What is the main function of ligases in molecular biology?

Ligases play a crucial role in molecular biology by catalyzing the joining of DNA or RNA fragments together. Their primary function is to form phosphodiester bonds between the nucleotides, effectively sealing nicks in the sugar-phosphate backbone of nucleic acids. This is an essential process during DNA replication and repair, where DNA strands need to be connected after fragments, such as Okazaki fragments during lagging strand synthesis, have been synthesized. The ability to join these fragments accurately is vital for maintaining the integrity of the genetic material and ensuring successful cell division and functionality in various biological processes.

By understanding the role of ligases in this context, it becomes clear why option B is the correct choice, as it accurately describes their main function in the molecular biology toolkit used for DNA manipulation and genetic engineering.