DNA ligase plays a crucial role in DNA replication by connecting Okazaki fragments, thereby sealing the gaps in the newly synthesized DNA strand.

DNA ligase is an essential enzyme involved in the replication of DNA, the process by which a cell duplicates its genetic material prior to division. During DNA replication, the double helix structure of DNA is unwound by an enzyme called DNA helicase. This unwinding generates two single strands of DNA that act as templates for synthesizing new complementary strands.

The enzyme DNA polymerase is responsible for synthesizing the new DNA by sequentially adding nucleotides to the growing strand. However, DNA polymerase can only add nucleotides in a specific direction, from $5'$ to $3'$. On the leading strand, this synthesis occurs continuously, while on the lagging strand, it is discontinuous, resulting in the formation of short DNA segments known as Okazaki fragments.

This is where DNA ligase becomes vital. Its primary function is to connect these Okazaki fragments, forming a continuous DNA strand. It achieves this by creating a phosphodiester bond between the $3'$ end of one fragment and the $5'$ end of the next. This process, referred to as ligation, effectively seals the gaps within the newly synthesized DNA strand.

In the absence of DNA ligase, the replication process would remain incomplete, leaving the new DNA molecules fragmented. Such fragmentation could lead to errors in the DNA sequence, potentially resulting in mutations and associated diseases. Thus, DNA ligase is essential for preserving the integrity of genetic information during replication.

Beyond its role in DNA replication, DNA ligase is also involved in other critical cellular processes, including DNA repair and recombination. In these contexts, it continues to function by joining together fragments of DNA, thereby ensuring the continuity and stability of the genetic material.