---
title: "DNA Polymerase I in Cell Biology"
description: "DNA polymerase I removes RNA primers and fills the gaps with DNA during replication and repair in Cell Biology, especially on the lagging strand."
canonical: "https://fiveable.me/cell-biology/key-terms/dna-polymerase-i"
type: "key-term"
subject: "Cell Biology"
unit: "Unit 13"
---

# DNA Polymerase I in Cell Biology

## Definition

DNA polymerase I is a bacterial DNA replication enzyme that removes RNA primers and replaces them with DNA. In Cell Biology, it works on Okazaki fragments on the lagging strand and also helps repair damaged DNA.

## What It Is

DNA polymerase I is a prokaryotic enzyme that cleans up after DNA synthesis by removing RNA primers and replacing them with DNA nucleotides. In Cell Biology, you usually see it during lagging-strand replication in bacteria such as E. coli.

Here is the basic sequence: primase lays down a short RNA primer, DNA polymerase III extends from that primer, and then DNA polymerase I takes over to remove the RNA and fill in the missing DNA. That cleanup step matters because DNA polymerase cannot start a new strand on its own. It needs a free 3' OH group, and the primer provides that starting point.

DNA polymerase I is a little unusual because it does more than one job. It has 5' to 3' polymerase activity, which lets it add DNA nucleotides, and 3' to 5' exonuclease activity, which lets it proofread and remove mispaired bases. It also has 5' to 3' exonuclease activity, which is what allows it to strip away RNA primers as it moves through an Okazaki fragment.

That makes DNA polymerase I different from the main fast-copying polymerase in bacterial replication. It is not the enzyme that builds the bulk of the new strand. Instead, it acts as a cleanup and replacement enzyme, swapping RNA for DNA so the lagging strand becomes a continuous DNA molecule after ligase seals the remaining nick.

A useful way to picture it is this: DNA polymerase III does the long stretch of copying, DNA polymerase I edits out the primer and patches the gap, and DNA ligase finishes the job by joining the fragments together. If any one of those steps is missing, the lagging strand stays incomplete.

## Why It Matters

DNA polymerase I shows how DNA replication is a coordinated process, not one enzyme doing everything at once. In Cell Biology, that distinction helps you trace the order of events at the replication fork instead of treating replication like a single step.

It also gives you a clean example of why the lagging strand is more complicated than the leading strand. Because lagging-strand DNA is made in short Okazaki fragments, each fragment starts with an RNA primer that later has to be removed. DNA polymerase I is the enzyme that turns those temporary RNA pieces into permanent DNA.

This term also connects replication to DNA repair. Since DNA polymerase I can remove and replace nucleotides, it helps fix small problems in bacterial DNA as well as clean up replication intermediates. That makes it a good example of how cells reuse molecular tools for more than one pathway.

If you are studying replication diagrams, this enzyme helps you identify which step happens after elongation but before final sealing. It is one of the easiest places to lose the sequence, so knowing its job makes the whole process easier to map.

## Connections

### Okazaki Fragments

DNA polymerase I works directly on Okazaki fragments because those short lagging-strand segments start with RNA primers. After DNA polymerase III extends each fragment, DNA polymerase I removes the primer and replaces it with DNA. Without that cleanup, the fragments would still contain RNA at their beginnings.

### [DNA Ligase](/cell-biology/key-terms/dna-ligase)

DNA ligase acts after DNA polymerase I has filled in the gap left by primer removal. Polymerase I replaces RNA with DNA, but it does not fully seal the sugar-phosphate backbone. Ligase closes the final nick between fragments, so the lagging strand becomes one continuous strand.

### [dna polymerase iii](/cell-biology/key-terms/dna-polymerase-iii)

DNA polymerase III is the main enzyme that builds most of the new bacterial DNA strand. DNA polymerase I is more of a finishing enzyme, removing primers and patching gaps. If you mix them up, the replication sequence stops making sense, especially on the lagging strand.

### Leading Strand

The leading strand gives you a helpful contrast because it is synthesized continuously, so it needs far fewer primer-removal steps. DNA polymerase I is much more noticeable on the lagging strand, where repeated priming creates many short fragments that must be cleaned up before the DNA can be sealed.

## On the AP Exam

A quiz question might ask you to label a replication fork diagram, and you would identify DNA polymerase I as the enzyme that removes RNA primers and replaces them with DNA on lagging-strand fragments. In a short-answer prompt, you may need to trace the order of replication events: primase adds the primer, DNA polymerase III extends the strand, DNA polymerase I replaces the primer, and DNA ligase seals the nick.

You may also see it in comparison questions where you choose the enzyme with proofreading or primer-removal activity. If a question mentions E. coli, Okazaki fragments, or RNA primer removal, DNA polymerase I is usually the match. In lab or discussion work, the main task is often explaining why replication needs both a building enzyme and a cleanup enzyme instead of just one polymerase.

## dna polymerase i vs dna polymerase iii

DNA polymerase III is the main enzyme that adds most of the nucleotides during bacterial DNA replication, while DNA polymerase I mainly removes RNA primers and fills in the resulting gaps. A lot of students mix them up because both are DNA polymerases, but their jobs are different. Think of polymerase III as the builder and polymerase I as the finisher.

## Key Takeaways

- DNA polymerase I is a bacterial enzyme that removes RNA primers and replaces them with DNA during replication.
- It is especially active on the lagging strand, where each Okazaki fragment begins with a primer that has to be removed.
- DNA polymerase I can also proofread and participate in DNA repair, so it is not only a replication enzyme.
- It does not make most of the new strand, DNA polymerase III does that work first.
- After DNA polymerase I fills the gap, DNA ligase seals the final break in the sugar-phosphate backbone.

## FAQs

### What is DNA polymerase I in Cell Biology?

DNA polymerase I is a bacterial enzyme that removes RNA primers and replaces them with DNA during replication. It works after DNA polymerase III has extended an Okazaki fragment, especially on the lagging strand. It also has proofreading and repair functions.

### What does DNA polymerase I do on the lagging strand?

On the lagging strand, DNA polymerase I removes the RNA primer from each Okazaki fragment and fills the gap with DNA nucleotides. That step turns a short RNA-DNA hybrid into a fully DNA segment. DNA ligase then seals the remaining nick between fragments.

### How is DNA polymerase I different from DNA polymerase III?

DNA polymerase III is the main enzyme that synthesizes most of the new DNA strand in bacteria. DNA polymerase I mainly removes RNA primers and patches the gaps with DNA. So polymerase III does the bulk copying, and polymerase I does the cleanup.

### Does DNA polymerase I proofread DNA?

Yes, DNA polymerase I has 3' to 5' exonuclease activity, which lets it proofread and remove some mismatched bases. That is separate from its 5' to 3' exonuclease activity, which removes RNA primers. Those two directions get confused a lot, so it helps to match each one to the right job.

## Related Study Guides

- [13.2 Mechanisms of DNA replication](/cell-biology/unit-13/mechanisms-dna-replication/study-guide/Y0mPHx1NnedMzLAH)

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