DNA Ligase

Okazaki fragments in eukaryotic cells typically range from 100 to 200 nucleotides in length. In contrast, in prokaryotic cells, such as bacteria, Okazaki fragments are larger, generally around 1,000 to 2,000 nucleotides long. These fragments are short stretches of DNA produced on the lagging strand during DNA replication, as the replication process on this strand occurs in a discontinuous manner, opposite to the direction of the replication fork movement. Types of DNA Ligase The DNA ligase enzyme has many forms in different organisms, which include: E. coli DNA ligase: Utilizes nicotinamide adenine dinucleotide (NAD) to form a phosphodiester bond between DNA strands. However, it doesn’t join strands with blunt ends. Furthermore, the performance of E. coli DNA ligase can be enhanced under in vitro conditions by adding DNA polymerase in the right amount to the reaction mixture. T4 DNA ligase: Isolated from bacteriophages and is one of the most commonly used DNA ligases for research purposes. It can join oligonucleotides, blunt and cohesive ends of DNA, RNA and RNA/DNA hybrids. Unlike E. coli, T4 DNA ligase uses adenosine triphosphate (ATP) as a cofactor rather than NAD. Mammalian DNA ligase DNA ligase I: After ribonuclease H removes the RNA primer from the Okazaki fragments, DNA ligase I joins the nascent DNA.‍ DNA ligase II: Produced after DNA ligase III is degraded by proteases (an enzyme that breaks down long polypeptides into short amino acid fragments). ‍ DNA ligase III: The only ligase that is present in the mitochondria. It facilitates DNA repair by the process of nucleotide excision repair in association with XRCC1 protein.‍ DNA ligase IV: Associates with XRCC4 protein and facilitates the repair of double-stranded DNA breaks through the non-homologous end-joining (NHEJ) pathway.‍ Thermostable DNA ligase: Found in a thermophilic bacterium and can withstand a higher temperature of 95 Celcius, unlike all other DNA ligases.