What is DNA Ligase?
Many enzymes play an important role in enhancing the activity of the DNA during various processes like replication and in Recombinant DNA Technology. DNA Ligase is one among them which play an important role in combing the two strands or molecules.
Features of DNA Ligase
DNA ligases are considered as one of the basic class of enzymes which is needed for all entities attain the structural integrity of the genome.
DNA ligase plays an important role in connecting the two strands where the phosphate group of one strand and the deoxyribose group of another strand is joined together.
This enzyme is activated only on combining with okazagi fragments which takes shape during DNA replication on the lagging strand.
The joining of two fragments between phosphodiester bond with the help of an energy molecule.
In some cases, like cellular phenomenon of recombination, in DNA replication processes and in repair mechanisms a break occurs in the phosphate backbone of the DNA.
It results in loss of stability of the genome and ends in loss of genetic fragments of the Genome and at times it may also lead to chromosomal mutations.
In such cases, the DNA ligases induces its catalytic activity that helps in repairing the breaks in the DNA by forming the phosphodiester bonds between adjacent nucleotides in the EDNA duplex.
This activity of DNA ligase helps in necessitating the co factor of nucleotide.
This activity is being performed in 3 steps which involves the mechanism of covalent bond modification.
This process involves the modification of covalent bond of the DNA substrate and the ligase enzyme.
DNA ligase enzymes are used in two ways such that one for the introduction of specific gene into the plasmid vector and on the other hand it can be used for fusing the gene by uniting all the genes together.
This phenomenon is generally referred to as ligation. Ligation can be carried throughout the length of the DNA which has its sticky or blunt ends and follows the process of restriction digestion.
The DNA fragments which are located at the blunt ends are joined the enzyme DNA ligase.
Types of DNA Ligase
There are four types of DNA ligases found in eukaryotes. They are as follows;
I. DNA Ligase I: I helps in ligating the nascent DNA on the lagging strand specially to fill the gap between the fragments of okazagi.
II. DNA Ligase II: These are not considered as true ligases because these enzymes do not have their own genes. The eukaryotic DNA ligases II is formed by coding DNA ligase III. It participates primarily in repairing the DNA pathway.
III. DNA Ligase III: This enzyme plays an important role in repairing the DNA by particularly acting on the excision repair of nucleotide. This ligase can also be found at the DNA of mitochondria.
IV. DNA Ligase IV: This DNA ligase plays an important role in joining the double strands of the DNA. And also takes part in the repairing process of breakage in double stranded DNA. It also activates especially during joining the ends of non- homologous pairs.
Role of DNA Ligase
DNA ligases play an important role in maintaining the integrity of the genomes effectively.
It performs the action by joining the breaks present in the phosphodiester bond of DNA. This breaks usually occurs during the process of recombination and the replication.
In most cases it also leads to joining of bonds during the damage of DNA and also in the repairing processes.
DNA ligase plays a primitive role in ligating the two strands of DNA either a single or a double stranded.
DNA ligases are also used in various purposes like in vitro and in vivo processes.
Role of DNA Ligase in DNA Replication
During the process of replication four different daughter strands of single stranded DNA are produced in a single duplex of DNA.
To perform a replication in a complete manner, various enzymes are required to perform variety of specific functions.
The replication of DNA is initiated by introducing an RNA primer through the enzyme primase. 3’ end of the primase is used to add nucleotides in the initial point, this performance is being carried out by DNA polymerase in the leading strand.
This process ends up in the lagging strand by synthesizing the okazaki fragments. At the time of completion of process the extraction of primase is carried out by loading the gaps of the okazagi fragments with nucleotides with the help of DNA polymerase.
Further the strands produced will be joined. Now, DNA ligase plays an important role in filling the gaps. This is carried out by producing the phosphodiester bonds in the gaps present in-between when the okazagi fragments are removed.
By ligation the ends of the strand 5’ and 3’ are joined together by eliminating the pyrophosphate from the triphosphate.
On the other hand, DNA replication use ligation, However DNA LIGATION is not applicable foe strands having blunt ends or if it is with double strand.
Role of DNA Ligase in Recombinant DNA Technology
Mostly the DNA ligases named I, II, and IV are used in most of the cloning experiments. But the DNA ligase from eukaryotic cells is not used.
Where as the ligase named T4DNA are used most of the times to perform the activity of ligases. During the process of restriction digestion two types of ends are produced, namely blunt or sticky ends where as in other biological techniques
DNA Ligase Citations
- DNA ligase: getting a grip to seal the deal. Curr Biol . 2005 Feb 8;15(3):R90-2.
- DNA ligase III: a spotty presence in eukaryotes, but an essential function where tested. Cell Cycle . 2011 Nov 1;10(21):3636-44.
- DNA ligase I, the replicative DNA ligase. Subcell Biochem . 2012;62:327-41.
- Altered DNA ligase activity in human disease. Mutagenesis . 2020 Feb 13;35(1):51-60.