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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# DNA helicases

Helicases are motors that separate the two DNA strands using ATP; they open the replication fork and help repair DNA.

Updated 2026-11-01

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## Key facts

- They break the hydrogen bonds between bases using energy from ATP hydrolysis.
- The eukaryotic replicative helicase is the MCM2-7 ring, active as the CMG complex (Cdc45-MCM-GINS). In bacteria: DnaB.
- RecQ helicases: BLM (Bloom syndrome), WRN (Werner syndrome, premature ageing), RECQL4 (Rothmund-Thomson).
- TFIIH holds two helicases (XPB, XPD) that open DNA in transcription and in nucleotide excision repair.
- Their loss causes genome instability and cancer predisposition.

**Also called**
MCM helicase, RecQ helicases (BLM, WRN)

**EC number**
[5.6.2.4](https://www.enzyme-database.org/query.php?ec=5.6.2.4)

**Genes**
MCM2–MCM7, BLM, WRN, RECQL4

**Structure (PDB)**
[4O3M](https://www.rcsb.org/structure/4O3M)

## Regulation

- MCM is loaded inactive in G1 (licensing) and only activated in S by CDK and DDK kinases.
- BLM forms the 'dissolvasome' with topoisomerase IIIα, RMI1 and RMI2, which undoes double Holliday junctions without crossing over.
- RPA proteins coat the single strand the helicase leaves behind.

## Inhibitors and toxins

- No drugs in clinical use; WRN inhibitors under study for microsatellite-unstable tumours

## Clinical notes

- In Bloom syndrome, loss of BLM greatly increases sister chromatid exchanges, a diagnostic sign.
- XPD (ERCC2) mutations can give xeroderma pigmentosum or trichothiodystrophy depending on how they affect repair and transcription.
- Microsatellite-unstable tumour cells depend on WRN to survive (synthetic lethality).

## Related conditions

- [Bloom syndrome](https://durgical.com/en/conditions/bloom-syndrome/)
- [Xeroderma pigmentosum](https://durgical.com/en/conditions/xeroderma-pigmentosum/)

## Related

- [DNA replication: the replication fork](https://durgical.com/en/biochemistry/topics/dna-replication-the-replication-fork/)
- [DNA polymerases](https://durgical.com/en/biochemistry/enzymes/dna-polymerases/)
- [Topoisomerases (I and II)](https://durgical.com/en/biochemistry/enzymes/topoisomerases/)
- [DNA repair](https://durgical.com/en/biochemistry/topics/dna-repair/)
- [Transcription](https://durgical.com/en/biochemistry/topics/transcription/)

## Test yourself

Think of the answer, then tap the question to see it.

## What energy source do helicases use to separate DNA?

ATP (or another NTP) hydrolysis, which moves the enzyme along the strand.

## Which helicase is missing in Bloom syndrome, and what cytogenetic sign does it give?

The RecQ helicase BLM; there are many more sister chromatid exchanges.

## What is the active eukaryotic replicative helicase called?

The CMG complex: Cdc45, the MCM2-7 ring and GINS.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [BLM: RecQ-like DNA helicase BLM (human)](https://www.uniprot.org/uniprotkb/P54132/entry) (UniProt, 2026); [MCM6: DNA replication licensing factor MCM6 (human)](https://www.uniprot.org/uniprotkb/Q14566/entry) (UniProt, 2026); [4O3M: Ternary complex of Bloom's syndrome helicase](https://www.rcsb.org/structure/4O3M) (RCSB Protein Data Bank, 2026); [DNA Replication](https://reactome.org/PathwayBrowser/#/R-HSA-69306) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/dna-helicases/