Question

Which of the following statements are not true regarding restriction endonucleases?
A. They are called molecular scissors.
B. These are the enzymes responsible for restricting the growth of bacteriophages in {E. coli}.
C. They cut the DNA only at the centre of the palindromic sites.
D. They remove nucleotides only from the ends of DNA fragments.
E. They recognise specific palindromic base-pair sequences. Choose the answer from the options given below:

• A. A and E only
• B. D and E only
• C. A and B only
• D. C and D only

Hint:

\textbf{Exo} means \textbf{Exit/End} (removes from ends). \textbf{Endo} means \textbf{Inside} (cuts within the strand).

Answer 1

The Correct Option is D

Step 1 : Understanding the Question
The objective is to identify the false statements concerning the characteristics and functions of restriction endonucleases, which are key tools in gene cloning.
Step 2 : Key Formulas and approach
The approach is to evaluate the biochemical definition of "endonuclease." We must determine if they cut "inside" or at the "ends" and their specific logic for recognizing and cleaving palindromic DNA sequences.
Step 3 : Detailed Explanation

Statement A: They are indeed known as (A) "molecular scissors" because they cleave DNA strands. This is true.

Statement B: Their natural role in bacteria is to (B) restrict the growth of viruses by cutting viral DNA. This is true.

Statement C: They do not always cut at the (C) exact centre. Many enzymes cut slightly away from the centre to create "sticky ends." This statement is false.

Statement D: This describes (D) exonucleases, which remove nucleotides from the ends. {Endonucleases} cut {within} the strand. This statement is false.

Statement E: They recognize specific (E) palindromic sequences (sequences that read same 5' to 3' on both strands). This is true.

Step 4 : Final Answer
Statements C and D are incorrect. Therefore, the correct option is (D).