Question

Suppose a DNA fragment from a human gene has been cut such that it has sticky ends. A DNA fragment from bacteria with sticky ends complementary to those of the human DNA is added to the mix. What would prevent the complementary ends of the human DNA from rejoining, and thus prevent the formation of a recombinant DNA molecule?

Answer 1

To prevent the complementary sticky ends of human DNA from rejoining and preventing recombinant DNA formation, the absence or inhibition of DNA ligase is key. Low DNA concentration and imperfectly complementary or damaged ends can also inhibit recombination.

Step-by-step explanation:

To prevent the complementary sticky ends of human DNA from rejoining and thus prevent the formation of a recombinant DNA molecule, certain conditions must be altered or inhibiting factors must be present. The enzyme DNA ligase is normally responsible for reattaching the DNA backbones, thus creating a continuous double-stranded DNA molecule.

Therefore, if DNA ligase is not present or its activity is inhibited, the sticky ends may temporarily anneal due to hydrogen bonding but will not form a stable, covalent bond required for the formation of a permanent recombinant DNA molecule. Additionally, if the concentration of the mixed DNA is too low, the likelihood of the complementary sticky ends finding each other and annealing decreases, which could also prevent recombination. Lastly, if the ends are not perfectly complementary or are damaged, they might not be able to anneal properly, thus inhibiting the formation of recombinant DNA.