A helicase is an enzyme that unwinds the double-stranded structure of DNA or RNA, separating the two intertwined strands so that cells can copy, repair, or read their genetic information. The human genome encodes 95 different helicase proteins, 31 that work on DNA and 64 that work on RNA, making them one of the largest and most essential enzyme families in your body.
How Helicases Unwind DNA
Your DNA exists as two strands twisted around each other, held together by weak chemical bonds between matching base pairs. Before a cell can copy its DNA or use a gene to build a protein, those strands need to be pulled apart. That’s the helicase’s job. The enzyme latches onto a single strand and moves along it, prying the two strands apart as it goes.
Helicases run on ATP, the same energy molecule that powers nearly every active process in your cells. For a long time, scientists assumed ATP worked like fuel, directly powering the enzyme forward along the DNA strand. But recent research published in Nature reveals something more elegant: ATP actually acts as a kind of brake or lock. While ATP is bound, it holds part of the helicase in tension, preventing movement. When the cell breaks down that ATP (a process called hydrolysis), the tension releases and the enzyme ratchets forward by one position, like a clicking turnstile. This “entropy switch” mechanism means the energy doesn’t shove the helicase along so much as it removes the thing holding it in place.
Many ring-shaped helicases advance one base pair for every ATP molecule consumed. Some, like the bacterial DnaB helicase, may take two-base-pair steps. The exact step size can even vary depending on how tightly bonded the DNA sequence is at that particular spot.
Direction of Travel
DNA strands have a chemical directionality, labeled 3′ and 5′ (referring to specific carbon atoms on the sugar backbone). Every helicase moves in one direction or the other along the strand it’s bound to, and this polarity is a core part of how scientists classify them. Some travel 3′ to 5′ (called Type A), while others travel 5′ to 3′ (Type B). The direction matters because it determines which strand the helicase rides on at a replication fork and how it coordinates with other molecular machinery.
The Six Superfamilies
Scientists group all known helicases into six superfamilies, labeled SF1 through SF6, based on their structure and shared sequence patterns. The biggest structural divide is shape. SF1 and SF2 helicases work as single molecules or small units without forming a ring. SF3 through SF6 form ring-shaped structures, typically assembling six protein subunits into a donut that threads the DNA strand through its central hole.
SF1 and SF2 are the largest and most diverse groups. They share a core structure built from two protein domains that resemble an ancient protein involved in DNA recombination, along with at least 12 characteristic sequence patterns. Within SF2 alone, researchers have identified nine distinct families. These two superfamilies also include helicases that travel in both directions, while the ring-shaped families tend to be more uniform: all known SF3 helicases move 3′ to 5′, SF4 and SF5 move 5′ to 3′, and SF6 moves 3′ to 5′.
DNA Helicases in Replication and Repair
The most prominent DNA helicase in human cells is the MCM2-7 complex, a ring of six different protein subunits that serves as the main engine of DNA replication. It unwinds the double helix ahead of the replication fork, creating single-stranded templates that the DNA-copying machinery can read. Loading and activating this complex is one of the key checkpoints cells use to control when and where DNA replication begins.
Other DNA helicases specialize in repair. When DNA is damaged by ultraviolet light, chemicals, or simple replication errors, specific helicases unwind the damaged region so repair enzymes can access and fix it. Still others help untangle DNA strands that become knotted during recombination, the process by which chromosomes exchange segments during cell division.
RNA Helicases and Protein Production
Nearly two-thirds of human helicases work on RNA rather than DNA, reflecting how central RNA processing is to cell function. The largest RNA helicase family is the DEAD-box group (named for a shared amino acid sequence). These enzymes play essential roles in splicing (editing RNA messages before they’re used), ribosome assembly (building the cellular machines that make proteins), and translation initiation (getting the protein-building process started).
Some RNA helicases work differently from their DNA counterparts. Rather than traveling along a strand while unwinding it, certain DEAD-box helicases can clamp onto a short section of double-stranded RNA and pry it open locally, without threading through the strand. This localized unwinding lets them act more like molecular pliers than molecular trains.
One RNA helicase called DHX33 illustrates how critical these enzymes are for basic cell function. When researchers reduced DHX33 levels in cells, the assembly of ribosomes on messenger RNA dropped sharply, causing a near-global shutdown of protein production. Restoring normal DHX33 rescued translation, but adding back a version with a broken helicase domain did not, confirming that the unwinding activity itself is what matters.
What Happens When Helicases Fail
Because helicases are involved in DNA copying and repair, mutations in helicase genes can cause serious genetic disorders. Three well-characterized examples involve helicases from the RecQ family.
Bloom syndrome results from mutations in the BLM helicase gene. The most striking feature is proportionally small body size. Babies born with Bloom syndrome weigh an average of about 1.9 kg (4.2 pounds), compared to the typical 3.2 to 3.3 kg (7 to 7.3 pounds). Their cells show dramatically increased chromosomal breakage and excessive recombination between chromosomes, which drives a high risk of developing many of the same cancer types that affect the general population, just at much younger ages.
Werner syndrome comes from mutations in the WRN helicase and looks very different. Rather than a developmental size problem, it causes premature aging. People with Werner syndrome develop features typically associated with old age, including graying hair, skin changes, and age-related diseases, decades earlier than expected. It is classified as a segmental progeroid syndrome, meaning it accelerates some but not all aspects of aging.
Rothmund-Thomson syndrome, caused by mutations in yet another RecQ-family helicase, shares the small body size of Bloom syndrome but produces its own distinct pattern of skin abnormalities and cancer susceptibility. All three syndromes feature genomic instability and increased cancer risk, but the specific types of instability and the cancers involved differ in each condition.
Helicases as Drug Targets
The same properties that make helicases essential to healthy cells also make them attractive targets for drugs. Cancer cells and viruses both depend heavily on helicase activity, and blocking the right helicase could, in theory, slow or stop their spread.
One of the most developed examples is a small molecule called RK-33, designed to block an RNA helicase called DDX3. In cancer, DDX3 unwinds double-stranded RNA that controls tumor cell growth, so inhibiting it can slow cancer progression by preventing that RNA from being read. The same helicase also turns out to be hijacked by RNA viruses. HIV, respiratory syncytial virus, dengue, Zika, West Nile, and several SARS-CoV-2 variants all exploit the host cell’s DDX3 to replicate their own genetic material.
In lab studies, RK-33 reduced SARS-CoV-2 viral loads by as much as a thousandfold across four different variants. Because the drug targets the host cell’s helicase rather than a viral protein like the spike protein, its effectiveness isn’t undermined by the mutations that create new variants. This approach, hitting a host enzyme that many different viruses rely on, could in principle work as a broad-spectrum antiviral rather than one tailored to a single pathogen.