A DNA helicase is a motor protein that converts ATP hydrolysis into mechanical work, translocating along nucleic acid and separating the two strands of a duplex. That much appears in every textbook. What appears less often is that helicases have left the textbook: one is the motor inside every nanopore sequencer, another is the enzyme that makes isothermal amplification possible, and a third is among the more actively pursued targets in oncology. This article covers the biology, then the three places a working laboratory actually encounters one.
At a glance
Helicases are ATP-dependent nucleic acid motor proteins, classified into six superfamilies (SF1 to SF6) on sequence and structural grounds. SF1 and SF2 are generally non-hexameric and include the repair and recombination enzymes; SF3 to SF6 are ring-shaped hexamers and include the replicative helicases. In bacteria the replicative helicase is DnaB, loaded by DnaC. In eukaryotes it is the CMG complex — CDC45, MCM2-7 and GINS — with MCM2-7 loaded first as an inactive double hexamer and activated when the other components join. Mutations in the RecQ family cause Bloom and Werner syndromes, and defects in XPB, XPD and FANCJ cause xeroderma pigmentosum and Fanconi anaemia respectively.
How a helicase works
The mechanism is the same in outline across the superfamilies, and the differences between them are in how the machine is built rather than what it does.
- ATP binds and is hydrolysed. The nucleotide-binding site sits between two RecA-like or AAA+ domains, and hydrolysis drives a conformational change that closes and reopens them.
- That cycle produces translocation. The conformational change is coupled to movement along one strand of the nucleic acid — an inchworm mechanism in non-hexameric enzymes, a rotary or sequential mechanism in the hexameric rings.
- The duplex is separated at the fork. Some helicases act as passive wedges, waiting for thermal fraying and capturing the open state; others actively destabilise base pairs ahead of them. The distinction determines how much resistance a given helicase can push through.
- Directionality is set by which strand the enzyme grips. A helicase translocating 3′ to 5′ on one strand and one translocating 5′ to 3′ are doing the same thing on opposite strands, which is why the two replicative helicases have opposite stated polarities.
- Single-strand binding proteins prevent re-annealing behind the enzyme, and topoisomerases relieve the positive supercoiling that accumulates ahead of it. A helicase working alone would stall against its own torsional load.
The six superfamilies
The comparison the sheet asks for. Structural organisation is the most useful axis, because it predicts both mechanism and function.
Family | Organisation | Direction | Representative enzymes | Principal role |
SF1 | Non-hexameric, RecA-like core | 3′→5′ or 5′→3′ | UvrD, Rep, PcrA, Dda, Pif1 | Repair, recombination, replication restart |
SF2 | Non-hexameric, RecA-like core | Both, family dependent | RecQ (BLM, WRN), XPB, XPD, RecG, NS3 | Genome stability, nucleotide excision repair, transcription |
SF3 | Hexameric ring, AAA+ | 3′→5′ | Viral E1, SV40 large T antigen | Viral replication |
SF4 | Hexameric ring, RecA-like | 5′→3′ | DnaB, T7 gp4 | Bacterial and phage replication |
SF5 | Hexameric ring | 5′→3′ | Rho | Transcription termination (RNA) |
SF6 | Hexameric ring, AAA+ | 3′→5′ | MCM2-7, RuvB | Eukaryotic and archaeal replication |
The practical shorthand: if it is a ring, it is probably a replicative or replication-adjacent helicase, and it encircles the strand it translocates on. If it is not a ring, it is probably a repair, recombination or transcription enzyme, and it works on an exposed single strand or a specific structure. The disease-associated helicases are overwhelmingly in SF2.
Replication: what unwinds the fork
In bacteria. DnaA opens the origin, DnaC loads DnaB onto the exposed single strands, and DnaB — a hexameric SF4 ring — translocates 5′ to 3′ along the lagging strand template, unwinding ahead of the replisome. DnaB also recruits primase, coupling unwinding to priming.
In eukaryotes, and this is the part most summaries get wrong. The origin recognition complex, with Cdc6 and Cdt1, loads MCM2-7 as an inactive head-to-head double hexamer during G1. It is not yet a helicase. Activation in S phase requires CDC45 and the GINS complex to assemble onto each MCM2-7 ring, forming the CMG complex — CDC45, MCM, GINS — which is the active replicative helicase. The two CMGs then pass each other and translocate 3′ to 5′ along the leading strand templates, moving in opposite directions to establish a bidirectional fork.
The separation of loading from activation is the licensing mechanism that ensures each origin fires once per cell cycle. Describing MCM2-7 as "the replicative helicase" collapses that distinction and misses why the two-step assembly exists.
And the consequences downstream. Unwinding generates positive supercoiling ahead of the fork, relieved by topoisomerases. It exposes single strands, protected from re-annealing and nuclease attack by RPA. And it provides the template for continuous leading-strand synthesis and discontinuous lagging-strand synthesis in Okazaki fragments — the asymmetry that follows directly from the fact that both polymerases can only extend 5′ to 3′.
Where laboratories actually meet a helicase
Three places, and the first two are instrumentation and reagents rather than biology.
1. The motor in a nanopore sequencer
There is a helicase in there. DNA pulled through a nanopore by an electric field moves far too fast to read base by base — a helicase grips the strand and feeds it through in steps slow enough for the signal to resolve, which is what made the technology work.
Flow (2026)
Nanopore sequencing has a problem that a helicase solves. DNA driven through a pore by an electric field moves far too fast for the ionic current signal to be resolved base by base. The solution is to attach a motor protein that grips the strand and feeds it through in discrete steps against the electrophoretic pull, slowing translocation to a rate the detector can follow.
Lab Quality Management Certificate
The Lab Quality Management certificate is more than training—it’s a professional advantage.
Gain critical skills and IACET-approved CEUs that make a measurable difference.
The enzyme that made this work was a helicase. Experiments using Hel308 with the MspA protein pore resolved ATP-dependent DNA processing as discrete current steps — half base-pair resolution at millisecond timing, entirely label-free — and the combination of an ATP-driven motor protein with a protein nanopore sensor became the basis of today’s commercial nanopore sequencers.
Two practical consequences for anyone running the platform. Read length is limited in part by motor processivity — when the helicase dissociates, the read ends, which is one reason solid-state nanopore development is pursuing protein-free ratcheting mechanisms. And because the motor is an ATP-dependent enzyme, library preparation chemistry, temperature and buffer composition all affect it. A translocation-speed problem on a nanopore run is an enzyme problem.
2. Helicase-dependent amplification
PCR uses heat to separate strands. HDA uses a helicase instead. That single substitution removes the thermal cycling requirement, which removes the thermal cycler — the reaction proceeds isothermally, with a helicase opening the duplex, single-strand binding proteins holding it open, primers annealing, and a polymerase extending. The published method uses helicases including UvrD, Dda, T7 phage helicase and RecQ, paired with polymerases such as Bst, and thermophilic variants operate at a single elevated temperature.
Interested in life sciences?
The attraction is instrumentation. An isothermal reaction needs a heat block rather than a cycler, which makes it viable in a cartridge, a handheld reader, or a setting with no laboratory infrastructure — the reason HDA and its isothermal relatives underpin a good deal of point-of-care molecular diagnostics. The trade-off is amplicon length: HDA is suited to short targets, with published thermophilic implementations working below a few hundred base pairs.
For a laboratory evaluating assay formats, this is the version of "what does a helicase do" that has a purchase order attached. Our overview of analytical techniques every lab should master covers where amplification sits in the wider method landscape.
3. A drug target, and a synthetic-lethal one
Helicase inhibition has been pursued for decades with limited success, because these are essential enzymes and inhibiting them tends to be toxic. The interesting recent development is a synthetic-lethal approach: rather than inhibiting a helicase everywhere, target one that a specific tumour genotype has become dependent on.
WRN — the RecQ-family helicase mutated in Werner syndrome — is the leading example. Cancers with microsatellite instability arising from mismatch repair deficiency accumulate expanded repeat sequences that form secondary structures, and resolving those structures appears to require WRN in a way that repair-proficient cells do not. That creates a dependency present in the tumour and absent in normal tissue, which is the shape of a tractable target. Several programmes have pursued WRN inhibitors on that basis.
The other side of the same biology is diagnostic. Mismatch repair status and microsatellite instability are already routine in oncology pathology, so the biomarker infrastructure for selecting patients exists ahead of the drugs. Our overview of the stages of pharmaceutical discovery and development covers the pathway such a programme follows, and genomics in precision medicine the diagnostic side.
Helicase defects and disease
The disease genes cluster in SF2, and specifically in the repair and recombination enzymes — which is what you would expect, since losing a replicative helicase is not survivable.
Disorder | Gene | Helicase family | Presentation |
Bloom syndrome | BLM | RecQ, SF2 | Growth deficiency, sun sensitivity, marked genomic instability and high cancer predisposition across many tumour types |
Werner syndrome | WRN | RecQ, SF2 | Features of premature ageing from adolescence onward, with elevated cancer risk. The same gene is the synthetic-lethal target above |
Rothmund-Thomson syndrome | RECQL4 | RecQ, SF2 | Skin changes, skeletal abnormalities, osteosarcoma risk |
Xeroderma pigmentosum | XPB (ERCC3), XPD (ERCC2) | SF2, TFIIH subunits | Extreme UV sensitivity and very high skin cancer risk. Both proteins are subunits of the TFIIH complex, so they serve both transcription and nucleotide excision repair |
Fanconi anaemia | FANCJ (BRIP1) | SF2 | Bone marrow failure, congenital abnormalities, cancer predisposition |
The common thread is genome instability rather than a failure of replication itself. These enzymes resolve structures that stall or break forks — G-quadruplexes, Holliday junctions, hairpins at repeats. Without them replication still proceeds, but errors accumulate, which is why the phenotypes are cancer predisposition and premature ageing rather than developmental failure.
Resources and further reading
- Nanopores as a tool to study protein dynamics — includes the account of Hel308 processing DNA through an MspA pore, and how motor-protein-plus-nanopore became the basis of commercial sequencing.
- Bloom syndrome and Werner syndrome — NORD summaries for the two best-characterised RecQ disorders.
- Xeroderma pigmentosum and Fanconi anaemia — StatPearls reviews.
- CRISPR and gene editing in practice — the editing context, noting that Cas9 is not itself a helicase.
This article was produced under Lab Manager’s AI Editorial Guidelines
