Helicase - wikipedia Helicase Structure of E. coli helicase RuvA DNA helicase Identifiers EC number 3.6. 4.12 Databases IntEnz IntEnz view BRENDA BRENDA entry ExPASy NiceZyme view KEGG KEGG entry MetaCyc metabolic pathway PRIAM profile PDB structures RCSB PDB PDBe PDBsum Search PMC articles PubMed articles NCBI proteins RNA helicase Identifiers EC number 3.6. 4.13 Databases IntEnz IntEnz view BRENDA BRENDA entry ExPASy NiceZyme view KEGG KEGG entry MetaCyc metabolic pathway PRIAM profile PDB structures RCSB PDB PDBe PDBsum Search PMC articles PubMed articles NCBI proteins Helicases are a class of enzymes vital to all living organisms. Their main function is to unpackage an organism 's genes. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands ( i.e., DNA, RNA, or RNA - DNA hybrid ) using energy derived from ATP hydrolysis. There are many helicases resulting from the great variety of processes in which strand separation must be catalyzed. Approximately 1 % of eukaryotic genes code for helicases. The human genome codes for 95 non-redundant helicases : 64 RNA helicases and 31 DNA helicases. Many cellular processes, such as DNA replication, transcription, translation, recombination, DNA repair, and ribosome biogenesis involve the separation of nucleic acid strands that necessitates the use of helicases. Contents 1 Function 1.1 Activation barrier in helicase activity 1.2 Active and passive helicases 2 History of DNA helicases 3 Structural features 4 Superfamilies 5 Helicase disorders and diseases 5.1 ATRX helicase mutations 5.2 XPD helicase point mutations 5.3 RecQ family mutations 6 Meiotic recombination 7 RNA helicases 8 Diagnostic tools for helicase measurement 8.1 Measuring and monitoring helicase activity 8.2 Determining helicase polarity 9 See also 10 References 11 External links Function Helicase action in DNA replication Helicases are often used to separate strands of a DNA double helix or a self - annealed RNA molecule using the energy from ATP hydrolysis, a process characterized by the breaking of hydrogen bonds between annealed nucleotide bases. They also function to remove nucleic acid - associated proteins and catalyze homologous DNA recombination. Metabolic processes of RNA such as translation, transcription, ribosome biogenesis, RNA splicing, RNA transport, RNA editing, and RNA degradation are all facilitated by helicases. Helicases move incrementally along one nucleic acid strand of the duplex with a directionality and processivity specific to each particular enzyme. Helicases adopt different structures and oligomerization states. Whereas DnaB - like helicases unwind DNA as ring - shaped hexamers, other enzymes have been shown to be active as monomers or dimers. Studies have shown that helicases may act passively, waiting for uncatalyzed unwinding to take place and then translocating between displaced strands, or can play an active role in catalyzing strand separation using the energy generated in ATP hydrolysis. In the latter case, the helicase acts comparably to an active motor, unwinding and translocating along its substrate as a direct result of its ATPase activity. Helicases may process much faster in vivo than in vitro due to the presence of accessory proteins that aid in the destabilization of the fork junction. Activation barrier in helicase activity Enzymatic helicase action, such as unwinding nucleic acids is achieved through the lowering of the activation barrier ( B ( displaystyle B ) ) of each specific action. The activation barrier is a result of various factors, and can be defined using the following equation, where N ( displaystyle N ) = number of unwound base pairs ( bps ), Δ G b p ( displaystyle Delta G_ ( bp ) ) = free energy of base pair formation, G i n t ( displaystyle G_ ( int ) ) = reduction of free energy due to helicase, and G f ( displaystyle G_ ( f ) ) = reduction of free energy due to unzipping forces. B = N ( Δ G b p − G i n t − G f ) ( displaystyle B = N ( Delta G_ ( bp ) - G_ ( int ) - G_ ( f ) ) ) Factors that contribute to the height of the activation barrier include : specific nucleic acid sequence of the molecule involved, the number of base pairs involved, tension present on the replication fork, and destabilization forces. Active and passive helicases The size of the activation barrier to overcome by the helicase contributes to its classification as an active or passive helicase. In passive helicases, a significant activation barrier exists ( defined as B > k B T ( displaystyle B > k_ ( B ) T ), where k B ( displaystyle k_ ( B ) ) is Boltzmann 's constant and T ( displaystyle T ) is temperature of the system ). Because of this significant activation barrier, its unwinding progression is affected largely by the sequence of nucleic acids within the molecule to unwind, and the presence of destabilization forces acting on the replication fork. Certain nucleic acid combinations will decrease unwinding rates ( i.e. guanine and cytosine ), while various destabilizing forces can increase the unwinding rate. In passive systems, the rate of unwinding ( V u n ( displaystyle V_ ( un ) ) ) is less than the rate of translocation ( V t r a n s ( displaystyle V_ ( trans ) ) ) ( translocation along the single - strand nucleic acid, ssNA ). Another way to view the passive helicase is its reliance on the transient unraveling of the base pairs at the replication fork to determine its rate of unwinding. In active helicases, B < k B T ( displaystyle B < k_ ( B ) T ), where the system lacks a significant barrier, as the helicase is able to destabilize the nucleic acids, unwinding the double - helix at a constant rate, regardless of the nucleic acid sequence. In active helicases, V u n ( displaystyle V_ ( un ) ) is approximately equal to V t r a n s ( displaystyle V_ ( trans ) ). Another way to view the active helicase is its ability to directly destabilize the replication fork to promote unwinding. Active helicases show similar behavior when acting on both double - strand nucleic acids, dsNA, or ssNA, in regards to the rates of unwinding and rates of translocation, where in both systems V u n ( displaystyle V_ ( un ) ) and V t r a n s ( displaystyle V_ ( trans ) ) are approximately equal. These two categories of helicases may also be modelled as mechanisms. In such models the passive helicases are conceptualized as Brownian ratchets, driven by thermal fluctuations and subsequent anisotropic gradients across the DNA lattice. The active helicases, in contrast, are conceptualized as stepping motors - also known as powerstroke motors - utilizing either a conformational `` inch worm '' or a hand - over-hand `` walking '' mechanism to progress. Depending upon the organism, such helix - traversing progress can occur at rotational speeds in the range of 5,000 to 10,000 R.P.M. History of DNA helicases DNA helicases were discovered in E. coli in 1976. This helicase was described as a `` DNA unwinding enzyme '' that is `` found to denature DNA duplexes in an ATP - dependent reaction, without detectably degrading ''. The first eukaryotic DNA helicase was in 1978 in the lily plant. Since then, DNA helicases were discovered and isolated in other bacteria, viruses, yeast, flies, and higher eukaryotes. To date, at least 14 different helicases have been isolated from single celled organisms, 6 helicases from bacteriophages, 12 from viruses, 15 from yeast, 8 from plants, 11 from calf thymus, and approximately 25 helicases from human cells. Below is a history of helicase discovery : 1976 -- Discovery and isolation of E. coli - based DNA helicase 1978 -- Discovery of the first eukaryotic DNA helicases, isolated from the lily plant 1982 -- `` T4 gene 41 protein '' is the first reported bacteriophage DNA helicase 1985 -- First mammalian DNA helicases isolated from calf thymus 1986 -- SV40 large tumor antigen reported as a viral helicase ( 1st reported viral protein that was determined to serve as a DNA helicase ) 1986 -- ATPaseIII, a yeast protein, determined to be a DNA helicase 1988 -- Discovery of seven conserved amino acid domains determined to be helicase motifs 1989 -- Designation of DNA helicase Superfamily I and Superfamily II 1989 -- Identification of the DEAD box helicase family 1990 -- Isolation of a human DNA helicase 1992 -- Isolation of the first reported mitochondrial DNA helicase ( from bovine brain ) 1996 -- Report of the discovery of the first purified chloroplast DNA helicase from the pea 2002 -- Isolation and characterization of the first biochemically active malarial parasite DNA helicase -- Plasmodium cynomolgi. Structural features The common function of helicases accounts for the fact that they display a certain degree of amino acid sequence homology ; they all possess sequence motifs located in the interior of their primary structure, involved in ATP binding, ATP hydrolysis and translocation along the nucleic acid substrate. The variable portion of the amino acid sequence is related to the specific features of each helicase. The presence of these helicase motifs allows putative helicase activity to be attributed to a given protein, but does not necessarily confirm it as an active helicase. Conserved motifs do, however, support an evolutionary homology among enzymes. Based on these helicase motifs, a number of helicase superfamilies have been distinguished. Superfamilies Helicases are classified in 6 groups ( superfamilies ) based on their shared sequence motifs. Helicases not forming a ring structure are in superfamilies 1 and 2, and ring - forming helicases form part of superfamilies 3 to 6. Helicases are also classified as α or β depending on if they work with single or double - strand DNA ; α helicases work with single - strand DNA and β helicases work with double - strand DNA. They are also classified by translocation polarity. If translocation occurs 3 ' - 5 ' the helicase is type A ; if translocation occurs 5 ' - 3 ' it is type B. Superfamily 1 ( SF1 ) : This superfamily can be further subdivided into SF1A and SF1B helicases. In this group helicases can have either 3 ' - 5 ' ( SF1A subfamily ) or 5 ' - 3 ' ( SF1B subfamily ) translocation polarity. The most known SF1A helicases are Rep and UvrD in gram - negative bacteria and PcrA helicase from gram - positive bacteria. The most known Helicases in the SF1B group are RecD and Dda helicases. Superfamily 2 ( SF2 ) : This is the largest group of helicases that are involved in varied cellular processes. They are characterized by the presence of nine conserved motifs : Q, I, Ia, Ib, and II through VI. This group is mainly composed of DEAD - box RNA helicases. Some other helicases included in SF2 are the RecQ - like family and the Snf2 - like enzymes. Most of the SF2 helicases are type A with a few exceptions such as the XPD family. Superfamily 3 ( SF3 ) : Superfamily 3 consists of helicases encoded mainly by small DNA viruses and some large nucleocytoplasmic DNA viruses. They have a 3 ' - 5 ' translocation directionality, meaning that they are all type A helicases. The most known SF3 helicase is the papilloma virus E1 helicase. Superfamily 4 ( SF4 ) : All SF4 family helicases have a type B polarity ( 5 ' - 3 ' ). The most studied SF4 helicase is gp4 from bacteriophage T7. Superfamily 5 ( SF5 ) : Rho proteins conform the SF5 group. Superfamily 6 ( SF6 ) : They contain the core AAA+ that is not included in the SF3 classification. Some proteins in the SF6 group are : mini chromosome maintenance MCM, RuvB, RuvA, and RuvC. Helicase disorders and diseases ATRX helicase mutations The ATRX gene encodes the ATP - dependent helicase, ATRX ( also known as XH2 and XNP ) of the SNF2 subgroup family, that is thought to be responsible for functions such as chromatin remodeling, gene regulation, and DNA methylation. These functions assist in prevention of apoptosis, resulting in cortical size regulation, as well as a contribution to the survival of hippocampal and cortical structures, affecting memory and learning. This helicase is located on the X chromosome ( Xq13. 1 - q21. 1 ), in the pericentromeric heterochromatin and binds to heterochromatin protein 1. Studies have shown that ATRX plays a role in rDNA methylation and is essential for embyonic development. Mutations have been found throughout the ATRX protein, with over 90 % of them being located in the zinc finger and helicase domains. Mutations of ATRX can result in X-linked - alpha - thalassaemia - mental retardation ( ATR - X syndrome ). Various types of mutations found in ATRX have been found to be associated with ATR - X, including most commonly single - base missense mutations, as well as nonsense, frameshift, and deletion mutations. Characteristics of ATR - X include : microcephaly, skeletal and facial abnormalities, mental retardation, genital abnormalities, seizures, limited language use and ability, and alpha - thalassemia. The phenotype seen in ATR - X suggests that the mutation of ATRX gene causes the downregulation of gene expression, such as the alpha - globin genes. It is still unknown what causes the expression of the various characteristics of ATR - X in different patients. XPD helicase point mutations XPD ( Xeroderma pigmentosum factor D, also known as protein ERCC2 ) is a 5 ' - 3 ', Superfamily II, ATP - dependent helicase containing iron - sulphur cluster domains. Inherited point mutations in XPD helicase have been shown to be associated with accelerated aging disorders such as Cockayne syndrome ( CS ) and trichothiodystrophy ( TTD ). Cockayne syndrome and trichothiodystrophy are both developmental disorders involving sensitivity to UV light and premature aging, and Cockayne syndrome exhibits severe mental retardation from the time of birth. The XPD helicase mutation has also been implicated in xeroderma pigmentosa ( XP ), a disorder characterized by sensitivity to UV light and resulting in a several 1000-fold increase in the development of skin cancer. XPD is an essential component of the TFIIH complex, a transcription and repair factor in the cell. As part of this complex, it facilitates nucleotide excision repair by unwinding DNA. TFIIH assists in repairing damaged DNA such as sun damage. A mutation in the XPD helicase that helps form this complex and contributes to its function causes the sensitivity to sunlight seen in all three diseases, as well as the increased risk of cancer seen in XP and premature aging seen in trichothiodystrophy and Cockayne syndrome. XPD helicase mutations leading to trichothiodystrophy are found throughout the protein in various locations involved in protein - protein interactions. This mutation results in an unstable protein due to its inability to form stabilizing interactions with other proteins at the points of mutations. This, in turn, destabilizes the entire TFIIH complex, which leads to defects with transcription and repair mechanisms of the cell. It has been suggested that XPD helicase mutations leading to Cockayne syndrome could be the result of mutations within XPD, causing rigidity of the protein and subsequent inability to switch from repair functions to transcription functions due to a `` locking '' in repair mode. This could cause the helicase to cut DNA segments meant for transcription. Although current evidence points to a defect in the XPD helicase resulting in a loss of flexibility in the protein in cases of Cockayne syndrome, it is still unclear how this protein structure leads to the symptoms described in Cockayne syndrome. In xeroderma pigmentosa, the XPD helicase mutation exists at the site of ATP or DNA binding. This results in a structurally functional helicase able to facilitate transcription, however it inhibits its function in unwinding DNA and DNA repair. The lack of cell 's ability to repair mutations, such as those caused by sun damage, is the cause of the high cancer rate in xeroderma pigmentosa patients. RecQ family mutations RecQ helicase RecQ helicases ( 3 ' - 5 ' ) belong to the Superfamily II group of helicases, which help to maintain stability of the genome and suppress inappropriate recombination. Deficiencies and / or mutations in RecQ family helicases display aberrant genetic recombination and / or DNA replication, which leads to chromosomal instability and an overall decreased ability to proliferate. Mutations in RecQ family helicases BLM, RECQL4, and WRN, which play a role in regulating homologous recombination, have been shown to result in the autosomal recessive diseases Bloom syndrome ( BS ), Rothmund - Thomson syndrome ( RTS ), and Werner syndrome ( WS ), respectively. Bloom syndrome is characterized by a predisposition to cancer with early onset, with a mean age - of - onset of 24 years. Cells of Bloom syndrome patients show a high frequency of reciprocal exchange between sister chromatids ( SCEs ) and excessive chromosomal damage. There is evidence to suggest that BLM plays a role in rescuing disrupted DNA replication at replication forks. Werner syndrome is a disorder of premature aging, with symptoms including early onset of atherosclerosis and osteoporosis and other age related diseases, a high occurrence of sarcoma, and death often occurring from myocardial infarction or cancer in the 4th to 6th decade of life. Cells of Werner syndrome patients exhibit a reduced reproductive lifespan with chromosomal breaks and translocations, as well as large deletions of chromosomal components, causing genomic instability. Rothmund - Thomson syndrome, also known as poikiloderma congenitale, is characterized by premature aging, skin and skeletal abnormalities, rash, poikiloderma, juvenile cataracts, and a predisposition to cancers such as osteosarcomas. Chromosomal rearrangements causing genomic instability are found in the cells of Rothmund - Thomson syndrome patients. Meiotic recombination During meiosis DNA double - strand breaks and other DNA damages in a chromatid are repaired by homologous recombination using either the sister chromatid or a homologous non-sister chromatid as template. This repair can result in a crossover ( CO ) or, more frequently, a non-crossover ( NCO ) recombinant. In the yeast Schizosaccharomyces pombe the FANCM - family DNA helicase FmI1 directs NCO recombination formation during meiosis. The RecQ - type helicase Rqh1 also directs NCO meiotic recombination. These helicases, through their ability to unwind D - loop intermediates, promote NCO recombination by the process of synthesis - dependent strand annealing. In the plant Arabidopsis thaliana, FANCM helicase promotes NCO and antagonizes the formation of CO recombinants. Another helicase, RECQ4A / B, also independently reduces COs. It was suggested that COs are restricted because of the long term costs of CO recombination, that is, the breaking up of favorable genetic combinations of alleles built up by past natural selection. RNA helicases Human DEAD - box RNA helicase This image represents the different promoter sequences and accessory domains that aid in RNA unwinding ( local strand separation ). The regions in red are ATP binding domains and the regions in yellow are RNA interaction domains. Specific sequences termed DEAD box proteins are also present that help catalyze reactions in which ATP does not need to be directly hydrolyzed, as long as it binds to the domains on the strand. RNA helicases are essential for most processes of RNA metabolism such as ribosome biogenesis, pre-mRNA splicing, and translation initiation. They also play an important role in sensing viral RNAs. RNA helicases are involved in the mediation of antiviral immune response because they can identify foreign RNAs in vertebrates. About 80 % of all viruses are RNA viruses and they contain their own RNA helicases. Defective RNA helicases have been linked to cancers, infectious diseases and neuro - degenerative disorders. Some neurological disorders associated with defective RNA helicases are : amyotrophic lateral sclerosis, spinal muscular atrophy, spinocerebellar ataxia type - 2, Alzheimer disease, and lethal congenital contracture syndrome. RNA helicases and DNA helicases can be found together in all the helicase superfamilies except for SF6. All the eukaryotic RNA helicases that have been identified up to date are non-ring forming and are part of SF1 and SF2. On the other hand, ring - forming RNA helicases have been found in bacteria and viruses. However, not all RNA helicases exhibit helicase activity as defined by enzymatic function, i.e., proteins of the Swi / Snf family. Although these proteins carry the typical helicase motifs, hydrolize ATP in a nucleic acid - dependent manner, and are built around a helicase core, in general, no unwinding activity is observed. RNA helicases that do exhibit unwinding activity have been characterized by at least two different mechanisms : canonical duplex unwinding and local strand separation. Canonical duplex unwinding is the stepwise directional separation of a duplex strand, as described above, for DNA unwinding. However, local strand separation occurs by a process wherein the helicase enzyme is loaded at any place along the duplex. This is usually aided by a single - strand region of the RNA, and the loading of the enzyme is accompanied with ATP binding. Once the helicase and ATP are bound, local strand separation occurs, which requires binding of ATP but not the actual process of ATP hydrolysis. Presented with fewer base pairs the duplex then dissociates without further assistance from the enzyme. This mode of unwinding is used by the DEAD / DEAH box helicases. An RNA helicase database is currently available online that contains a comprehensive list of RNA helicases with information such as sequence, structure, and biochemical and cellular functions. Diagnostic tools for helicase measurement Measuring and monitoring helicase activity Various methods are used to measure helicase activity in vitro. These methods range from assays that are qualitative ( assays that usually entail results that do not involve values or measurements ) to quantitative ( assays with numerical results that can be utilized in statistical and numerical analysis ). In 1982 - 1983, the first direct biochemical assay was developed for measuring helicase activity. This method was called a `` strand displacement assay ''. Strand displacement assay involves the radiolabeling of DNA duplexes. Following helicase treatment, the single - strand DNA is visually detected as separate from the double - strand DNA by non-denaturing PAGE electrophoresis. Following detection of the single - strand DNA, the amount of radioactive tag that is on the single - strand DNA is quantified to give a numerical value for the amount of double - strand DNA unwinding. The strand displacement assay is acceptable for qualitative analysis, its inability to display results for more than a single time point, its time consumption, and its dependence on radioactive compounds for labeling warranted the need for development of diagnostics that can monitor helicase activity in real time. Other methods were later developed that incorporated some, if not all of the following : high - throughput mechanics, the use of non-radioactive nucleotide labeling, faster reaction time / less time consumption, real - time monitoring of helicase activity ( using kinetic measurement instead of endpoint / single point analysis ). These methodologies include : `` a rapid quench flow method, fluorescence - based assays, filtration assays, a scintillation proximity assay, a time resolved fluorescence resonance energy transfer assay, an assay based on flashplate technology, homogenous time - resolved fluorescence quenching assays, and electrochemiluminescence - based helicase assays ''. With the use of specialized mathematical equations, some of these assays can be utilized to determine how many base paired nucleotides a helicase can break per hydrolysis of 1 ATP molecule. Commercially available diagnostic kits are also available. One such kit is the `` Trupoint '' diagnostic assay from PerkinElmer, Inc. This assay is a time - resolved fluorescence quenching assay that utilizes the PerkinEmer `` SignalClimb '' technology that is based on two labels that bind in close proximity to one another but on opposite DNA strands. One label is a fluorescent lanthanide chelate, which serves as the label that is monitored through an adequate 96 / 384 well plate reader. The other label is an organic quencher molecule. The basis of this assay is the `` quenching '' or repressing of the lanthanide chelate signal by the organic quencher molecule when the two are in close proximity - as they would be when the DNA duplex is in its native state. Upon helicase activity on the duplex, the quencher and lanthanide labels get separated as the DNA is unwound. This loss in proximity negates the quenchers ability to repress the lanthanide signal, causing a detectable increase in fluorescence that is representative of the amount of unwound DNA and can be used as a quantifiable measurement of helicase activity. The execution and use of single - molecule fluorescence imaging techniques, focusing on methods that include optical trapping in conjunction with epifluorescent imaging, and also surface immobilization in conjunction with total internal reflection fluorescence visualization. Combined with microchannel flow cells and microfluidic control, allow individual fluorescently labeled protein and DNA molecules to be imaged and tracked, affording measurement of DNA unwinding and translocation at single - molecule resolution. Determining helicase polarity Helicase polarity, which is also deemed `` directionality '', is defined as the direction ( characterized as 5 ' → 3 ' or 3 ' → 5 ' ) of helicase movement on the DNA / RNA single - strand along which it is moving. This determination of polarity is vital in f. ex. determining whether the tested helicase attaches to the DNA leading strand, or the DNA lagging strand. To characterize this helicase feature, a partially duplex DNA is used as the substrate that has a central single - strand DNA region with different lengths of duplex regions of DNA ( one short region that runs 5 ' → 3 ' and one longer region that runs 3 ' → 5 ' ) on both sides of this region. Once the helicase is added to that central single - strand region, the polarity is determined by characterization on the newly formed single - strand DNA. See also Chromodomain helicase DNA binding protein : CHD1, CHD1L, CHD2, CHD3, CHD4, CHD5, CHD6, CHD7, CHD8, CHD9 DEAD box / DEAD / DEAH box helicase : DDX3X, DDX5, DDX6, DDX10, DDX11, DDX12, DDX58, DHX8, DHX9, DHX37, DHX40, DHX58 ASCC3, BLM, BRIP1, DNA2, FBXO18, FBXO30, HELB, HELLS, HELQ, HELZ, HFM1, HLTF, IFIH1, NAV2, PIF1, RECQL, RTEL1, SHPRH, SMARCA4, SMARCAL1, WRN, WRNIP1 References Jump up ^ Wu Y ( 2012 ). `` Unwinding and rewinding : double faces of helicase ? ''. J Nucleic Acids. 2012 : 140601. doi : 10.1155 / 2012 / 140601. PMC 3409536. PMID 22888405. Jump up ^ Umate P, Tuteja N, Tuteja R ( January 2011 ). `` Genome - wide comprehensive analysis of human helicases ''. Commun Integr Biol. 4 ( 1 ) : 118 -- 37. doi : 10.4161 / cib. 13844. PMC 3073292. PMID 21509200. ^ Jump up to : Patel, S.S. ; Donmez, I ( 2006 ). `` Mechanisms of Helicases ''. Journal of Biological Chemistry. 281 ( 27 ) : 18265 -- 18268. doi : 10.1074 / jbc. R600008200. ISSN 0021 - 9258. PMID 16670085. 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DNA Helicases at the US National Library of Medicine Medical Subject Headings ( MeSH ) RNA Helicases at the US National Library of Medicine Medical Subject Headings ( MeSH ) DNA replication ( comparing Prokaryotic to Eukaryotic ) Initiation Prokaryotic ( initiation ) Pre-replication complex dnaC Cdc6 Helicase dnaA dnaB T7 Primase dnaG Eukaryotic ( preparation in G1 phase ) Pre-replication complex Origin recognition complex ORC1 ORC2 ORC3 ORC4 ORC5 ORC6 Cdc6 Cdt1 Minichromosome maintenance MCM2 MCM3 MCM4 MCM5 MCM6 MCM7 Licensing factor Autonomously replicating sequence Single - strand binding protein SSBP2 SSBP3 SSBP4 RNase H RNASEH1 RNASEH2A Helicase : HFM1 Primase : PRIM1 PRIM2 Both Origin of replication / Ori / Replicon Replication fork Lagging and leading strands Okazaki fragments Primer Replication Prokaryotic ( elongation ) DNA polymerase III holoenzyme dnaC dnaE dnaH dnaN dnaQ dnaT dnaX holA holB holC holD holE Replisome DNA ligase DNA clamp Topoisomerase DNA gyrase Prokaryotic DNA polymerase : DNA polymerase I Klenow fragment Eukaryotic ( synthesis in S phase ) Replication factor C RFC1 Flap endonuclease FEN1 Topoisomerase Replication protein A RPA1 Eukaryotic DNA polymerase : alpha POLA1 POLA2 PRIM1 PRIM2 delta POLD1 POLD2 POLD3 POLD4 epsilon POLE POLE2 POLE3 POLE4 DNA clamp PCNA Control of chromosome duplication Both Movement : Processivity DNA ligase Termination Telomere : Telomerase TERT TERC DKC1 DNA repair Excision repair Base excision repair / AP site DNA glycosylase Uracil - DNA glycosylase Poly ADP ribose polymerase Nucleotide excision repair / ERCC XPA XPB XPC XPD / ERCC2 XPE / DDB1 XPF / DDB1 XPG / ERCC5 ERCC1 RPA RAD23A RAD23B Excinuclease DNA mismatch repair MLH1 MSH2 Other forms of repair Transcription - coupled repair ERCC6 ERCC8 Homology directed repair Non-homologous end joining Ku Microhomology - mediated end joining Postreplication repair Photolyase CRY1 CRY2 Other / ungrouped proteins Ogt PcrA Proliferating Cell Nuclear Antigen Homologous recombination RecA / RAD51 Sgs1 Slx4 Regulation SOS box SOS response Other / ungrouped 8 - Oxoguanine Adaptive response Meiotic recombination checkpoint RecF pathway DNA helicase : BLM WRN FANC proteins : core protein complex FANCA FANCB FANCC FANCE FANCF FANCG FANCL FANCM FANCD1 FANCD2 FANCI FANCJ FANCN Transferases : phosphorus - containing groups ( EC 2.7 ) 2.7. 1 - 2.7. 4 : phosphotransferase / kinase ( PO ) 2.7. 1 : OH acceptor Hexo - Gluco - Fructo - Hepatic Galacto - Phosphofructo - Liver Muscle Platelet Riboflavin Shikimate Thymidine ADP - thymidine NAD Glycerol Pantothenate Mevalonate Pyruvate Deoxycytidine PFP Diacylglycerol Phosphoinositide 3 Class I PI 3 Class II PI 3 Sphingosine Glucose - 1, 6 - bisphosphate synthase 2.7. 2 : COOH acceptor Phosphoglycerate Aspartate kinase 2.7. 3 : N acceptor Creatine 2.7. 4 : PO acceptor Phosphomevalonate Adenylate Nucleoside - diphosphate Uridylate Guanylate Thiamine - diphosphate 2.7. 6 : diphosphotransferase ( P O ) Ribose - phosphate diphosphokinase Thiamine diphosphokinase 2.7. 7 : nucleotidyltransferase ( PO - nucleoside ) Polymerase DNA polymerase DNA - directed DNA polymerase II III IV V RNA - directed DNA polymerase Reverse transcriptase Telomerase DNA nucleotidylexotransferase / Terminal deoxynucleotidyl transferase RNA nucleotidyltransferase RNA polymerase / DNA - directed RNA polymerase RNA polymerase I II III IV V Primase RNA - dependent RNA polymerase PNPase Phosphorolytic 3 ' to 5 ' exoribonuclease RNase PH PNPase Nucleotidyltransferase UTP -- glucose - 1 - phosphate uridylyltransferase Galactose - 1 - phosphate uridylyltransferase Guanylyltransferase mRNA capping enzyme Other Recombinase ( Integrase ) Transposase 2.7. 8 : miscellaneous Phosphatidyltransferases CDP - diacylglycerol -- glycerol - 3 - phosphate 3 - phosphatidyltransferase CDP - diacylglycerol -- serine O - phosphatidyltransferase CDP - diacylglycerol -- inositol 3 - phosphatidyltransferase CDP - diacylglycerol -- choline O - phosphatidyltransferase Glycosyl - 1 - phosphotransferase N - acetylglucosamine - 1 - phosphate transferase 2.7. 10 - 2.7. 13 : protein kinase ( PO ; protein acceptor ) 2.7. 10 : protein - tyrosine see tyrosine kinases 2.7. 11 : protein - serine / threonine see serine / threonine - specific protein kinases 2.7. 12 : protein - dual - specificity see serine / threonine - specific protein kinases 2.7. 13 : protein - histidine Protein - histidine pros - kinase Protein - histidine tele - kinase Histidine kinase Hydrolases : acid anhydride hydrolases ( EC 3.6 ) 3.6. 1 Pyrophosphatase Inorganic Thiamine Apyrase Thiamine - triphosphatase 3.6. 2 Adenylylsulfatase Phosphoadenylylsulfatase 3.6. 3 - 4 : ATPase 3.6. 3 Cu++ ( 3.6. 3.4 ) Menkes / ATP7A Wilson / ATP7B Ca+ ( 3.6. 3.8 ) SERCA ATP2A1 ATP2A2 ATP2A3 Plasma membrane ATP2B1 ATP2B2 ATP2B3 ATP2B4 SPCA ATP2C1 ATP2C2 Na+ / K+ ( 3.6. 3.9 ) ATP1A1 ATP1A2 ATP1A3 ATP1A4 ATP1B1 ATP1B2 ATP1B3 ATP1B4 H+ / K+ ( 3.6. 3.10 ) ATP4A Other P - type ATPase ATP8B1 ATP10A ATP11B ATP12A ATP13A2 ATP13A3 3.6. 4 Dynein Kinesin Myosin Katanin 3.6. 5 : GTPase 3.6. 5.1 : Heterotrimeric G protein GNAI1 GNAI2 GNAI3 GNAQ GNA11 GNA12 GNA13 Transducin GNAT1 GNAT2 Gustducin GNAT3 3.6. 5.2 : Small GTPase > Ras superfamily Rho family of GTPases : Cdc42 CDC42 TC10 TCL RhoUV RhoU RhoV Rac Rac1 RhoG RhoBTB RhoH Rho Rnd RhoDF RhoF RhoD other : Ras HRAS KRAS NRAS Rab RAB23 RAB27 Arf ARF6 SAR1B ARL13B ARL6 Ran Rheb Rap RGK 3.6. 5.3 : Protein - synthesizing GTPase Prokaryotic IF - 2 EF - Tu EF - G Eukaryotic 3.6. 5.5 - 6 : Polymerization motors Dynamin Tubulin Enzymes Activity Active site Binding site Catalytic triad Oxyanion hole Enzyme promiscuity Catalytically perfect enzyme Coenzyme Cofactor Enzyme catalysis Regulation Allosteric regulation Cooperativity Enzyme inhibitor Classification EC number Enzyme superfamily Enzyme family List of enzymes Kinetics Enzyme kinetics Eadie -- Hofstee diagram Hanes -- Woolf plot Lineweaver -- Burk plot Michaelis -- Menten kinetics Types EC1 Oxidoreductases ( list ) EC2 Transferases ( list ) EC3 Hydrolases ( list ) EC4 Lyases ( list ) EC5 Isomerases ( list ) EC6 Ligases ( list ) Molecular and Cellular Biology portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Helicase&oldid=829954200 '' Categories : EC 3.6. 4 DNA replication Talk Search Contents About Wikipedia Bosanski Català Čeština Dansk Deutsch Eesti Ελληνικά Español فارسی Français Galego Bahasa Indonesia Italiano עברית ქართული Қазақша Lietuvių Македонски Nederlands 日本 語 Norsk nynorsk Occitan Polski Português Română Русский کوردی Српски / srpski Srpskohrvatski / српскохрватски Svenska Türkçe Українська 中文 25 more Edit links This page was last edited on 11 March 2018, at 20 : 45. 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Question: where does helicase separate the strands of dna
Answer: directionally along a nucleic acid phosphodiester backbone