Question

The role of DNA ligase in the construction of a recombinant DNA molecule is

• A. Formation of phosphodiester bond between two
  DNA fragments
• B. All of the above
• C. Ligation of all purine and pyrimidine bases
• D. Formation of hydrogen bonds between sticky ends
  of DNA fragments

Answer 1

The correct option is A Formation of phosphodiester bond between two

DNA fragments
Recombinant DNA
Recombinant DNA is formed by the introduction of a foreign DNA (containing the gene of interest) into the vector DNA. This process involves the use of restriction endonucleases and ligases.

DNA ligase
DNA ligase is the enzyme that creates the phosphodiester bond between the nucleotides of DNA fragments and joins them together. For example, a DNA ligase in rDNA technology is used to join DNA of interest and vector DNA, thus helping in the formation of recombinant DNA molecules.

Hydrogen bonds between DNA fragments
When complementary sticky ends of DNA fragments meet, hydrogen bonds are formed between the complementary bases which results in annealing of the ends of the fragments. The occurrence of this reaction does not need the catalytic influence of any enzyme.

Ligation of purines and pyrimidines
Purines and pyrimidines of one strand of DNA pair up with the complementary bases of the other strand via hydrogen bonds, thus holding the two strands of the DNA helix together. No enzymes are required for this complementary pairing.

Final answer
(A) Formation of phosphodiester bond between two DNA fragments