Which type of enzyme is responsible for joining DNA fragments together?

DNA ligase is the enzyme responsible for joining DNA fragments together. This enzyme plays a crucial role in various biological processes, including DNA replication, repair, and recombination. During DNA replication, for instance, the leading strand is synthesized continuously, while the lagging strand is formed in short segments known as Okazaki fragments. DNA ligase is essential in sealing the gaps between these fragments, ensuring that the DNA strand is continuous and intact.

In addition to its role in replication, DNA ligase is also vital in repairing breaks in the DNA strand, thus maintaining the integrity of the genetic material. This function is particularly important in cellular processes that involve DNA manipulation, such as genetic engineering and cloning.

The other options represent different types of enzymes with distinct functions. For instance, restriction enzymes cut DNA at specific sequences, DNA polymerase synthesizes new DNA strands by adding nucleotides complementary to the template strand, and helicase unwinds the DNA double helix to allow for replication or transcription to occur. While these enzymes are critical for various tasks within the cell, they do not fulfill the specific role of joining DNA fragments, which highlights the unique function of DNA ligase.