Question

Identify the incorrect statement given below.

• A. Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA
• B. Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strand
• C. The ends cut by restriction enzymes can be joined together (end-to-end) using DNA ligases
• D. Endonucleases remove nucleotides from the ends of the DNA whereas, exonucleases make cuts at specific positions within the DNA

Answer 1

The correct option is D Endonucleases remove nucleotides from the ends of the DNA whereas, exonucleases make cuts at specific positions within the DNA

The nucelases are of two types.

• Endonuclease, which makes cuts at specific sites within the DNA
• Exonuclease, which removes nucleotides from the ends.

Restriction endonucleases are nucleic acid degrading enzymes which cuts double stranded DNA into fragments after recognizing specific nucleotide sequences. The recognition sequence is palindromic which means that it reads the same when read from either side.

For example, recognition sequence for EcoRI restriction endonuclease is
5'- GAATTC- 3'
3'- CTTAAG- 5'
Also, restriction enzymes make the cut in the DNA strand a little away from the palindromic sites. For example,
in case of EcoRI, "|" would denote the cut.
5'- G|AATTC- 3'
3'- CTTAA|G- 5'
Also, DNA ligases join the ends of DNA which are cut by restriction enzymes.