Introducing the identified DNA into the host.
(iii) Maintaining the introduced DNA in the host and transfer of the DNA to its progeny.