Actually I developed this doubt while solving some questions(they are poorly framed I suspect). According to my notes and my institute modules, 1 RNA primer is required on the leading strand as well to start DNAP activity, and at the end repairing enzyme will remove the primer and add the complementary nucleotides. Now, here's the problem i am facing, will the DNA ligase join the newly added nucleotides by the correction enzyme or it is not required? The questions that created this doubt say in the solutions that DNA ligase is not a requirement in replication on leading strand but i think it should be required to join the nucleotides added by the correction enzyme as is the case on lagging strand. Am i correct?


1 Answer 1


The work of DNA ligase is to form "phospho-ester bond" or "phospho-diester bond"(bond joining phosphate of one nucleotide with OH of another nucleotide).

by forming this bond the DNA ligase seals gap b/w 2 nucleotide sequence.

Now in case of lagging strand okazaki fragments are formed and gap b/w two okazaki fragments are filled by DNA polymerase and then the final joint is done by DNA ligase by forming phospho-ester bond b/w a nucleotide(last nucleotide which was placed by DNA Polymerase)and another nucleotide (which is the first nucleotide in the nucleotide sequence of second okazaki fragment which is being joined together)

now there is no need of all this mechanism in case of leading strand because there is no okazaki fragment present in leading strand. So DNA ligase is NOT required in case of leading strand.

hope this helps!!


as discussed in the comments below, the DNA ligase might be required in joining two leading strands of two replication fork of a replicon

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    $\begingroup$ Thanks bro, actually i talked to my teacher also about this and he admitted that actually DNA ligase IS required on leading strand when the nucleotides are finally added in the place of one Primer that is required at the starting of leading strand. However since its role is so small it is usually neglected and NCERT also has mentioned DNA ligase with only lagging strand so that's why... $\endgroup$ Nov 22, 2021 at 5:22
  • $\begingroup$ @Vaibhav Agarwal yes bro i got your point , DNA ligase might be required in joining the leading strands of two replication fork correct? $\endgroup$ Nov 22, 2021 at 6:34
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    $\begingroup$ Actually the DNA ligase kinda works like this- we know everytime DNA polymerase has to start its activity, RNA primase is added . RNA primase consists of a few ribonucleotides added complementary to the DNA strand which serve as kind of a launch pad for the DNA polymerase . now when the DNA polymerase is done with its work after replicating the entire strand , the DNA polymerase 1 ( mainly, however other enzymes are involved as well) comes and removes the ribonucleotides and adds the complementary deoxyribonucleotides instead. Now the DNA ligase comes and seals the newly added deoxynucleotides $\endgroup$ Nov 22, 2021 at 6:52
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    $\begingroup$ As a primer is added on the leading strand , some DNA ligase activity is required there. However it is generally ignored (as NCERT has done and we will follow for NEET) due to the very small role it plays there. $\endgroup$ Nov 22, 2021 at 6:53
  • $\begingroup$ @Vaubhav ya bro you are correct in explaining that how DNA ligase and DNA polymerase work. $\endgroup$ Nov 22, 2021 at 7:12

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