---
title: "DNA Ligase | Biological Chemistry I"
description: "DNA ligase joins DNA fragments by sealing phosphodiester bonds, a core enzyme in replication and repair in Biological Chemistry I."
canonical: "https://fiveable.me/biological-chemistry-i/key-terms/dna-ligase"
type: "key-term"
subject: "Biological Chemistry I"
unit: "Unit 12"
---

# DNA Ligase | Biological Chemistry I

## Definition

DNA ligase is the enzyme that seals breaks in DNA by forming phosphodiester bonds between adjacent nucleotides. In Biological Chemistry I, you see it as the enzyme that makes newly copied DNA continuous.

## What It Is

DNA ligase is the enzyme that closes the gaps in DNA by sealing the sugar-phosphate backbone with a phosphodiester bond. In Biological Chemistry I, it shows up right after DNA polymerase has copied a strand but left it in pieces, especially on the lagging strand.

That lagging strand problem happens because DNA polymerase can only build DNA in one direction. As the replication fork opens, the lagging strand is made in short stretches called Okazaki fragments, not as one smooth piece. Ligase does not add new nucleotides. Instead, it connects the finished fragments so the strand becomes continuous.

Mechanistically, ligase fixes a nick in the backbone, meaning the bases are already paired but the bond between nucleotides is missing. The enzyme uses energy from ATP in eukaryotes and many laboratory settings, or NAD+ in some bacteria, to drive the bonding step. That energy use matters because sealing DNA is not just a passive sticking-together job, it is a controlled chemical reaction.

A helpful way to picture it is to think of DNA replication as building a zipper. DNA polymerase makes the teeth, but ligase zips up the tiny breaks between sections. Without ligase, the DNA molecule would stay fragmented, which is a problem for chromosome stability and for any cell that needs to copy its genetic material accurately.

Ligase also shows up in DNA repair. If DNA is damaged and a repair pathway cuts out a bad section, ligase is often the enzyme that finishes the job by sealing the repaired strand back together. So when you see DNA ligase in this course, think “final seal” rather than “builder.” It does not synthesize the strand from scratch, it finishes what other enzymes started.

## Why It Matters

DNA ligase matters because it connects the chemistry of bond formation to the biology of genome copying. In Biological Chemistry I, replication is not just about base pairing, it is about whether a chemical chain can be rebuilt into one stable molecule. Ligase is the step that turns separate DNA pieces into a strand that can survive in the cell.

It also helps you make sense of why the lagging strand is more complicated than the leading strand. Since the lagging strand is made in Okazaki fragments, ligase is the enzyme that removes the “temporary” look of replication and makes the new DNA usable. That makes it a clean example of how enzyme order matters in a pathway.

This term also connects to repair chemistry. DNA gets nicked during normal metabolism and after damage, so ligase appears again when the cell patches DNA. If ligase fails, breaks can remain, and that raises the chance of mutation, chromosome instability, or cell death. In a biochemistry course, that links a single enzyme to the bigger idea that chemical accuracy protects genetic information.

If you are tracing a replication diagram, ligase is often the last enzyme you identify in the strand-building sequence. If you are explaining a mutation or repair problem, ligase is one of the enzymes that tells you whether the DNA backbone can be restored fully.

## Connections

### Okazaki fragments

DNA ligase is the enzyme that joins Okazaki fragments on the lagging strand. Those fragments are made because DNA polymerase can only synthesize in one direction, so the strand is built in short sections. Ligase does not create the fragments, it seals the gaps between them after synthesis.

### DNA polymerase

DNA polymerase and DNA ligase do different jobs during replication. Polymerase adds nucleotides to build the new strand, while ligase connects the completed pieces and seals nicks in the backbone. If you confuse them, remember that polymerase writes the DNA sequence and ligase finishes the join.

### Nucleotide excision repair

Ligase also appears in repair pathways like nucleotide excision repair, where damaged DNA is removed and replaced. After the missing section is rebuilt, ligase seals the final nick. That makes it a good example of how replication enzymes can also support maintenance of genetic information.

### [semiconservative replication](/biological-chemistry-i/key-terms/semiconservative-replication)

Semiconservative replication means each daughter DNA molecule keeps one original strand and one newly made strand. DNA ligase matters because the new strand has to become a complete backbone before the molecule can function normally. It helps the copied DNA finish as a stable double helix instead of a set of fragments.

## On the AP Exam

A quiz question may show a replication fork diagram and ask you to identify the enzyme that seals the gaps between Okazaki fragments. That is DNA ligase, and the correct move is to connect it to the lagging strand, not to nucleotide addition. On a problem set, you might trace the order of enzymes in replication and explain that ligase acts after DNA polymerase has already built each fragment.

If you get a short-answer or lab-style question about DNA repair, look for the final step where the backbone is restored after a section has been cut out and replaced. The best answer usually mentions phosphodiester bond formation, since that is the chemical action ligase performs. A good response shows that you know ligase seals a nick, it does not copy the template strand.

## dna ligase vs DNA polymerase

These enzymes are easy to mix up because both act during DNA replication, but they do different chemistry. DNA polymerase builds the new DNA strand by adding nucleotides, while DNA ligase seals the backbone after the pieces are already made. If the question asks about joining fragments or closing nicks, the answer is ligase.

## Key Takeaways

- DNA ligase is the enzyme that seals breaks in the DNA backbone by forming phosphodiester bonds.
- In replication, it is best known for joining Okazaki fragments on the lagging strand.
- Ligase uses ATP or NAD+ as an energy source, depending on the organism and enzyme type.
- It does not build new DNA from the template, it finishes and stabilizes DNA that has already been synthesized or repaired.
- If ligase fails, DNA can stay fragmented, which threatens genomic stability.

## FAQs

### What is DNA ligase in Biological Chemistry I?

DNA ligase is the enzyme that seals breaks in DNA by joining adjacent nucleotides in the sugar-phosphate backbone. In Biological Chemistry I, it usually appears in replication and repair, especially when you are tracing how DNA becomes one continuous strand.

### What does DNA ligase do to Okazaki fragments?

It joins them together on the lagging strand after they have been made by DNA polymerase. The fragments already have matching bases, but ligase closes the missing bond in the backbone so the strand is continuous.

### Is DNA ligase the same as DNA polymerase?

No. DNA polymerase adds nucleotides to build new DNA, while DNA ligase seals the completed pieces together. A simple way to remember it is that polymerase makes the strand and ligase finishes the join.

### Where does DNA ligase show up besides replication?

It also shows up in DNA repair pathways, including nucleotide excision repair, where damaged DNA is removed and replaced. After the missing section is filled in, ligase seals the final nick in the backbone.

## Related Study Guides

- [12.1 DNA replication mechanisms and enzymes](/biological-chemistry-i/unit-12/dna-replication-mechanisms-enzymes/study-guide/KgWBroNs3LXE68pc)

## About This Document

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