16. (a) What is a palindromic sequence in DNA?
(b) How do restriction endonucleases act on palindromic sites to produce ‘sticky ends’?
(c) What is the significance of such ‘sticky ends’ in recombinant DNA technology?
a) Generally palindrome means the words which read same whether they are read forward or backwards.( Eg: HANNAH). A palindromic sequence (inverted reverse sequence) is a nucleic acid sequence when we read from 5' to 3' on one strand matches the sequence 5' to 3' on the other complementary strand with which it forms a double helix. It can form a hair pin structure.
b) A restriction enzyme will only cut at specific sequences and it recognise palindromic sequences. It cuts the opposite strands of DNA between the same bases on both the strands. A sticky end is generated by a staggered cut. A sticky end has protruding single stranded strands with unpaired nucleotides whiah can anneal together by hydrogen bonding.
After restriction enzyme digestion,
- Sticky ends are more useful in molecular cloning because human DNA can only fit into a bacterial plasmid in one direction. In contrast, if both the human DNA and bacterial plasmid have blunt ends the human DNA can be inserted head-to-tail or tail-to-head into the plasmid.
- Ligating sticky ends requires less DNA because they find each other due to their stickiness.
- Different Enzymes can create the same sticky ends eventhough they recognise different restriction sequences. This increases the likelihood that there will be sticky end restriction sites that flank the human gene of interest.
(a) Palindromic sequence of base pair that reads the same on to strand while orientation of reading kept same (as palindromic word are read same from both side eg- MALAYALAM )
(b) restriction endonuclease cut strand of DNA a little away from centre of palindromic site but between the same bases on two opposite strands. Leaving single stranded portion at end. Overhanging streches called "sticky ends".
(c) sticky ends cause formation of H-bonding with their complementary cut counter part stickiness of end facilitates action of enzyme DNA ligase.