Question

Which of the following statements regarding restriction enzymes is NOT true?

A) Most restriction enzymes recognize sequences of 20-25 bp in length.
B) Bacteria can add methyl groups to their DNA to prevent digestion by endogenous restriction enzymes.
C) Most restriction enzymes recognize palindromic sequences in host DNA.
D) Restriction enzymes may produce either sticky or blunt ends when digesting host DNA.

Answer 1

The correct option is option A

Step-by-step explanation:

Restriction enzymes are endocleases that cleave DNA fragment (of usually four, five or six nucleotide long) at specific sites to produce blunt or sticky ends. They recognize palindromic sequences of host DNA when cleaving the specific sites. The sequences below (on complementary strands) give an example of a palindromic sequences.

5'-CCC║GGG-3'

3'-GGG║CCC-5'

As can be seen above, when read from 5' to 3', the two sequences are the same despite being on opposing strands. And when cut between the guanine (G) and cytosine (C) (as shown above), it produces a blunt end. But when cut as shown below produces a sticky end.

5'- G║AATTC -3'

3'- CTTAA║G -5'

The explanation above shows options C and D are right while option A is wrong (hence the correct option).

Also, bacteria prevent their own DNA from been digested by restriction enzymes by adding methyl group to their restriction sites which prevents restriction enzymes from recognizing restriction sites of their DNA; this generally makes bacterial DNA to be highly methylated. This explanation makes option B right also.