Biology · Biotechnology: Principles and Processes · NEET
The enzyme cuts a little away from the centre of the palindrome, so each cut piece is left with a short single-stranded stretch that hangs out (an overhang). Because the two overhangs on the two cut pieces are complementary, they can pair up again by hydrogen bonds, that is, they can stick to each other. This is why they are called sticky ends or cohesive ends. Blunt ends have no overhang, so nothing hangs out to stick.
BLUNT ends form when the enzyme cuts exactly at the centre of the palindrome, so both strands are cut at the same point and the ends are flush. STICKY ends form when the enzyme cuts a little away from the centre (but between the same two bases on the opposite strands), leaving overhangs. Students often flip this in the exam, so learn it as: centre cut = blunt, off-centre cut = sticky.
EcoRI makes STICKY ends. It recognises the palindrome 5'-GAATTC-3' and cuts between G and A on both strands, which is off-centre, leaving a single-stranded AATT overhang. EcoRV, in contrast, cuts its site right at the centre and gives blunt ends. Do not mix up EcoRI (sticky) with EcoRV (blunt).
Yes. DNA ligase is needed for BOTH sticky and blunt ends, because ligase seals the sugar-phosphate backbone. The difference is only in how easy joining is: sticky ends first base-pair with each other (they hold themselves in place), so ligation is easier and more efficient. Blunt ends have nothing holding them together, so blunt-end ligation works but is slower and less specific.
If you cut the foreign gene and the plasmid vector with the same restriction enzyme, both get the same sticky ends. Same overhangs are complementary, so the gene fits neatly into the vector and ligase joins them. If you used different enzymes with different overhangs, the ends would not match and the recombinant molecule would not form. This is a favourite one-mark idea in NEET.
Which of the following restriction enzymes produces blunt ends?
Statement I: Restriction endonucleases recognise specific sequences to cut DNA known as palindromic nucleotide sequences. Statement II: Restriction endonucleases cut the DNA strand a little away from the centre of the palindromic site. Choose the most appropriate answer.
A foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For most cloning, sticky ends are more convenient because their complementary overhangs pair up and hold the two pieces together, so ligation is easier and joins the right pieces. Blunt ends can join any blunt end to any other blunt end, which is less specific, but they are useful when the sequences do not share a common enzyme site.
Yes. Both sticky and blunt ends need DNA ligase to seal the sugar-phosphate backbone. Sticky ends just pair first, making the job easier; blunt ends have no pairing help, so ligation is slower.
They are the same thing. Sticky ends are also called cohesive ends because the single-stranded overhangs are complementary and cohere (stick) to matching overhangs.
EcoRI, Sal I, Xho I and Hind III give sticky ends (off-centre cut). EcoRV and Hind II (in the classic example) give blunt ends (centre cut).
Because the recognition site is a palindrome, the enzyme cuts between the same two bases on both strands. The cut is off-centre, so each of the two resulting pieces carries a matching single-stranded overhang, and the two overhangs are complementary to each other.
