How does DNA ligase seal a nick in the sugar-phosphate backbone?

DNA ligase plays a crucial role in sealing nicks in the sugar-phosphate backbone of DNA by forming a phosphodiester bond between adjacent nucleotides. When DNA is synthesized or repaired, there can be interruptions or nicks in the phosphodiester backbone, which consists of a sugar (deoxyribose) linked to a phosphate group by a bond.

DNA ligase facilitates the joining of these nicks by catalyzing the reaction that links the 3' hydroxyl (OH) group of one nucleotide's sugar with the 5' phosphate group of the next nucleotide. This process effectively restores the continuity of the backbone by creating a covalent bond, thus stabilizing the DNA molecule and ensuring the integrity of the genetic information.

The other approaches mentioned in the question's options do not contribute to this specific action of sealing the DNA fragment. For instance, forming hydrogen bonds primarily occurs between complementary base pairs during DNA annealing rather than linking the sugar-phosphate backbone. Adding a methyl group pertains to DNA methylation, which plays a role in gene regulation and DNA protection rather than repairing nicks. Lastly, producing a new RNA primer is relevant during the DNA replication process, particularly for initiating synthesis but does not directly involve the sealing