Question

Which of the following statements is not correct for Restriction enzymes?

• A. Exonucleases remove nucleotides from ends of the DNA.
• B. Endonucleases make cuts at specific positions within the DNA.
• C. Ligases join sticky ends of DNA together.
• D. The first restriction endonuclease was Hind II.

Answer 1

The Correct Option is D

Restriction enzymes are proteins used in molecular biology to cut DNA at specific sequences. There are several types of restriction enzymes, primarily classified into exonucleases and endonucleases. Let's evaluate each statement to identify inaccuracies:  

• Exonucleases remove nucleotides from ends of the DNA. Exonucleases function by cleaving nucleotides one at a time from the end of a DNA molecule, confirming this statement as correct.
• Endonucleases make cuts at specific positions within the DNA. Endonucleases create cuts at defined sites within DNA, which is essential for various genetic engineering processes. This statement is also correct.
• Ligases join sticky ends of DNA together. DNA ligases are crucial enzymes that repair, replicate, and recombine DNA by joining the sugar-phosphate backbone of DNA strands. They effectively join sticky or blunt ends, accurately described here.
• The first restriction endonuclease was Hind II. The first identified and characterized restriction endonuclease was EcoRI. Hind II was among the first restriction endonucleases discovered, but not the very first. Therefore, this statement is incorrect.

Therefore, the incorrect statement is: "The first restriction endonuclease was Hind II."

Answer 2

Restriction endonucleases are enzymes that cut DNA at specific sequences. They are also known as restriction enzymes.

• Statement (1): Exonucleases remove nucleotides from the ends of the DNA. This is true. Exonucleases degrade DNA by removing nucleotides from the ends of the strand.
• Statement (2): Endonucleases make cuts at specific positions within the DNA. This is true. Endonucleases, including restriction enzymes, cut DNA at specific internal sites within the sequence.
• Statement (3): Ligases join sticky ends of DNA together. This is true. DNA ligase is an enzyme that joins DNA fragments, including those with sticky ends, by forming phosphodiester bonds.
• Statement (4): The first restriction endonuclease was not Hind II. The first restriction enzyme discovered was EcoRI, which comes from Escherichia coli. Hind II is one of the many restriction enzymes discovered later.