Abstract:
Provided are an adaptor element in a bubble shape, a method of constructing a sequencing library with such an adapter element. The adaptor element is a hybrid formed with a long-chain nucleic acid A and a short-chain nucleic acid B. The hybrid is in the bubble shape with paired regions at two terminals and a non-paired region in the middle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is the U.S. national phase of PCT Application No. PCT/CN2014/091952 filed with the State Intellectual Property Office of P. R. China on Nov. 21, 2014, the entire content of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to the field of biotechnology, in particular, to an adaptor element in a bubble shape, a method of constructing a sequencing library with such an adapter element, a constructed sequencing library, and a use thereof. 
       BACKGROUND 
       [0003]    It has been undergoing dramatic changes with unimaginable pace for Deoxyribonucleic acid (DNA) sequencing technology since a capillary electrophoresis sequencer was launched by AB Company in the nineties of last century. The second- and third-generation of sequencer have also been brought to market in succession recently. 
         [0004]    It has great prospects for the Blackbird sequencing platform from Complete Genomics Company (referred to as CG for brevity), among other next generation sequencing platform, when applied in the field of clinical research such as molecular diagnosis, relying on their advantages of higher accuracy (99.9998%) and larger throughput over others. However, it is an urgent and important task to simplify processes for library construction and shorten period consumed by library construction by means of technique optimization due to overlong period consumed by the CG sequencing based library construction, which impedes progress of research and program development badly, and challenges caused by rapid rise of various new-generation sequencing technologies, thereby shortening the consumed period for CG platform and reducing manufacture cost. 
         [0005]    An adaptor is a specially designed DNA sequencing, which can be recognized as a start site for sequencing after ligated at two ends of a target DNA fragment, such that sequence information thereafter can be obtained by a sequencer. For guaranteeing that the sequence information obtained is easy to be analysis, one target DNA fragment should be ligated with two adaptors having two different sequences respectively at two terminals (i.e., 5′-terminal and 3′-terminal). 
         [0006]    In general, a conventional process for constructing a sequencing library based on CG sequencing platform is achieved with four adaptors, thus leading to cumbersome processes and long term for library construction. The method of constructing a sequencing library is improved by CG with only two adaptors, such that the period for library construction is reduced by about a half. Such a method, mainly including: fragmenting a genomic DNA sample; ligating an adaptor and performing Polymerase Chain Reaction (PCR) for first time; cyclizing a double stranded DNA fragment followed by digestion; and ligating an adaptor and performing PCR for second time; and isolating a single stranded DNA fragment followed by cyclization, can ensure the adaptor to be ligated in a certain direction and minimize interconnection between DNA fragments by means of the specially designed sequence, which still requires six steps of enzyme digestion for each time of the adaptor ligation. As a result, this library constructing method above is also over cumbersome, reflected by too many steps of enzyme digestion and purification which bring disadvantages such as low efficiency, high cost and low sample recovery. Accordingly, in order to obtain enough amount for sequencing on a DNA sequencer, two times of PCR amplification are needed, which further increases the cost for library construction. In addition, for enriching a specific single stranded DNA fragment during the isolating step, two times of enriching and isolating are also required, which also decreases efficiency of library construction. Therefore, although four adaptors based library constructing method is simplified into two adaptors based, it still needs to shorten the process, and reduce the high cost and initial DNA amount for the library construction. 
         [0007]    The present disclosure is proposed to reduce steps of ligating adaptors and cycles of PCR amplification in the library construction based on CG sequencing platform, and thus shortening overall time consumed for the library construction, and reducing cost and initial DNA amount for the library construction. 
       SUMMARY 
       [0008]    In view of the above deficiencies of the related art, it is objects of the present disclosure to provide an adaptor element in an adaptor element in a bubble shape, a method of constructing a sequencing library with such an adapter element, a constructed sequencing library, and a use thereof. In the method proposed by the present disclosure, the cyclic double stranded nucleic acid (after first cyclization) is captured with avidin-coated magnetic beads before or after digestion with class III restriction endonuclease, after which digested linear double stranded nucleic acid is always combined on the magnetic beads, where the subsequent steps including ligation of the adaptor element 2 and enzyme digestion to remove the fragment B1 at the 5′-terminal of the adaptor element 2 are performed. Subsequently, the magnetic beads combined with the resulting DNA product are placed on a magnetic separator for adsorption, followed by discarding supernatant obtained and washing off residual impurity from the magnetic beads using a proper buffer, such that the purpose of purifying the resulting DNA product is achieved. As a result, ligating steps, PCR amplification cycles, overall time-consuming and cost-consuming, as well as initial DNA amount for library construction are reduced for the GC sequencing based library construction, thereby improving efficiency thereof. As compared with the related art, the overall period and the cost-consuming are reduce by a half, and the initial DNA amount for library construction was decreased from 3 μg to 1 μg. 
         [0009]    In a first aspect, the present disclosure provides in embodiments an adaptor element. The adaptor element is a hybrid formed with a long-chain nucleic acid A and a short-chain nucleic acid B, in which the hybrid is in a bubble shape with paired regions at two terminals and a non-paired region in the middle, and the hybrid has a phosphorylated base at the 5′-terminal of the long-chain nucleic acid A and an overhung Thymine (T) at the 3′-terminal of the short-chain nucleic acid B, 
         [0010]    optionally, the hybrid contains a recognition site of class III restriction endonuclease, 
         [0011]    preferably, the recognition site of class III restriction endonuclease is 0-2 bp away from where the adaptor element is ligated to a target DNA fragment. 
         [0012]    As to the adaptor element, it is preferred that the hybrid comprises three regions: 
         [0013]    a first region, including a fragment A1 at the 3′-terminal of the long-chain nucleic acid A and a fragment B1 at the 5′-terminal of the short-chain nucleic acid B, wherein part of the fragment A1 is complementary combined with the fragment B1, thereby forming first paired double strands and a non-paired single strand at the 3′-terminal of the long-chain nucleic acid A, 
         [0014]    a second region, including a fragment A2 in the middle of the long-chain nucleic acid A and a fragment B2 in the middle of the short-chain nucleic acid B, wherein the fragment A2 is not complementary to the fragment B2, thereby forming the bubble shape consisting of two non-paired single strands; and 
         [0015]    a third region, including a fragment A3 at the 5′-terminal of the long-chain nucleic acid A and a fragment B3 at the 3′-terminal of the short-chain nucleic acid B, wherein
       the fragment A3 has the phosphorylated base at the 5′-terminal thereof;   the fragment B3 has the overhung Thymine (T) at the 3′-terminal thereof;   the fragment B3, except the overhung Thymine (T) at the 3′-terminal, are complementary to the fragment A3, thereby forming second paired double strands and the overhung Thymine (T) at the 3′-terminal of the short-chain nucleic acid,       
 
         [0019]    optionally, the third region contains a sequence capable of being recognized by class III restriction endonuclease. 
         [0020]    Further preferably, the fragment A1 has a length of 10 nt to 20 nt, the fragment B1 has a length of 8 nt to 16 nt, the fragment A2 has a length of 11 to 36 nt, the fragment B2 has a length of 11 nt to 21 nt, the fragment A3 has a length of 6 nt to 8 nt, and the fragment B3 has a length of 7 nt to 9 nt. 
         [0021]    Further preferably, the fragment B1 contains a site, preferably Uracil (U) or deoxyuridine (dU), recognizable and digestible by an enzyme, preferably a USER enzyme. 
         [0022]    Further preferably, the fragment A2 includes a barcode sequence. 
         [0023]    Further preferably, the class III restriction endonuclease is Acu I, Bpm I, BceA I, Bbv I, BciV I, BpuE I, BseM II, BseR I, Bsg I, BsmF I, BtgZ I, Eci I, EcoP15 I, Eco57M I, Fok I, Hga I, Hph I, Mbo II, Mnl I, SfaN I, TspDT I, TspDW I or Taq II. 
         [0024]    In a preferable embodiment, the long-chain nucleic acid A of the adaptor element has a sequence of 5′-/Phos/CTGCTGACGTACTG(N)AGCACGAGACGTTCTCGACT/ddC/-3′, where /Phos/ represents a phosphorylation modification, N is a barcode sequence, /ddC/ represents dideoxycytidine; preferably, the barcode sequence has a length of 6nt to 10nt, more preferably, the barcode sequence is 5′-TGTCATAAAT-3′. That is, in a more preferable embodiment, the long-chain nucleic acid A of the adaptor element is of a sequence of 5′-/Phos/CTGCTGACGTACTGTGTCATAAATAGCACGAGACGTTCTCGACT/ddC/-3′ (SEQ ID NO:1). The short-chain nucleic acid B (SEQ ID NO:2) has a sequence of 5′-GAGAACGUCTCGTGCUACGTTCTCGACTCAGCAGT-3′. 
         [0025]    In another preferable embodiment, the long-chain nucleic acid A of the adaptor element has a sequence of 5′-/Phos/AGTCGGAGGCCAAGCGTGCTTAGGACAT-3′, where /Phos/ represents a phosphorylation modification, and 
         [0026]    the short-chain nucleic acid B has a sequence of 5 ‘-GTCCTAAGCACUGTAGTGTACGATCCGACTT-3’. 
         [0027]    In a second aspect, the present disclosure provides in embodiments a method of ligating an adaptor element, i.e., ligating the adaptor element described in the first aspect to a DNA fragment of interest respectively at both terminals. 
         [0028]    In embodiments, the adaptor element is ligated to the DNA fragment of interest respectively at two terminals by means of ligating reaction. 
         [0029]    Preferably, the method further includes: subjecting the DNA fragment of interest to end-repairing, phosphorylation at the 5 ‘-terminal and adenine (A) addition to the 3’-terminal prior to ligating the adaptor element. 
         [0030]    In some embodiments, for example, the DNA fragment of interest was end-repaired with DNA polymerase, preferably T4 DNA polymerase. In some embodiments, for example, the DNA fragment of interest was subjected to phosphorylation at the 5′-terminal with nucleoside kinase, preferably T4 polynucleotide kinase. In some embodiments, for example, the DNA fragment of interest was subjected to adenine (A) addition to the 3′-terminal with polymerase without 3′→5′ exonuclease activity, i.e., Klenow exo-. 
         [0031]    In a third aspect, the present disclosure provides in embodiments a method of constructing a sequencing library, including ligating the adaptor element described in the first aspect, or ligating the adaptor element by the method described in the second aspect. 
         [0032]    Preferably, the method of constructing the sequencing library including the following steps: 
         [0033]    1) subjecting a double stranded DNA fragment to end-repairing, phosphorylation at the 5′-terminal and adenine (A) addition to the 3′-terminal; 
         [0034]    preferably, the double stranded DNA fragment is prepared by: 
         [0035]    1a) fragmenting an mRNA sample, to obtain fragmented mRNA; 
         [0036]    1b) subjecting the fragmented mRNA to reverse transcription, so as to obtain a cDNA amplification product as the double stranded DNA fragment, 
         [0037]    optionally, the double stranded DNA fragment is obtained by fragmentation directly from a DNA sample, 
         [0038]    preferably, the double stranded DNA sample is fragmented in such a physical or chemical way, more preferably physical ultrasound or enzyme digestion, that the double stranded DNA sample is randomly broken or cleaved, 
         [0039]    preferably, the double stranded DNA fragment is end-repaired with T4 DNA polymerase, 
         [0040]    preferably, the double stranded DNA fragment after end-repaired is phosphorylated with nucleotide kinase, preferably T4 polynucleotide kinase, 
         [0041]    preferably, the double stranded DNA fragment after end-repaired and phosphorylated is added with adenine (A) at the 3′-terminal using Klenow polymerase without 3′ to 5′exonuclease activity, 
         [0042]    2) ligating an adaptor element 1 to both ends of the DNA fragment obtained in step 1), 
         [0043]    in a preferable embodiment, the long-chain nucleic acid A of the adaptor element 1 has a sequence of 5′-/Phos/CTGCTGACGTACTG(N)AGCACGAGACGTTCTCGACT/ddC/-3′, where /Phos/ represents a phosphorylation modification, N is a barcode sequence, /ddC/ represents dideoxycytidine, 
         [0044]    preferably, the barcode sequence has a length of 6nt to 10nt, more preferably, the barcode sequence is 5′-TGTCATAAAT-3′, 
         [0045]    i.e., in a more preferable embodiment, the long-chain nucleic acid A of the adaptor element 1 has a sequence of 5′-/Phos/CTGCTGACGTACTGTGTCATAAATAGCACGAGACGTTCTCGACT/ddC/-3′ (SEQ ID NO:1), 
         [0046]    the short-chain nucleic acid B of the adaptor element 1 has a sequence of 5′-GAGAACGUCTCGTGCUACGTTCTCGACTCAGCAGT-3′, 
         [0047]    3) subjecting the DNA fragment obtained in step 2) as a template to PCR amplification with a first pair of primers, one of which is complementary to the 3′-terminal of the long-chain nucleic acid of the adaptor element 1, and the other one of which is complementary to at least one of the fragment B2 and part of the fragment B3, 
         [0048]    the first pair of primers each contains a site in the middle thereof, preferably Uracil (U) or deoxyuridine (dU), recognizable and digestible by an enzyme, preferably a USER enzyme, 
         [0049]    one of the first pair of primers is labeled with a biotin, 
         [0050]    when the adaptor element 1 having the specific sequence described above is used, the first pair of primer includes: 
         [0051]    a primer 1 having a sequence of AGTCGAGAACGUCTCG/iBiodT/GCT (SEQ ID NO:5), where /iBiodT/ represents a biotin-labeled dT; and 
         [0052]    a primer 2 having a sequence of ACGTTCTCGACUCAGCAG (SEQ ID NO:6), 
         [0053]    4) cyclizing the amplified fragment obtained in step 3) with two sticky ends thereof generated by enzyme-digestion, to obtain a cyclic double stranded nucleic acid; 
         [0054]    5) digesting the cyclic double stranded nucleic acid obtained in step 4) with the class III restriction endonuclease, and then capturing digested DNA fragments with avidin-coated magnetic beads, preferably streptavidin-coated magnetic beads, or 
         [0055]    5′) capturing the cyclic double stranded nucleic acid obtained in step 4) with avidin-coated magnetic beads, preferably streptavidin-coated magnetic beads, and digesting captured cyclic double stranded nucleic acid with the class III restriction endonuclease, 
         [0056]    6) subjecting the digested double stranded DNA fragment obtained in step 5) or 5′) to end-repairing and adenine (A) addition to the 3′-terminal; 
         [0057]    7) ligating an adaptor element 2 to both ends of the DNA fragment obtained in step 6), wherein 
         [0058]    the adaptor element 2 has a sequence different from that of the adaptor element 1, 
         [0059]    the fragment B1 of the adaptor element 2 contains a site, preferably Uracil (U) or deoxyuridine (dU), recognizable and digestible by an enzyme, preferably a USER enzyme, 
         [0060]    when the adaptor element 1 having the specific sequence described above is used, preferably, the the long-chain nucleic acid A of the adaptor element 2 has a sequence of 5′-/Phos/AGTCGGAGGCCAAGCGTGCTTAGGACAT-3′, where /Phos/ represents a phosphorylation modification; and 
         [0061]    the short-chain nucleic acid B of the adaptor element 2 has a sequence of 5′-GTCCTAAGCACUGTAGTGTACGATCCGACTT-3′, 
         [0062]    8) digesting the DNA fragment ligated with the adaptor element 2 obtained in step 7) at the site recognizable and digestible by the enzyme in the fragment B1, followed by phosphorylation at the 5′-terminal of the adaptor element 2 after removing the fragment B1; 
         [0063]    9) denaturing phosphorylated DNA fragment obtained in step 8), preferably in an alkaline solution or under a high temperature, and then isolating non-biotin-labeled single stranded nucleic acid; 
         [0064]    10) cyclizing the non-biotin-labeled single stranded nucleic acid obtained in step 9) to form a cyclic single stranded nucleic acid, i.e., a sequencing library, 
         [0065]    preferably, the non-biotin-labeled single stranded nucleic acid is cyclized with a bridge fragment having two ends capable of complementary combining with two terminals of the non-biotin-labeled single stranded nucleic acid, 
         [0066]    when the adaptor element 2 having the specific sequence described above is used, preferably, the bridge fragment has a sequence of 5′-ATCGTACACTACATGTCCTAAGCA-3′ (SEQ ID NO:7), 
         [0067]    preferably, the method further includes: digesting the non-biotin-labeled single stranded nucleic acid which is not cyclized after cyclization in step 10), more preferably with at least one of exonuclease 1 and exonuclease 3. 
         [0068]    The cyclized single stranded nucleic acid obtained in step 10) can be directly subjected to the subsequent sequencing step, such that DNA nanoballs are formed after rolling circle amplification, for nucleic acid information acquirement. 
         [0069]    In a fourth aspect, the present disclosure provides in embodiments a sequencing library, constructed by the method described in the third aspect. Such a sequencing library constructed is a single stranded nucleic acid sequencing library. 
         [0070]    In a fifth aspect, the present disclosure provides in embodiments a use of the sequencing library of claim  7  in sequencing a genome, preferably targeted sequencing. 
         [0071]    Preferably, the sequencing library is sequenced on a platform that is used for sequencing cyclic single stranded nucleic acid libraries, preferable the CG sequencing platform. 
         [0072]    In a sixth aspect, the present disclosure provides in embodiments a method of sequencing a nucleic acid, including: 
         [0073]    sequencing a sequencing library described in the fourth aspect, preferably on a platform that is used for sequencing cyclic single stranded nucleic acid libraries, more preferably on the CG sequencing platform, 
         [0074]    preferably, the method further includes at least one of assembling and connecting the sequencing results. 
         [0075]    In a seventh aspect, the present disclosure provides in embodiments a kit for constructing a sequencing library, including the adaptor element described in the first aspect. 
         [0076]    Preferably, the kit further includes nucleoside kinase, preferably T4 polynucleotide kinase; DNA polymerase, preferably T4 DNA polymerase or polymerase without 3′→5′ exonuclease activity, eg., Klenow exo-; USER enzyme; and class III restriction endonuclease, for example, Acu I, Bpm I, BceA I, Bbv I, BciV I, BpuE I, BseM II, BseR I, Bsg I, BsmF I, BtgZ I, Eci I, EcoP15 I, Eco57M I, Fok I, Hga I, Hph I, Mbo II, Mnl I, SfaN I, TspDT I, TspDW I or Taq II. 
       Advantageous Effects 
       [0077]    As compared with the conventional method of constructing a sequencing library with two different adaptors in each step, the method proposed in the present disclosure includes ligating an adaptor element in a bubble shape in a simple way and enzyme-digesting with magnetic beads. 
         [0078]    The conventional method includes ligating the adaptor to the DNA fragment of interest in a certain direction, which ensures the adaptor to be ligated in a certain direction and minimizes interconnection between DNA fragments by means of the specially designed sequence, i.e., designing two different adaptors respectively targeting the 5′-terminal and the 3′-terminal, which are ligated in separate steps. As a result, adaptor ligation should be achieved with two adaptor sequences, a blocking sequence, and tow primer sequences together for each ligating step. Accordingly, 6 steps of enzyme digestion and 5 steps of purification are required, leading to cumbersome processes and highly consumed cost for library construction (cost for sequence, enzyme-digesting reagent, and purification), long period and high sample loss. 
         [0079]    However, the method including ligating adaptor element in the bubble shape proposed by the present disclosure ensures the adaptor to be ligated in a certain direction, improves efficiency of library construction, and reduces cost and initial DNA sample for the library construction. 
         [0080]    As it is designed that the DNA fragment of interest is added with “A”, the adaptor element in a bubble shape with an overhung Thymine (T) at the 3′-terminal can be effectively ligated to the DNA fragment of interest in a certain direction. As compared with the conventional method in the prior art including multiple ligating steps, the method proposed in the present disclosure achieves adaptor ligation in one step only with 3 to 4 steps of enzyme digestion, 3 steps of purification, which simplifies processes, shortens the period for library construction, and decreases initial DNA fragment from 3 μg to 1 μg because of less times of purification leading to less sample loss. 
         [0081]      FIG. 3  shows comparison between a method of ligating an adaptor element in a bubble shape according to an embodiment of the present disclosure and a conventional method of ligating an adaptor in a certain direction in the related art. As can be seen from  FIG. 3 , in the conventional method, the adaptor A is ligated to the DNA fragment of interest respectively at two terminals in a certain direction through dephosphorylating, end-repairing, ligating an adaptor at the 5′-terminal, extension with a primer, ligating an adaptor at the 3′-terminal, nick translation and connection, i.e., involving 6 steps of enzyme digestion and 5 steps of purification. However, in method proposed by the present disclosure, the adaptor element in the bubble shape is ligated to the DNA fragment of interest respectively at two terminals in a certain direction through one step of phosphorylating and end-repairing, adding “A”, ligating an adaptor element in the bubble shape respectively at the 5′-terminal and the 3′-terminal, digesting with USER enzyme (optional), i.e., involving 4 (or 3) steps of enzyme digestion, such that two adaptors can be ligated to the DNA fragment of interest rapidly. 
         [0082]    Besides, conventional enzyme digestion is performed in a solution. Accordingly, digested DNA fragment obtained is dissolved in the resulting reaction solution. In order to purified the digested DNA fragment after the enzyme reaction, remaining enzyme and other impurity are removed by silica column (eg., Qiagen PCR Purification Kit) or conventional magnetic beads (eg., Agencourt AMPure XP beads). However, such purification needs high cost reagents and long operation period, and causes 10% to 30% DNA sample loss after each step of purification. In the contrast, in the method proposed by the present disclosure, the cyclic double stranded nucleic acid (after first cyclization) is captured with avidin-coated magnetic beads before or after digestion with class III restriction endonuclease, after which digested linear double stranded nucleic acid is always combined on the magnetic beads, where the subsequent steps including ligation of the adaptor element 2 and enzyme digestion to remove the fragment B1 at the 5′-terminal of the adaptor element 2 are performed. Subsequently, the magnetic beads combined with the resulting DNA product are placed on a magnetic separator for adsorption, followed by discarding supernatant obtained and washing off residual impurity from the magnetic beads using a proper buffer, such that the purpose of purifying the resulting DNA product is achieved. Such a process is more convenient in that not only costs in purification reagent and DNA sample loss (about 1% loss only) are reduced, but also automatic operation is achieved, thereby improving work efficiency. 
         [0083]      FIG. 4  shows comparison between enzyme digestion in solution and enzyme digestion with magnetic beads, general steps from digesting a cyclic double stranded DNA fragment by class III restriction endonuclease to isolating a linear single stranded DNA fragment. In specific, it can be seen from  FIG. 4  that, two steps of purification with magnetic beads are required for the enzyme digestion in solution, so as to select digested DNA fragments with a desired size. However, such a method for selecting digested DNA fragment will also result in incorporation of some non-target digested DNA fragment with the same length. In the contrast, the enzyme digestion with magnetic beads used in the present disclosure includes combining the target DNA fragment with the magnetic beads by means of specific conjunction between biotin labeled in the adaptor A ligated to the target DNA fragment and streptavidin coated on the magnetic beads, such that the target digested DNA fragment can be enriched accurately. Additional two adaptors are ligated to the DNA fragment of interest through 6 steps of enzyme digestion and 6 steps of purification with magnetic beads by the conventional method; but through 2 steps of enzyme digestion and 2 steps of purification with magnetic beads by the method proposed by the present disclosure. 
         [0084]    In addition, in the conventional method, the targeted single stranded DNA fragment is obtained by ligating two additional adaptors, PCR amplification to enhance amount of target DNA fragment, isolating a single stranded nucleic acid, and capturing the specific single stranded nucleic acid. In the contrast, in the method proposed by the present disclosure, without PCR amplification after ligating the adaptor element 2, a non-target adaptor sequence (i.e., the fragment B1) is removed by digesting with USER enzyme while a phosphorylated base is formed at the 5′-terminal, after which the target single stranded DNA fragment is eluted by denaturation. Accordingly, the DNA sample loss is greatly reduced as enzyme digestion is performed on magnetic beads, such that sufficient amount of the target digested DNA fragment is obtained as a sequencing library for sequencing on a DNA sequencer without PCR amplification (PCR-free) after ligating the adaptor element 2. This not only omits a step of PCR amplification, but also simplifies a step of isolating a target digested single stranded nucleic acid, such that steps of ligating the adaptor element 2 and isolating the single stranded nucleic acid for library construction has been simplified to a great extent. In addition, as the cycles of PCR amplification are reduced, the volume of the sequencing library is increased, the number of repeated reads in the sequencing library is decreased, and the efficiency of obtaining sequencing data of the library is improved. 
         [0085]    In all, according to embodiments of the present disclosure, the method of constructing a sequencing library has been improved and optimized with the novel adaptor element in the bubble shape, by means of ligating the adaptor in a solution or with magnetic beads, such that the first time of ligating the adaptor is simplified from 6 steps of enzyme digestion and 5 steps of purification with magnetic bead to 4 steps of enzyme digestion and 3 steps of purification; and the second time of ligating the adaptor is simplified from 6 steps of enzyme digestion and 6 steps of purification with magnetic bead to 2 steps of enzyme digestion and 2 steps of washing magnetic beads, thereby omitting the second time of PCR amplification and one step of isolating and enriching the target single stranded nucleic, reducing overall period and cost consumed for the library construction, decreasing initial DNA amount for the library construction from 3 μg to 1 μg, and finally facilitating achievement of automation of high-throughput library construction and improvement of the efficiency of obtaining library sequencing data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0086]      FIG. 1  is a flow chart illustrating a method of constructing a sequencing library according to an embodiment of the present disclosure, in which 
           [0087]    1 fragmenting genomic DNA after fragment selection; 
           [0088]    2 end-repairing and phosphorylation; 
           [0089]    3 adding adenine (“A”) at the 3′ terminal; 
           [0090]    4 ligating the adaptor elements 1 respectively at two terminals; 
           [0091]    5 digesting with a USER enzyme (i.e., the fragment B1 is removed); (such a step is optional); 
           [0092]    6 PCR amplification; 
           [0093]    7 enzyme digesting/cyclizing the double stranded DNA wherein obtained cyclic double stranded nucleic acid labeled with a biotin can be captured by streptavidin-coated beads; 
           [0094]    8 digesting with class III restriction endonucleases; 
           [0095]    9 end-repairing and adding “A”; 
           [0096]    10 ligating the adaptor element 2 and digesting with a USER enzyme; 
           [0097]    11 eluting the single stranded DNA; 
           [0098]    12 cyclizing the single stranded DNA. 
           [0099]      FIG. 2  is a flow chart illustrating steps from ligating an adaptor element to PCR amplification in the method of constructing a sequencing library according to an embodiment of the present disclosure, in which 
           [0100]    1 is to ligate an adaptor element with an overhung Thymine (T) at the 3′-terminal (consisting of the first, second and third regions) to a DNA fragment with an overhung Thymine (T) at the 3′-terminal; 
           [0101]    2 is to digest a non-target adaptor sequence (i.e., the fragment B1) in a forward strand of the adaptor element 1 by a USER enzyme (such a step is optional); 
           [0102]    3 is to amplify the DNA fragment containing a target adaptor sequence with a pair of primer targeting forward and reverse strands of the target adaptor sequence respectively. 
           [0103]      FIG. 3  shows comparison between a method of ligating an adaptor element according to an embodiment of the present disclosure and a method of ligating an adaptor in a certain direction in the related art. 
           [0104]      FIG. 4  shows comparison between enzyme digestion in solution and enzyme digestion with beads, general steps from digesting a cyclic double stranded DNA fragment by class III restriction endonuclease to isolating a linear single stranded DNA fragment. 
           [0105]      FIG. 5  is an electrophoresis result showing the DNA fragment ligated with the adaptor element 1 obtained according to embodiments of the present disclosure, in which 
           [0106]    a sample loaded in a lane marked with “M” is a DNA ladder D2000 (Tiangen); 
           [0107]    a sample loaded in a lane marked with “C” is a product obtained in step 2, i.e., the DNA fragment without the adaptor element 1; 
           [0108]    samples loaded in lanes respectively marked with “1L” and “2L” are products 1 and 2 obtained in step 5, i.e., the DNA fragment ligated with the adaptor element 1; 
           [0109]    samples loaded in lanes respectively marked with “1P” and “2P” are products 1 and 2 obtained in step 6, i.e., PCR amplification products of the DNA fragment ligated with the adaptor element 1. 
           [0110]      FIG. 6  is an electrophoresis result showing the DNA fragment ligated with the adaptor element 2, in which 
           [0111]    a sample loaded in a lane marked with “M1” is a 500 bp DNA ladder (Tiangen); 
           [0112]    a sample loaded in a lane marked with “M2” is a DNA ladder D2000 (Tiangen); 
           [0113]    samples loaded in the lanes respectively marked “1 to 7” are products obtained in step 13, i.e., PCR amplification products 1 to 7 of the DNA fragment ligated with the adaptor element 2. 
           [0114]      FIG. 7  is an electrophoresis result showing the final product, in which 
           [0115]    a sample loaded in a lane marked with “M” is RiboRulerHigh Range RNA Ladder (ready-to-use) (Fermentas); 
           [0116]    samples loaded in the lanes respectively marked “1 to 7” are products obtained in step 16, i.e., the cyclic single stranded DNA libraries 1 to 7. 
       
    
    
     DETAILED DESCRIPTION 
       [0117]    For the purpose of understanding the present disclosure, the present disclosure is exemplified as follows. It will be appreciated for those skilled in the art that the examples are merely illustrative and should not be construed as a particular limit of the disclosure. 
       Example 1 the Process of Constructing a Sequencing Library Proposed by the Present Disclosure 
       [0118]    1. Genomic DNA Fragmentation 
         [0119]    The genomic DNA sample derived from Yanhuang 1# can be fragmented in several ways, such as physical ultrasonication and enzyme digestion, both of which has well-established procedures commercially available. In the present example, the physical ultrasonication was used for fragmentation. 
         [0120]    To a 96-well PCR plate, one polytetrafluoroethylene wire, 1 μg genomics DNA and Tris-EDTA (TE) buffer or nuclease-free water were added up to 100 μl for each well. After sealing, the 96-well PCR plate was placed onto Covaris E220 for fragmentation under conditions as below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Filling factor 
                 21% 
               
               
                   
                 Pressure (PIP) 
                 500 
               
               
                   
                 Pulse factor 
                 500 
               
               
                   
                 Fragmentation period 
                 20 s, twice 
               
               
                   
                   
               
             
          
         
       
     
         [0121]    2. Selection of Fragmented Genomic DNA 
         [0122]    The fragmented genomic DNA can be selected out by magnetic beads purification or gel-recovery. In the present example, the magnetic beads purification was used for selection. 
         [0123]    The fragmented genomic DNA was mixed with 45 μl Ampure XP magnetic beads to be uniform, followed by still standing for 7 min to 15 min. The first supernatant collected after placed onto magnetic separator for a while was mixed with 18 μl fresh Ampure XP magnetic beads to be uniform, followed by still standing for 7 min to 15 min After still standing on the magnetic separator for another while and thus removing the second supernatant, the remaining beads were washed with 75% ethanol twice. Those dried magnetic beads were then dissolved in 50 μl TE buffer to retrieve fragmented DNA, followed by still standing for 7 min to 15 min. 
         [0124]    3. Phosphorylating and End-Repairing Those Selected DNA Fragments in One Reaction 
         [0125]    A first solution was formulated with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 9.33 μl 
               
               
                   
                 10 × NEBNext end-repairing buffer 
                 7.00 μl 
               
               
                   
                 T4 DNA Polymerases (3 U/μl) 
                 1.87 μl 
               
               
                   
                 T4 Polynucleotide kinases (10 U/μl) 
                 1.40 μl 
               
               
                   
                 bovine serum albumin (20 mg/ml) 
                 0.40 μl 
               
               
                   
                 Total 
                 20.00 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0126]    Those selected DNA fragments obtained in step 3 was mixed with the first solution to be uniform followed by incubation at 20° C. for 30 minutes. After reaction, resulting product was purified with 70 μl Ampure XP magnetic beads and dissolved in 40 μl TE buffer. The resulting product can be purified in several ways, i.e., using magnetic beads, passing through a column, running a gel and isolating a target product therefrom, etc, which are used exchangeable. In the present example, the resulting product was purified with magnetic beads, unless otherwise specified. 
         [0127]    4. Adding “A” to the Phosphorylated and End-Repaired DNA Fragment Respectively at Two Terminals 
         [0128]    A second solution was formulated with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 11.45 μl  
               
               
                   
                 10 × NEB buffer 2 
                 6.00 μl 
               
               
                   
                 0.1M triphosadenine 
                 0.15 μl 
               
               
                   
                 Klenow fragment (3′-5′ Exo) (5 U/μl) 
                 2.40 μl 
               
               
                   
                 Total 
                 20.00 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0129]    The phosphorylated and end-repaired DNA fragment obtained in step 3 was mixed with the second solution to be uniform, followed by incubation at 37° C. for 30 minutes. The resulting product was purified with 60 μl Ampure XP magnetic beads and dissolved in 40 μl TE buffer. 
         [0130]    5. Ligating the Adaptor Element 1 
         [0131]    In the present example, the used adaptor element 1 has a long-chain nucleic acid 1 of /Phos/CTGCTGACGTACTGTGTCATAAATAGCACGAGACGTTCTCGACT/ddC/, and a short-chain nucleic acid 1 of GAGAACGUCTCGTGCUACGTTCTCGACTCAGCAGT. It should be note that the sequence is written from the 5′-end to the 3′-end in a left-to-right manner; “//” means a group therein is a modifying group for a terminal nucleotide, or a terminal nucleotide therein has been modified; “phos” indicates phosphorylation; “dd” indicates dideoxy; and “bio” represents biotin; and bases in bold font forms the barcode sequence. 
         [0132]    A third solution containing the adaptor element 1 (25 μM) was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 long-chain nucleic acid 1 (200 μM) 
                 12.5 μl 
               
               
                 short-chain nucleic acid 1 (200 μM) 
                 12.5 μl 
               
               
                 Sodium chloride (5M) 
                  1.2 μl 
               
               
                 trihydroxymethyl aminomethane - HCl (1M, pH 7.8) 
                  1.2 μl 
               
               
                 ethylenediamine tetraacetic acid disodium salt (20 mM) 
                  0.5 μl 
               
               
                 Enzyme-free water 
                 72.1 μl 
               
               
                 Total 
                 100.0 μl  
               
               
                   
               
             
          
         
       
     
         [0133]    A fourth solution for ligation was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 13.12 μl 
               
               
                   
                 Ligating buffer 1 
                 15.00 μl 
               
               
                   
                 T4 DNA Ligase (Rapid) (600 U/μl) (enzymatics) 
                  1.88 μl 
               
               
                   
                 Total 
                 30.00 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0134]    In the present example, the ligating buffer 1 used was formulated as below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 trihydroxymethyl aminomethane - HCl (1M, pH 7.8) 
                 150 mM  
               
               
                   
                 Polyethylene glycol 8000 
                 15% 
               
               
                   
                 Magnesium chloride 
                 30 mM 
               
               
                   
                 ribonucleotide triphosphate 
                  3 mM 
               
               
                   
                   
               
             
          
         
       
     
         [0135]    The DNA fragment added with “A” obtained in step 4 was mixed with 6 μl of the third solution (25 μM) to be uniform, thereby obtaining a first mixture. Such a first mixture was then mixed with the fourth solution for ligation to be uniform, followed by incubation at 20° C. for 30 minutes, thereby obtaining a second mixture. After the reaction, the second mixture was further mixed with 35 μl of ethylenediamine tetraacetic acid disodium salt (35 μM) to be uniform, thereby obtaining a third mixture. Such a third mixture was purified with 50 μl Ampure XP magnetic beads, and then dissolved in 52 μl TE buffer to retrieve the DNA product ligated with the adaptor element 1. 
         [0136]    Such steps achieve ligation of the adaptor element 1 to a target nucleic acid fragment. The electrophoresis result showing the DNA fragment ligated with and without the adaptor element 1 as well as PCR amplified product for each is shown in  FIG. 5 . 
         [0137]    6. First PCR Amplification 
         [0138]    A primer 1 has a sequence of AGTCGAGAACGUCTCG/iBiodT/GCT (SEQ ID NO:5). 
         [0139]    A primer 2 has a sequence of ACGTTCTCGACUCAGCAG (SEQ ID NO:6). 
         [0140]    A fifth solution was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 142 μl 
               
               
                   
                 2× PfuTurbo Cx Buffer (Agilent) 
                 225 μl 
               
               
                   
                 PfuTurbo Cx Hot start nucleic acid 
                  11 μl 
               
               
                   
                 polymerase (2.5 U/μl) (Agilent) 
               
               
                   
                 20 μM Primer 1 
                  11 μl 
               
               
                   
                 20 μM Primer 2 
                  11 μl 
               
               
                   
                 Total volume 
                 400 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0141]    50 μl of the DNA fragment ligated with the adaptor element 1 obtained in step 5 was mixed with the fifth solution to be uniform, followed by first PCR amplification with the procedures as shown in the table below. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
             
             
               
                 95° C. 
                  3 min 
                   
               
             
          
           
               
                 95° C. 
                 30 s 
                 7 cycles 
               
               
                 56° C. 
                 30 s 
               
               
                 72° C. 
                  4 min 
               
             
          
           
               
                 68° C. 
                 10 min 
                   
               
             
          
           
               
                 cooling to 4° C. at a rate of 0.1° C./s 
               
             
          
           
               
                  4° C. 
                 Hold 
               
               
                   
               
             
          
         
       
     
         [0142]    After the PCR amplification, a resulting product was purified with 450 μl Ampure XP magnetic beads, and then dissolved in 65 μl TE buffer to retrieve a first amplified product. 1 μl of the first amplified product was quantified with a Qubit dsDNA HS assay kit (invitrogen). 2.4 μg of the first amplified product was used for the next step. 
         [0143]    7. Enzyme-Digesting at Uracil 
         [0144]    A sixth solution was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 10× Taq Buffer 
                 5 μl 
               
               
                   
                 USER enzyme (1 U/μl) 
                 8 μl 
               
               
                   
                 Total Volume 
                 13 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0145]    37 μl of the first amplified product (2.4 μg) was mixed with the sixth solution to be uniform, followed by incubation at 37° C. for 1 hour. 
         [0146]    8. Cyclizing the Double Stranded DNA with Sticky Terminals 
         [0147]    A reaction system 1 was formulated in advance with the components listed below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 355 μl 
               
               
                   
                 10×TA Buffer (epicenter) 
                  45 μl 
               
               
                   
                 Total Volume 
                 400 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0148]    A reaction system 2 was formulated in advance with the components listed below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 36 μl  
               
               
                   
                 10×TA Buffer (epicenter) 
                 5 μl 
               
               
                   
                 ATP (100 mM) 
                 5 μl 
               
               
                   
                 T4 DNA Ligase (Rapid) (600 U/μl) (enzymatics) 
                 4 μl 
               
               
                   
                 Total Volume 
                 50 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0149]    The DNA fragment after digested at the uracil site obtained in step 7 was mixed with the reaction system 1 to be uniform, followed by first incubation in a 60° C. water bath for 30 minutes and second incubation at 37° C. water bath for 20 minutes, thereby obtaining a fourth mixture. Such a fourth mixture was further mixed with the reaction system 2 to be uniform, followed by incubation at room temperature for 1 hour, thereby obtaining a fifth mixture. 
         [0150]    500 μl of the fifth mixture was mixed with 33 μl Ampure XP magnetic beads to be uniform, followed by still standing for 7 minutes to 15 minutes and then placed onto a magnetic separator for adsorption. Resulting supernatant was collected and mixed with additional 170 μl Ampure XP magnetic beads to be uniform, followed by still standing for 7 minutes to 15 minutes and placed onto the magnetic separator for adsorption. After discarding supernatant, the remaining magnetic beads were washed twice with 75% ethanol. Those dried magnetic beads were then dissolved in 68.1 μl TE buffer to retrieve the cyclized DNA. 
         [0151]    9. Removing Non-Cyclic Double Stranded DNA 
         [0152]    A seventh solution was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Plasmid-Safe 9× Reaction Buffer (Epicentre) 
                  8.9 μl 
               
               
                   
                 Plasmid-Safe ™ ATP-Dependent DNase (Epicentre) 
                   5 μl 
               
               
                   
                 Total Volume 
                 13.9 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0153]    The DNA product after cyclization obtained in step 8 was mixed with the seventh solution to be uniform, followed by incubation at 37° C. for 1 h, so as to digest and remove linear DNA which was not cyclized after cyclization. The cyclic double stranded DNA thus collected was purified with 80 μl Ampure XP magnetic beads, and dissolved in 82 μl TE buffer. 
         [0154]    10. Digesting the Cyclic Double Stranded DNA 
         [0155]    An eighth solution was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 233.2 μl 
               
               
                   
                 10× NEBuffer3.1 
                   36 μl 
               
               
                   
                 Ecop15 I Endonuclease (10 U/μl) 
                  10.8 μl 
               
               
                   
                 Total Volume 
                   280 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0156]    The cyclic double stranded DNA was mixed with the eighth solution to be uniform, followed by incubation at 37° C. for 16 hours. 
         [0157]    11. Binding with Streptavidin-Coated Magnetic Beads 
         [0158]    90 μl MyOne Streptavidin C1 magnetic beads was washed with 1× magnetic beads binding buffer twice (450 μl for each time of washing) (it was required to discard supernatant after each washing), and then resuspended in 90 μl magnetic beads binding buffer, which was mixed with 0.9 μl of 0.5% Tween 20, ready for use. 
         [0159]    360 μl digested cyclic double stranded DNA obtained in step 10 was mixed with 72 μl sodium chloride solution (3M), 90 μl MyOne Streptavidin C1 magnetic beads after washed to be uniform, followed by still standing at room temperature for 10 minutes and then placed onto a magnetic separator for 3 minutes allowing adsorption. After the resulting supernatant was discarded, the remaining beads were washed twice with 1× low-salt washing buffer (containing 0.05% Tween 20) (150 μl for each time of washing) (it was required to discard supernatant after each washing), and then resuspended in 90 μl 1× low-salt washing buffer. 
         [0160]    12. End-Repairing and Adding “A” to the Digested Product from the Cyclic Double Stranded DNA in One Reaction 
         [0161]    A ninth solution was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 0.4 μl 
               
               
                   
                 10× NEBuffer2 
                  12 μl 
               
               
                   
                 dATP (100 mM) 
                   4 μl 
               
               
                   
                 dNTP (25 mM) 
                 0.4 μl 
               
               
                   
                 Klenow Enzyme without 3′-5′ 
                 3.2 μl 
               
               
                   
                 exonuclease activity (5 U/μl) 
                   
               
               
                   
                 Total Volume 
                  20 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0162]    The magnetic beads conjugated with the digested cyclic double stranded DNA obtained in step 12 was mixed with the ninth solution to be uniform, followed by incubation at 37° C. for 1 hour, thereby obtaining a sixth mixture. 
         [0163]    After incubation, the sixth mixture was further mixed with 2.2 μl of ethylenediamine tetraacetic acid disodium salt (0.5 M) to be uniform, followed by still standing for 1 minute and discarding the resulting supernatant. The remaining magnetic beads were washed twice with 1× low-salt washing buffer (containing 0.05% Tween 20), and then resuspended in 80 μl 1× low-salt washing buffer. 
         [0164]    13. Ligating the Adaptor Element 2 
         [0165]    A long-chain nucleic acid 2 has a sequence of /Phos/AGTCGGAGGCCAAGCGTGCTTAGGACAT. 
         [0166]    A short-chain nucleic acid 2 has a sequence of GTCCTAAGCACUGTAGTGTACGATCCGACTT. 
         [0167]    A tenth solution containing the adaptor element 2 (10 μM) was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Long-chain nucleic acid 2 (100 μM) 
                 10 μl 
               
               
                   
                 Short-chain nucleic acid 2 (100 μM) 
                 10 μl 
               
               
                   
                 Sodium chloride (1M) 
                  5 μl 
               
               
                   
                 trihydroxymethyl aminomethane - HCl (0.2M, pH 7.8) 
                  5 μl 
               
               
                   
                 ethylenediamine tetraacetic acid disodium salt (2 mM) 
                  5 μl 
               
               
                   
                 Enzyme-free water 
                 65 μl 
               
               
                   
                 Total 
                 100 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0168]    An eleventh solution for ligation was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Ligating buffer 1 
                 56.8 μl 
               
               
                   
                 T4 DNA Ligase (Rapid) (600 U/μl) (enzymatics) 
                  4.2 μl 
               
               
                   
                 Total 
                   61 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0169]    The DNA product after end-repaired and added with “A” obtained in step 12 was mixed with 30 μl of the tenth solution containing the adaptor element 2 (10 μM) to be uniform, thereby obtaining a seventh mixture. 
         [0170]    The seventh mixture was mixed with the eleventh solution for ligation to be uniform, followed by incubation at room temperature for 30 minutes, thereby obtaining an eight mixture. Such an eighth mixture was then mixed with 3.42 μl of ethylenediamine tetraacetic acid disodium salt (0.5 M) to be uniform, followed by incubation for 1 minute. After the resulting supernatant was discarded, the remaining magnetic beads were washed twice with 1× low-salt washing buffer (containing 0.05% Tween 20), and then resuspended in 80 μl 1× low-salt washing buffer. 
         [0171]    Such steps achieve ligation of the adaptor element 2 to a target nucleic acid fragment. The ligation efficiency can be proven by eluting 3 μl of the single stranded nucleic acid, amplifying for 8 cycles and applying 6 μl of an amplified product to electrophoresis detection, as shown in  FIG. 6 . 
         [0172]    14. Enzyme-Digesting at Uracil and Isolating a Single Stranded Nucleic Acid 
         [0173]    The magnetic beads resuspended in step 13 were mixed with 1 μl USER enzyme to be uniform, followed by incubation at 37° C. for 1 hour, thereby obtaining a ninth mixture. After the incubation, the ninth mixture was mixed with 8.1 μl of ethylenediamine tetraacetic acid disodium salt (0.1 M) to be uniform, followed by incubation for 1 minute. After the resulting supernatant was discarded, the remaining magnetic beads were washed twice with 1× low-salt washing buffer (containing 0.05% Tween 20), and then resuspended in 75 μl sodium hydroxide (0.1 M), followed by incubation at room temperature for 5 minutes. The supernatant thus obtained was neutralized with 37.5 μl acidic buffer, thereby obtaining a single stranded product in a total volume of 112.5 μl. 
         [0174]    15. Cyclizing the Single Stranded DNA 
         [0175]    A reaction system 3 was formulated in advance with the components listed below, in which the bridge fragment has two ends capable of complementary combining with two terminals of the isolated single stranded nucleic acid obtained in step 14, and has a sequence of ATCGTACACTACATGTCCTAAGCA (SEQ ID NO:7). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 59 μl 
               
               
                   
                 Bridge fragment (100 μM) 
                  4 μl 
               
               
                   
                 Total 
                 63 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0176]    A reaction system 4 was formulated in advance with the components listed below, 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 135.3 μl  
               
               
                   
                 10× TA Buffer (epicentre) 
                  35 μl 
               
               
                   
                 100 mM ATP 
                 3.5 μl 
               
               
                   
                 T4 DNA Ligase (Rapid) (600 U/μl) (enzymatics) 
                 1.2 μl 
               
               
                   
                 Total 
                 175 μl  
               
               
                   
                   
               
             
          
         
       
     
         [0177]    The isolated single stranded nucleic acid obtained in step 14 was mixed with the reaction system 3, and then reaction system 4 to be uniform, followed by incubation at 37° C. for 1.5 hours. 
         [0178]    16. Purification by Removing the Non-Cyclic Single Stranded DNA 
         [0179]    A twelfth solution for digestion was formulated in advance with the components listed in the table below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Enzyme-free water 
                 1.5 μl 
               
               
                   
                 10× TA Buffer (epicentre) 
                 3.7 μl 
               
               
                   
                 Exonuclease 1 (20 U/μl) (NEB) 
                 11.1 μl  
               
               
                   
                 Exonuclease 3 (100 U/μl) (NEB) 
                 3.7 μl 
               
               
                   
                 Total 
                  20 μl 
               
               
                   
                   
               
             
          
         
       
     
         [0180]    350.5 μl of the DNA product after cyclization obtained in step 15 was mixed with 20 μl of the twelfth solution to be uniform, followed by incubation at 37° C. for 30 minutes, thereby obtaining a tenth mixture. Such a tenth mixture was mixed with 15.4 μl of ethylenediamine tetraacetic acid (500 mM) to be uniform, thereby obtaining an eleventh mixture, followed by purifying with 500 μl PEG32 magnetic beads and dissolving in 70 μl TE buffer to retrieve a final product. 
         [0181]    In the present example, seven final products were obtained, each have a concentration, a total amount and a molecular weight as shown below, and their electrophoresis results are shown in  FIG. 7 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Concentration 
                 Total amount 
                 molecular weight 
               
               
                   
                 (ng/μl) 
                 (ng) 
                 (pmol) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Product 1 
                 0.35 
                 24.50 
                 0.44 
               
               
                 Product 2 
                 0.34 
                 23.80 
                 0.42 
               
               
                 Product 3 
                 0.35 
                 24.50 
                 0.44 
               
               
                 Product 4 
                 0.28 
                 19.60 
                 0.35 
               
               
                 Product 5 
                 0.37 
                 25.90 
                 0.46 
               
               
                 Product 6 
                 0.39 
                 27.30 
                 0.49 
               
               
                 Product 7 
                 0.30 
                 21.00 
                 0.37 
               
               
                   
               
             
          
         
       
     
         [0182]    It can be seen from the results that each product is of a concentration and a total amount meeting requirement of the subsequent sequencing (molecular weight□0.12 pmol). The electrophoresis result shows that the DNA fragment finally obtained is concentrated, indicating that the sequencing library constructed is in good quality. It is thus proved in the above example that the technical solution proposed in the present disclosure is completely successful. 
         [0183]    The Applicant has stated that the present disclosure illustrates the detailed process equipment and process flow of the present disclosure by the examples described above, but the present disclosure is not limited to the detailed process equipment and process flow described above, that is it is not meant that the present disclosure has to rely on the detailed process equipment and process described above to be implemented. It will be apparent to those skilled in the art that any improvements to the present disclosure, equivalents of the raw materials of the present disclosure and addition of auxiliary ingredients, selection of specific means and the like all fall within the scope of the protection and disclosure of the present disclosure.