What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Helicase Structure and Function in DNA Replication

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Helicases

What Are Helicases?

Helicases are a class of molecular motor enzymes that catalyze the separation of duplex nucleic acid strands using the chemical energy derived from ATP hydrolysis. They are found in all domains of life—bacteria, archaea, and eukaryotes—and participate in virtually every process involving nucleic acid metabolism, including DNA replication, repair, recombination, transcription, and RNA processing. In the context of DNA replication, the primary function of a helicase is to unwind the parental double helix ahead of the replication machinery, generating single-stranded DNA (ssDNA) templates that can be copied by DNA polymerases.

The defining biochemical feature of a helicase is its ability to couple ATP binding and hydrolysis to directional movement along a nucleic acid strand. This movement is coupled to the physical disruption of base-pairing interactions between complementary strands. The energy released from ATP hydrolysis—approximately 30.5 kJ/mol under standard conditions—drives the conformational changes required for processive unwinding. Helicases are not nucleases; they do not cleave the phosphodiester backbone. They are also not topoisomerases, which change DNA topology by transiently breaking and rejoining strands. For a broader comparison of these enzyme classes, see Helicase a Topoisomerase.

Role in DNA Replication

During DNA replication, the double-stranded DNA (dsDNA) must be melted to expose the template strands. This unwinding occurs at a specialized region called the replication fork, a Y-shaped structure where the parental duplex is separated into two ssDNA templates. The helicase that operates at this site is often referred to as the replicative helicase, and it is the central enzymatic engine of the Replication Fork Helicase machinery. In Escherichia coli, the replicative helicase is DnaB, a hexameric ring-shaped enzyme that encircles the lagging-strand template and translocates 5′ to 3′. In eukaryotes, the replicative helicase is the CMG complex (Cdc45-MCM-GINS), which also encircles the leading-strand template and translocates 3′ to 5′.

The helicase does not work alone. It physically associates with other replication proteins—primase, DNA polymerase, and single-stranded DNA binding proteins—to form a dynamic assembly called the replisome. The helicase sets the pace of replication by controlling the rate of fork progression, typically 500–1000 base pairs per second in bacteria and 50–100 base pairs per second in eukaryotes. Without helicase activity, polymerases would stall at the first few base pairs of duplex DNA, as they are unable to displace the complementary strand on their own. Understanding helicase structure is therefore essential to understanding how replication is initiated, sustained, and regulated.

Structural Domains of Helicases

RecA-like Core

The structural foundation of nearly all helicases is the RecA-like fold, a conserved protein domain originally identified in the bacterial recombinase RecA. This fold consists of a central β-sheet flanked by α-helices, forming a nucleotide-binding pocket. The RecA-like core is composed of two subdomains that together create the ATP-binding site at their interface. In helicases, this core is typically present in one or two copies per polypeptide chain. The presence of this domain is so universal that helicases are defined by it: any enzyme containing a RecA-like ATPase domain that can translocate along nucleic acid is classified as a helicase.

The RecA-like core performs two essential functions. First, it binds ATP and hydrolyzes it to ADP and inorganic phosphate. Second, it undergoes large conformational changes in response to nucleotide binding and hydrolysis. These conformational changes are transmitted to DNA-binding elements, producing the mechanical force needed for strand separation. The core is therefore both a catalytic domain and a motor domain. Mutations that disrupt the RecA-like fold, such as substitutions in the Walker A motif (GXXXXGKT/S, where X is any residue), abolish ATP hydrolysis and eliminate helicase activity.

ATPase and Translocation Domains

Within the RecA-like core, several conserved sequence motifs coordinate ATP binding and hydrolysis. The Walker A motif (also called the P-loop) binds the phosphate groups of ATP, while the Walker B motif (hhhhDE, where h is hydrophobic) coordinates the catalytic magnesium ion and the attacking water molecule. Additional motifs—Q-motif, motif III, motif IV, and motif V—contribute to nucleotide recognition, conformational coupling, and DNA binding. Together, these motifs form the ATPase active site at the interface between the two RecA subdomains.

The ATPase cycle drives translocation through a mechanism known as the "inchworm" model for monomeric helicases or the "hand-over-hand" model for oligomeric helicases. In the inchworm model, the helicase alternates between two DNA-binding states: one subdomain binds tightly to the nucleic acid while the other releases and moves forward. ATP binding induces a conformational change that closes the cleft between subdomains, pulling the helicase forward by one nucleotide. ATP hydrolysis and phosphate release reopen the cleft, resetting the enzyme for the next cycle. Each ATP hydrolyzed moves the helicase one or two nucleotides along the strand, depending on the enzyme.

DNA Binding Domains

Helicases contain one or more DNA-binding domains that are distinct from the ATPase core. These domains recognize the sugar-phosphate backbone of nucleic acids, providing the track along which the helicase translocates. In SF1 and SF2 helicases, DNA binding is mediated by conserved motifs located on the surface of the RecA-like core, particularly motifs Ia, II, and III. These motifs contact the phosphodiester backbone and, in some cases, the bases of the nucleic acid.

In hexameric helicases, DNA binding occurs primarily through a central channel formed by the ring-shaped assembly. The inner surface of the channel is lined with positively charged residues that interact with the negatively charged DNA backbone. The channel diameter is approximately 20–30 Å, large enough to accommodate ssDNA but too narrow for dsDNA. This steric constraint is central to the unwinding mechanism: the helicase encircles one strand while excluding the complementary strand from the central pore. The excluded strand is thus physically displaced as the helicase translocates. For a detailed treatment of how helicases interact with their nucleic acid substrates, see Helicase Protein.

Classification of Helicases

Superfamily 1 and 2

Helicases are classified into six superfamilies (SF1–SF6) based on the presence and arrangement of conserved sequence motifs. The two largest groups, SF1 and SF2, contain helicases that function as monomers or dimers and translocate along single-stranded nucleic acids. SF1 and SF2 helicases share a common core of two RecA-like domains but differ in the specific motifs and in the directionality of translocation.

SF1 helicases include the E. coli enzymes Rep, UvrD, and PcrA. These enzymes translocate 3′ to 5′ along ssDNA and are involved in DNA repair and replication restart. SF1 helicases have a relatively simple architecture: two RecA-like domains flanked by N-terminal and C-terminal accessory domains that confer substrate specificity. SF2 helicases are the most numerous and diverse, including the eukaryotic transcription factor TFIIH, the RNA helicase eIF4A, and the recombination enzyme Rad54. SF2 helicases can translocate in either direction, and many members are RNA helicases. The structural distinction between SF1 and SF2 lies in the relative orientation of the two RecA domains and the placement of conserved motifs within them.

Hexameric Helicases

SF3–SF6 helicases are predominantly hexameric ring-shaped enzymes. The replicative helicases of bacteria (DnaB, SF4), archaea and eukaryotes (MCM, SF6), and many viral helicases (SF3) fall into this category. Hexameric helicases assemble into ring structures with a central pore that encircles one strand of the DNA duplex. The ring is composed of six identical or closely related subunits, each contributing a RecA-like ATPase domain. The ATPase active sites are located at the interfaces between adjacent subunits, creating a continuous motor around the ring.

The hexameric architecture provides several advantages. First, it allows for high processivity: the ring cannot easily dissociate from the DNA, so the helicase can unwind thousands of base pairs without falling off. Second, it enables coordinated ATP hydrolysis around the ring, producing a smooth, ratchet-like translocation. Third, the central channel provides a steric exclusion mechanism for strand separation, as described below. The E. coli DnaB helicase translocates at approximately 1000 nucleotides per second, while the eukaryotic CMG complex translocates at 50–100 nucleotides per second, reflecting differences in ring structure and regulatory complexity.

Mechanism of DNA Unwinding

ATP-Driven Conformational Changes

The unwinding reaction is driven by a series of conformational changes in the helicase that are coupled to the ATP hydrolysis cycle. The cycle proceeds through four major states: ATP-bound, pre-hydrolysis, ADP+Pi-bound, and ADP-bound. Each state has a distinct conformation of the RecA-like core, and these conformations are translated into movement along the DNA.

In the ATP-bound state, the two RecA subdomains are closed, forming a tight complex with the nucleotide. This closure positions DNA-binding motifs to grip the nucleic acid strand firmly. Upon ATP hydrolysis, the enzyme enters a transition state in which the subdomains begin to open. Release of inorganic phosphate triggers the largest conformational change: the subdomains open fully, releasing the DNA grip and allowing the helicase to move forward. ADP release then resets the enzyme to the open conformation, ready for the next ATP binding event.

For hexameric helicases, the ATPase cycle is coordinated around the ring. Adjacent subunits adopt different nucleotide states—ATP, ADP+Pi, or apo—creating a wave of conformational changes that propagates around the ring. This coordination ensures that at any given time, at least one subunit is tightly bound to the DNA, preventing backward slippage. The result is a processive, unidirectional motor that can generate forces of 10–20 piconewtons, sufficient to disrupt the hydrogen bonds between base pairs. For a molecular-level explanation of how these forces break base-pairing interactions, see Helicase Break Hydrogen Bonds.

Steric Exclusion and Translocation

The actual separation of the two DNA strands occurs by a mechanism called steric exclusion. In this model, the helicase binds to one strand of the duplex and translocates along it, while the complementary strand is physically excluded from the central channel or binding groove. As the helicase moves forward, it wedges itself between the two strands at the fork junction, forcing them apart.

For hexameric helicases, the steric exclusion mechanism is straightforward: the ring encircles one strand, and the other strand is simply too large to fit through the central pore. The excluded strand is displaced to the outside of the ring as the helicase advances. For SF1 and SF2 helicases, which do not form rings, the mechanism involves a wedge or pin domain that separates the strands. The helicase binds to the ssDNA-dsDNA junction, and a β-hairpin or loop structure physically splits the duplex as the enzyme translocates.

The directionality of translocation is determined by the orientation of the helicase on the DNA. A 3′ to 5′ helicase binds to ssDNA and moves toward the fork, unwinding the duplex as it goes. A 5′ to 3′ helicase does the same but approaches from the opposite direction. In DNA replication, the directionality of the replicative helicase determines which strand is used as the template for leading-strand synthesis. The E. coli DnaB helicase translocates 5′ to 3′ on the lagging-strand template, while the eukaryotic CMG complex translocates 3′ to 5′ on the leading-strand template. Both arrangements achieve the same result: the helicase moves ahead of the polymerase, unwinding the duplex and exposing ssDNA for copying.

Methods to Study Helicase Structure

X-ray Crystallography

X-ray crystallography has been the primary method for determining helicase structures at atomic resolution. The technique requires the formation of well-ordered crystals of the protein, which are then exposed to an X-ray beam. The diffraction pattern produced by the crystals is used to calculate the electron density map of the protein, from which the positions of individual atoms can be determined.

Crystallographic structures have been obtained for numerous helicases, including the SF1 helicase PcrA, the SF2 helicase NS3 from hepatitis C virus, and the bacterial hexameric helicase DnaB. These structures have revealed the arrangement of RecA-like domains, the architecture of the ATPase active site, and the mode of DNA binding. A major limitation of crystallography is that it provides a static snapshot of the protein, typically in a single conformational state. To capture different states of the ATPase cycle, researchers have used nucleotide analogs such as ADP·BeF₃ (a mimic of the ATP-bound state) or ADP·AlF₄ (a mimic of the transition state). These analogs trap the helicase in specific conformations, allowing the structural changes associated with ATP hydrolysis to be visualized.

Cryo-Electron Microscopy

Cryo-electron microscopy (cryo-EM) has revolutionized the study of large helicase complexes, particularly hexameric rings and their assemblies with other replication proteins. In cryo-EM, a sample of the protein complex is rapidly frozen in a thin layer of vitreous ice, preserving it in a near-native state. The frozen sample is imaged in an electron microscope, and thousands of individual particle images are averaged to produce a three-dimensional reconstruction.

Cryo-EM has been instrumental in determining the structure of the eukaryotic CMG helicase, a complex of 11 subunits with a molecular weight of approximately 1 MDa. The structure revealed how the MCM ring encircles DNA, how Cdc45 and GINS stabilize the ring, and how the complex interacts with other replisome components. Cryo-EM has also captured the CMG complex in different nucleotide states, showing how ATP hydrolysis drives conformational changes around the ring. The resolution of cryo-EM reconstructions has improved dramatically in recent years, with many structures now determined at better than 3 Å resolution, comparable to crystallography.

Single-Molecule Assays

While structural methods provide static pictures, single-molecule assays reveal the dynamic behavior of helicases in real time. These techniques include optical tweezers, magnetic tweezers, and fluorescence resonance energy transfer (FRET). In an optical tweezers experiment, a DNA molecule is tethered between two beads, one held in an optical trap and the other attached to a micropipette. A helicase is allowed to unwind the DNA, and the change in DNA length is measured by the movement of the bead in the trap. This approach measures the rate of unwinding, the step size per ATP hydrolyzed, and the force generated by the helicase.

Single-molecule FRET (smFRET) uses fluorescent labels attached to the helicase and the DNA to monitor conformational changes during unwinding. By measuring the efficiency of energy transfer between donor and acceptor fluorophores, researchers can track the opening and closing of the RecA domains, the movement of the helicase along DNA, and the separation of the two strands. These assays have shown that helicases can pause, slip backward, and restart, revealing a stochastic element to unwinding that is not apparent from bulk biochemical assays.

Helicases in the Replication Fork

Replisome Assembly

The replicative helicase is loaded onto DNA during the initiation phase of replication, a process that requires specialized loader proteins. In E. coli, DnaB is loaded onto the origin of replication (oriC) by the DnaC loader protein, which binds to DnaB and delivers it to the DnaA-bound origin complex. The loading process requires ATP binding but not hydrolysis by DnaC. Once loaded, DnaB encircles the lagging-strand template and begins translocating, recruiting the primase DnaG and the DNA polymerase III holoenzyme to form the replisome.

In eukaryotes, helicase loading is more complex and occurs in two steps. First, the MCM2-7 hexamer is loaded onto double-stranded DNA at origins of replication during the G1 phase of the cell cycle, a process requiring the origin recognition complex (ORC), Cdc6, and Cdt1. This loading produces a double hexamer of MCM2-7 encircling the DNA. During S phase, the kinases Cdc7 and CDK2 phosphorylate components of the pre-replication complex, triggering the recruitment of Cdc45 and GINS. The association of these factors converts the inactive MCM2-7 double hexamer into the active CMG helicase, which then unwinds the origin and establishes the replication fork. For a detailed view of the architecture of the replication fork, see Replication Fork Structure.

Coordination with Other Enzymes

At the replication fork, the helicase must coordinate its activity with several other enzymes. The primase synthesizes short RNA primers on the lagging-strand template, providing the 3′ hydroxyl groups required by DNA polymerase to initiate synthesis. In bacteria, the primase DnaG physically interacts with DnaB, and this interaction stimulates primer synthesis. The primase is recruited to the fork periodically, synthesizing a new primer every 1000–2000 nucleotides on the lagging strand.

The single-stranded DNA binding protein (SSB in bacteria, RPA in eukaryotes) coats the ssDNA produced by the helicase. SSB binding protects the ssDNA from nucleases, prevents the formation of secondary structures, and stimulates the activity of the helicase. In E. coli, SSB interacts directly with DnaB and with the χ subunit of the DNA polymerase III holoenzyme, helping to coordinate leading- and lagging-strand synthesis.

The helicase also interacts with the DNA polymerase. In bacteria, the τ subunit of the polymerase III holoenzyme binds to DnaB, tethering the polymerase to the helicase and ensuring that leading-strand synthesis keeps pace with unwinding. In eukaryotes, the CMG helicase interacts with DNA polymerase ε on the leading strand and with DNA polymerase δ on the lagging strand, although the precise architecture of these interactions is still being elucidated. The coordination between helicase and polymerase is essential: if the polymerase stalls, the helicase must also pause to prevent the accumulation of excessive ssDNA, which would trigger DNA damage responses.

Regulation of Helicase Activity

Phosphorylation

Helicase activity is tightly regulated to ensure that DNA replication occurs only once per cell cycle and only at the appropriate time. One of the primary regulatory mechanisms is phosphorylation by cell cycle kinases. In eukaryotes, the Dbf4-dependent kinase Cdc7 phosphorylates the MCM2-7 complex during S phase, a modification required for helicase activation. Cdc7 phosphorylates multiple subunits of MCM2-7, particularly the N-terminal tails of MCM2, MCM4, and MCM6. This phosphorylation promotes the recruitment of Cdc45 and GINS, converting the inactive double hexamer into the active CMG helicase.

Cyclin-dependent kinase (CDK) also phosphorylates helicase components, but its role is more complex. CDK phosphorylation of Cdc6 targets it for degradation after helicase loading, preventing re-replication. CDK also phosphorylates the MCM2-7 complex, and this modification is required for the firing of origins that have already been licensed. The combined action of Cdc7 and CDK ensures that helicase activation is coupled to S phase entry and that each origin fires only once per cell cycle.

Accessory Factors

Beyond phosphorylation, helicase activity is modulated by a variety of accessory proteins. In bacteria, the helicase loader DnaC acts as an inhibitor of DnaB after loading: DnaC remains bound to DnaB and prevents its translocation until the loader is released. The release of DnaC requires ATP hydrolysis, which is stimulated by the interaction of DnaC with DnaB and the origin DNA.

In eukaryotes, several proteins regulate CMG helicase activity at the fork. The TIMELESS-TIPIN complex binds to the CMG helicase and couples its activity to the replication checkpoint. When replication is stalled, the checkpoint kinase ATR phosphorylates downstream targets that stabilize the fork and prevent helicase-polymerase uncoupling. The MCM10 protein is required for CMG activation and may help to open the MCM ring during helicase loading. The fork protection complex (Timeless-Tipin in humans, Csm3-Tof1 in yeast) prevents the helicase from running ahead of the polymerase, maintaining the integrity of the replication fork.

Post-translational modifications other than phosphorylation also regulate helicase function. Ubiquitination of MCM7 by the Cul4-RING ubiquitin ligase promotes the unloading of CMG helicase after replication is complete, a process that is essential for preventing re-replication in the subsequent cell cycle. SUMOylation of various replisome components has also been reported, although the functional consequences are less well understood.

Common Pitfalls and Misconceptions

Directionality Confusion

A frequent source of error is confusing the directionality of helicase translocation with the direction of DNA synthesis. Helicase directionality refers to the polarity of the strand along which the helicase moves: a 3′ to 5′ helicase moves along a single strand from its 3′ end toward its 5′ end, while a 5′ to 3′ helicase moves in the opposite direction. This directionality is an intrinsic property of the enzyme and is determined by the orientation of its DNA-binding motifs.

Students often assume that a 3′ to 5′ helicase must be on the leading strand because leading-strand synthesis is continuous. This is incorrect. The E. coli DnaB helicase is 5′ to 3′ and is located on the lagging-strand template. The eukaryotic CMG helicase is 3′ to 5′ and is located on the leading-strand template. In both cases, the helicase moves toward the fork and unwinds the duplex ahead of the polymerases. The directionality of the helicase determines which strand it encircles, not which strand is synthesized continuously.

ATP Hydrolysis vs Binding

Another common misconception is that ATP binding alone is sufficient for helicase unwinding. In fact, ATP binding and ATP hydrolysis serve different purposes. ATP binding induces the conformational change that closes the RecA domains and tightens the grip on DNA. This is the "power stroke" of the motor. ATP hydrolysis and the subsequent release of inorganic phosphate trigger the conformational change that releases the DNA and resets the enzyme. Both steps are required for processive movement.

A helicase that can bind ATP but cannot hydrolyze it (for example, a mutant with a defective Walker B motif) will bind DNA tightly but will not translocate. Such mutants are often used as "dead" helicases in biochemical experiments to trap the enzyme on DNA. Conversely, a helicase that hydrolyzes ATP but cannot couple the energy to conformational changes will waste ATP without moving. The coupling between ATP hydrolysis and mechanical work is the essence of helicase function.

Structural Terminology

Students frequently misuse structural terms when describing helicases. The term "hexameric" refers to a complex of six subunits, but not all hexameric helicases are rings. Some hexameric helicases form planar arrays or other arrangements. The term "processivity" refers to the number of nucleotides unwound before the helicase dissociates from the DNA, not the rate of unwinding. A helicase can be highly processive but slow, or fast but poorly processive.

The distinction between "helicase" and "translocase" is also often blurred. A translocase moves along nucleic acid but does not necessarily unwind duplex DNA. Some translocases, such as the chromatin remodelers, move along DNA without separating strands. All helicases are translocases, but not all translocases are helicases. The defining feature of a helicase is the ability to couple translocation to strand separation.

Summary and Key Takeaways

Helicases are molecular motors that couple ATP hydrolysis to the directional unwinding of nucleic acid duplexes. Their structure is built around a conserved RecA-like core that contains the ATPase active site and DNA-binding motifs. The classification of helicases into superfamilies reflects differences in the arrangement of these core domains and the oligomeric state of the enzyme. The mechanism of unwinding involves ATP-driven conformational changes that produce directional movement along one strand while excluding the complementary strand. The replicative helicase is a central component of the replisome, coordinating its activity with primase, polymerase, and single-stranded DNA binding proteins. Helicase activity is regulated by phosphorylation, accessory proteins, and cell cycle checkpoints to ensure proper DNA replication.

Frequently Asked Questions

What is the basic structure of a helicase?

The basic structure of a helicase consists of one or two RecA-like core domains that contain the ATPase active site and DNA-binding motifs. SF1 and SF2 helicases are typically monomeric or dimeric and contain two RecA-like domains in a single polypeptide chain. Hexameric helicases (SF3–SF6) assemble into ring-shaped structures with a central pore that encircles one strand of DNA. The RecA-like core is composed of a central β-sheet flanked by α-helices, with conserved sequence motifs (Walker A, Walker B, Q-motif, and others) that coordinate ATP binding and hydrolysis.

How does helicase unwind DNA?

Helicase unwinds DNA by translocating along one strand of the duplex and physically displacing the complementary strand. The energy for this movement comes from ATP hydrolysis, which drives conformational changes in the RecA-like core. ATP binding closes the core domains and tightens the grip on DNA; ATP hydrolysis and phosphate release open the core and release the DNA, allowing the helicase to move forward. In hexameric helicases, the ring encircles one strand, and the complementary strand is excluded from the central pore. In monomeric helicases, a wedge or pin domain separates the strands at the fork junction.

What is the directionality of helicase movement?

Helicase directionality is defined by the polarity of the strand along which the enzyme translocates. A 3′ to 5′ helicase moves along a single strand from its 3′ end toward its 5′ end. A 5′ to 3′ helicase moves in the opposite direction. The replicative helicase in bacteria (DnaB) is 5′ to 3′, while the eukaryotic replicative helicase (CMG) is 3′ to 5′. The directionality determines which strand the helicase encircles but does not determine whether leading- or lagging-strand synthesis is continuous.

What are the superfamilies of helicases?

Helicases are classified into six superfamilies (SF1–SF6) based on conserved sequence motifs and structural features. SF1 and SF2 are the largest groups and include monomeric or dimeric helicases that translocate along single-stranded nucleic acids. SF3–SF6 are predominantly hexameric ring-shaped helicases. The replicative helicases of bacteria (SF4), archaea and eukaryotes (SF6), and many viruses (SF3) belong to the hexameric group. SF2 is the most diverse superfamily and includes both DNA and RNA helicases.

What techniques are used to determine helicase structure?

The primary techniques for determining helicase structure are X-ray crystallography and cryo-electron microscopy. X-ray crystallography provides atomic-resolution structures of helicases in different nucleotide states, using nucleotide analogs to trap specific conformations. Cryo-EM is used for large helicase complexes, such as the eukaryotic CMG helicase, and can capture the complex in near-native conditions. Single-molecule techniques, including optical tweezers and fluorescence resonance energy transfer, complement structural methods by measuring the dynamic behavior of helicases in real time.

Why is ATP hydrolysis important for helicase function?

ATP hydrolysis provides the chemical energy that drives the conformational changes required for helicase translocation. ATP binding induces a conformational change that tightens the helicase's grip on DNA, while ATP hydrolysis and phosphate release trigger a conformational change that releases the DNA and allows forward movement. Without ATP hydrolysis, the helicase would bind DNA but would not move. The energy from ATP hydrolysis is also used to generate the force required to disrupt base-pairing interactions between the two DNA strands.

What is the role of helicase in DNA replication?

The helicase unwinds the parental DNA duplex at the replication fork, generating single-stranded DNA templates for the polymerases. It is the central motor of the replisome and sets the pace of replication. The helicase also serves as a platform for the assembly of other replication proteins, including primase, polymerase, and single-stranded DNA binding proteins. Without helicase activity, DNA polymerases cannot copy duplex DNA because they require single-stranded templates.

Key Takeaways

Helicases are ATP-driven molecular motors that unwind nucleic acid duplexes by translocating along one strand and excluding the complementary strand.
The conserved RecA-like core contains the ATPase active site and DNA-binding motifs and is the defining structural feature of all helicases.
Helicases are classified into six superfamilies; SF1/SF2 are monomeric or dimeric, while SF3–SF6 are hexameric rings.
The unwinding mechanism involves ATP binding (tight DNA grip), hydrolysis (power stroke), and phosphate release (reset), producing directional movement.
The replicative helicase (DnaB in bacteria, CMG in eukaryotes) coordinates with primase, polymerase, and SSB/RPA at the replication fork.
Helicase activity is regulated by phosphorylation, accessory proteins, and cell cycle checkpoints to ensure proper replication timing and fork stability.
X-ray crystallography, cryo-EM, and single-molecule assays provide complementary views of helicase structure and dynamics.