Solution: Analyzing Restriction Endonuclease Statements

This question asks to identify the statements that are not true about restriction endonucleases.

Statement Analysis

• A. They are called molecular scissors.
  True. Restriction enzymes cut DNA strands at specific recognition sites, acting metaphorically like scissors.
• B. These are the enzymes responsible for restricting the growth of bacteriophages in E. coli.
  True. Bacteria like E. coli use restriction-modification systems, involving these enzymes, to defend against foreign DNA, such as that from bacteriophages.
• C. They cut the DNA only at the centre of the palindromic sites.
  False. While restriction enzymes recognize palindromic sequences, they cut within these sequences. The cut site is not always precisely at the center, and different enzymes have different cutting patterns (e.g., leaving sticky or blunt ends). The word "only" makes this statement inaccurate.
• D. They remove nucleotides only from the ends of DNA fragments.
  False. Restriction endonucleases are endonucleases, meaning they cleave the DNA backbone from *within* the DNA molecule. Enzymes that remove nucleotides from the ends are called *exo*nucleases.
• E. They recognise specific palindromic base-pair sequences.
  True. A key characteristic of restriction enzymes is their ability to recognize and bind to unique, often palindromic, DNA sequences.

Identifying Incorrect Statements

Based on the analysis, statements C and D are not true regarding restriction endonucleases.

Conclusion

The correct option includes only the false statements, which are C and D.