What do ligases catalyze in biochemical reactions?

Ligases are enzymes that play a critical role in catalyzing specific biochemical reactions, particularly those that involve the joining together of two large molecules. The key characteristic of ligases is that they facilitate addition or synthesis reactions, often requiring energy input in the form of ATP. This is particularly important in cellular processes such as DNA replication and repair, where ligases are responsible for forming new covalent bonds between nucleotides to create a continuous DNA strand.

In the context of biochemical pathways, ligases typically catalyze the formation of bonds or the synthesis of complex molecules, which can be critical for building cellular structures or storing genetic information. The requirement of ATP highlights the energetic costs associated with these synthesis reactions, as the process not only involves the substrates but also needs energy to proceed.

Other options do not accurately describe the primary function of ligases. For instance, decomposition of large molecules would involve enzymes such as hydrolases that break down compounds, while cleavage reactions often involve different enzymes. Electron transfer reactions are generally catalyzed by oxidoreductases rather than ligases. Thus, ligases are distinctly defined by their role in facilitating addition reactions that require energy, making option B the correct choice.