Given below are two statements :

Statement I:

Restriction endonucleases recognize a specific sequence to cut DNA known as the palindromic nucleotide sequence.

Statement II:

Restriction endonucleases cut the DNA strand a little away from the center of the palindromic site.

In the light of the above statements, choose the most appropriate answer from the options given below:

The correct answer is option 2.

Solution

Concept:

• Restriction enzymes act as molecular scissors.
• There are two kinds of restriction enzymes: endonucleases and exonucleases.
• Exonuclease helps in removing nucleotides from ends of DNA while endonuclease makes cuts at the specific sites in the DNA.

Explanation:

Let us understand the working of restriction endonuclease:

• Restriction endonuclease inspects the entire length of the DNA. 
• Upon finding a specific nucleotide sequence it will bind to it on the DNA and will slice the sequence from the DNA strand.
• It recognizes palindrome nucleotide sequences (the sequence of base pairs reads the same on the two strands of DNA when the reading orientation is kept the same).   
• They cut the DNA strand a little away from the centre of the palindrome nucleotide sequence but this occurs between the two same pairs on the opposite strands of DNA double helix.
• This leaves single-stranded portions at the ends. There are overhanging stretches called sticky ends on each strand.
• Sticky ends form hydrogen bonds with their complementary cut counterparts.

• This stickiness of the ends facilitates the action of the enzyme DNA ligase.

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Statement I: Restriction endonucleases recognise specific sequence to cut DNA known as palindromic nucleotide sequence; this is true. As restriction endonuclease recognizes palindrome sequence on the DNA.

Statement II: Restriction endonucleases cut the DNA strand a little away from the centre of the palindromic site; this is true. As restriction endonuclease cut the DNA strand a little away from the centre of the palindrome nucleotide sequence that occurs between the two same pairs on the DNA double helix.

So, the correct answer is option 2.