Generations of Sequencing Technologies

Department of Gene Technology, Royal Institute of Technology (KTH), AlbaNova University Center, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden.
Genomics (Impact Factor: 2.28). 12/2008; 93(2):105-11. DOI: 10.1016/j.ygeno.2008.10.003
Source: PubMed
ABSTRACT
Advancements in the field of DNA sequencing are changing the scientific horizon and promising an era of personalized medicine for elevated human health. Although platforms are improving at the rate of Moore's Law, thereby reducing the sequencing costs by a factor of two or three each year, we find ourselves at a point in history where individual genomes are starting to appear but where the cost is still too high for routine sequencing of whole genomes. These needs will be met by miniaturized and parallelized platforms that allow a lower sample and template consumption thereby increasing speed and reducing costs. Current massively parallel, state-of-the-art systems are providing significantly improved throughput over Sanger systems and future single-molecule approaches will continue the exponential improvements in the field.

Full-text

Available from: Joakim Lundeberg, May 12, 2016
Review
Generations of sequencing technologies
Erik Pettersson
, Joakim Lundeberg, Afshin Ahmadian
Department of Gene Technology, Royal Institute of Technology (KTH), AlbaNova University Center, Roslagstullsbacken 21, SE - 106 91 Stockholm, Sweden
abstractarticle info
Article history:
Received 2 September 20 08
Accepted 2 October 2008
Available online 21 November 2008
Keywords:
DNA sequencing
Next generation
Advancements in the eld of DNA sequencing are changing the scientic horizon and promising an era of
personalized medicine for elevated human health. Although platforms are improving at the rate of Moore's
Law, thereby reducing the sequencing costs by a factor of two or three each year, we nd ourselves at a point
in history where individual genomes are starting to appear but where the cost is still too high for routine
sequencing of whole genomes. These needs will be met by miniaturized and parallelized platforms that allow
a lower sample and template consumption thereby increasing speed and reducing costs. Current massively
parallel, state-of-the-art systems are providing signicantly improved throughput over Sanger systems and
future single-molecule approaches will continue the exponential improvements in the eld.
© 2008 Elsevier Inc. All rights reserved.
Contents
Introduction ................................................................ 105
Present generation of DNA sequencing technologies ............................................. 106
Terminating chains ........................................................... 106
Hybridization to tiling arrays ...................................................... 106
Parallelized Pyrosequencing ....................................................... 106
Reverse termination .......................................................... 108
Ligating degenerated probes....................................................... 108
Future generation of DNA sequencing technologies .............................................. 108
Single-molecule sequencing ....................................................... 108
Remarks .................................................................. 110
References ................................................................. 110
Introduction
The ability to swiftly and accurately gain knowledge of nucleic acid
composition is essential to many of the biological sciences. As the pace
of progress is high and we are moving towards an era of synthetic
genomics and personalized medicine, the demand for highly efcient
sequencing technologies is obvious, where effortless deciphering of
genetic sequences will shed light on novel biological functions and
phenotypic differences. Metagenomic endeavors [13] are providing
new tools in the art of genetic engineering, thereby enabling the
design of articial life in the service of humanity [4]. These future
synthetic organisms may produce petrol substitutes or provide
systems for mopping up excessive carbon dioxide in the atmosphere
[57]. Perhaps even more captivating is the possibility of resequencing
larger and larger fractions of human genomes at an ever decreasing
cost, an effort that will elucidate phenotypic variants, extending the
comprehension of disease susceptibility and pharmacogenomics,
permitting personalized medicine.
Although we have not yet reached the long envisioned $1000
genome [8], novel approaches and renements of existing methods
are reducing the cost per base by the day while increasing the
throughput. The establishment of a reference genome in the
beginning of this decade [9,10] is now permitting cost-effective
resequencing of ever larger fractions of human genomes. The
Advanced Sequencing Technology Development Awards initiated
by the National Human Genome Research Institute (NHGRI) in 2004
[11] are beginning to show results. Advancements for the next
generation sequencing methods include not only current state-of-
the-art systems from 454 [12,13], Illumina [14,15] and Applied
Biosystems [16] but also single-molecule detection approaches,
capable of recognizing incorporation or hybridization events on
single molecules. Further into the future lies more direct recognition
of unamplied material, i.e. nano pores or nano edges relying on
physical recognition of the bases in an unmodied DNA strand,
rather than detecting chemical incorporation.
Genomics 93 (2009) 105111
Corresponding author. Fax: +46 8 5537 8481.
E-mail address: eriq@kth.se (E. Pettersson).
0888-7543/$ see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.ygeno.2008.10.003
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Genomics
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Page 1
The drop in cost has led to the initiation of several sequencing
projects aiming at elucidating the variation not covered by SNP
arrays. In the Personal Genome Project [1719], the exon regions of
ten genomes are to be sequenced and compared. Researchers at the
Beijing Genomics Institute (BGI) [20] are determined to sequence
100 individuals of Han Chinese origin during the upcoming three
years in the Yanhuang Project and recently, an international
consortium announced the 1000 Genomes Project where the
sequence of 1000 individuals will provide A catalogue of human
genetic variation [21]. The improvements in sequencing technology
and reduction in cost have allowed the rst personal genomics
company [22] to begin the sequencing of customers' genomes.
To allow for a further reduction in cost the X PRIZE Foundation in
Santa Monica, CA, has introduced the Archon X PRIZE for Genomics
[23] and will award a sum of $10 million to the rst team that can
design a system capable of sequencing 100 human genomes in
10 days. Additional requirements are an error rate of no more than one
in 100,000 bases, a coverage of at least 98% and a cost of no more than
$10,000 for each sequenced genome. Representatives from many of
the different sequencing categories are represented in the Archon X
PRIZE challenge and the research world is closing in on the $1000
genome. The race is on.
Present generation of DNA sequencing technologies
There are many factors to consider in DNA sequencing such as read
length, bases per second and raw accuracy. All the work in the eld has
led to an exponential reduction in cost per base. Sanger sequencing
has been one of the most inuential innovations in biological research
since it was rst presented in 1977. A little more than 20 years later, a
bioluminescence sequencing-by-synthesis approach saw the light of
day [24]. Today, Pyrosequencing has evolved at 454 Life Sciences,
generating about ve hundred million bases of raw sequence in just a
few hours [12]. This throughput, although heavily rened and
improved during the years, is something Sanger sequencing in its
current form cannot easily match. However, during the last year,
Illumina and Applied Biosystems have introduced sequencing systems
offering even higher throughput than the systems provided by 454,
generating billions of bases in a single run. These novel methods all
rely on parallel, cyclic interrogation of sequences from spatially
separated clonal amplicons. Although with shorter read lengths and a
slower sequence extraction from individual features as compared to
the Sanger method, the parallelized process offers a much higher total
throughput and reduces cost signican tly by generatin g thousands
of bases per second. By shearing the template and parallel
sequencing of single fragments, over sampling may provide
improved coverage and the possibility of stitching together the
original sequence while increasing total accuracy. Already today
these high-throughput methods are expanding our knowledge, also
in the related elds of transcriptome and proteome research. Gene
expression analysis with whole-transcriptome sequencing is possi-
ble and furthermore, in proteome research, by sequencing DNA
extrac ted by antibodies targeting DNA-binding proteins (ChIP-Seq),
transcription factor binding sites and chromatin modications can
be investigated [25,26].
Terminating chains
Since 1977, a total nucleic acid polymer of approximately 10
11
bases
has been determined with Sanger's chain termination sequencing
method [27]. By halting the elongation with a labeled, and thereby
identiable, dideoxyribonucleotide triphosphate (ddNTP), the length
of the fragment can be utilized for interrogating the base identity of
the terminating base [28]. In its current form, uorescently labeled
ddNTPs [29,30] are mixed with regular, non labeled, non terminating
nucleotides in a cycle sequencing reaction [31,32] rendering elonga-
tion stops at all positions in the template. Capillary electrophoresis can
then be applied for separating sequences by length and providing
subsequent interrogation of the terminating base (see Fig. 1A).
Initially at a high cost, renements and automation have
improved cost effectiveness signicantly. In 1985, $10 allowed
reading one single base, while the same amount of money rendered
10,000 bases 20 years later [8,27]. Current instruments provided by
Applied Biosystems deliver read lengths of up to 1000 bases, high
raw accuracy and allow for 384 samples to be sequenced in parallel
generating 24 bases per instrument second. Projects of multiplexing
and miniaturization in order to reduce reagent volumes, lower
consumable co sts and increase throughput are being pursued
[33,34].
Hybridization to tiling arrays
The concept of allele-specic hybridization (ASH) has been used
for resequencing and genotyping purposes by expanding a probe set,
targeting a specic position in the genome, to include interrogation of
each of the four possible nucleotides [35]. A tiling array can be
fabricated with probe sets targeting each position in the reference
genome. Read length is given by the probe length (often 25 bp) and
base calling is performed by examining the signal intensities for the
different probes of each set. Accuracy is an issue and is dependent on
the ability of the assay to discriminate between exact matches and
those with a single base difference. Performance may vary signi-
cantly due to different base compositions (different thermal annealing
properties) of different regions, resulting in problems with false
positives as well as with large inaccessible regions composed of
repetitive sequence stretches [36,37].
The throughput is an obvious benet , since all bases are
interrogated simultaneously and the concept has been applied to
resequencing the human chromosome 21 by Perlgen [37] and HIV
[36]. By representing all possible sequences for a given probe length,
de novo sequencing can be performed and overlapping sequences
used for sequence assembly [38]. In a recent report, the genome of
Bacteriophage λ and Escherichia coli were resequenced by shotgun
sequencing by hybridization with an accuracy of 99.93% and a raw
throughput of 320 Mbp/day [39].
Parallelized Pyrosequencing
The Genome Sequencer FLX by 454 Life Sciences [13] and Roche
depends on an emulsion PCR followed by parallel and individual
Pyrosequencing of the clonally amplied beads in a PicoTiterPlate (see
Fig. 1B). Emulsion PCR is a clonal amplication performed in an oil-
aqueous emulsion. Unlike when digesting a genome with restriction
endonucleases, shearing will provide randomly fragmented pieces of
more or less similar length. By the addition of general adaptor
sequences to the fragments, only one primer pair is required for
amplication. In the emulsion PCR, a primer-coated bead, a DNA
fragment and other necessary components for PCR (including the
second general primer) are isolated in a water micro-reactor, favoring a
1:1 bead to fragment ratio. Once the emulsion is broken, beads not
carrying any amplied DNA are removed in an enrichment process
[12,40]. The amplied and enriched beads are then distributed on the
PicoTiterPlate, where a well (44 µm in diameter) allows xation of one
bead (28 µm in diameter) [12]. However, out of the 1.6 million wells,
not all will contain a bead and not all of those that do will give a useful
sequence.
Following the distribution of the DNA-carrying beads to the
PicoTiterPlate Pyrosequencing will be performed. Pyrosequencing is
a sequencing-by-synthesis method where a successful nucleotide
incorporation event is detected as emitted photons [41]. Since the
single-stranded DNA fragments on the beads have been amplied with
general tags, a general primer is annealed permitting an elongation
106 E. Pettersson et al. / Genomics 93 (2009) 105111
Page 2
towards the bead. The emission of photons upon incorporation
depends on a series of enzymatic steps. Incorporation of a nucleotide
by a polymerase releases a diphosphate group (PPi), which catalyzed
by ATP sulphurylase forms adenosine triphosphate (ATP) by the use of
adenosine phosphosulphate (APS). Finally, the enzyme luciferase
(together with D-luciferin and oxygen) can use the newly formed
ATP to emit light. Another enzyme, apyrase, is used for degradation of
unincorporated dNTPs as well as to stop the reaction by degrading ATP
[41].
In the 454 system, the Pyrosequencing technology is adapted as
follows. The enzymes luciferase and ATP sulphurylase are immobi-
lized on smaller beads surrounding the larger amplicon carrying
beads. All other reagents are supplied through a ow allowing
reagents to diffuse to the templates in the PicoTiterPlate. Polymerase
and one exclusive dNTP per cycle generate one or more incorporation
events and the emitted light is proportional to the number of
incorporated nucleotides. Photons are detected by a CCD camera
and after each round, apyrase is owed through in order to degrade
excess nucleotides. The washing procedure for the removal of by-
products permits read lengths of over 400 bp (250 bp in the GS FLX
system and over 400 bp in the recently upgraded instrument, the GS
FLX Tita nium). This limitation is due to negative frame shifts
(incorporation of nucleotides in each cycle is not 100% complete)
and positive frame shifts (the population of nucleotides that is not
fully degraded by the apyrase and can therefore be incorporated after
the next nucleotide) that eventually will generate high levels of noise.
Approximately 1.2 million wells will give one unique sequence of
400 bp, on average generating less than 500 million bases (Mb) in one
single run. Whole-genome sequencing has been performed on
bacterial genomes in single runs [12]. An oversampling of 20× permits
the identication of PCR-introduced errors and to call homopolymeric
errors [42].
Fig. 1. (A) Chain Termination. A DNA sequence of choice is prepared using a sequencing reaction where regular deoxynucleotides are combined with terminating dideoxynucleotides.
Each chain terminating nucleotide is labeled with a base-specic color. The sequencing reaction generates fragments of all lengths and separation can be made using a gel or capillary
electrophoresis where the labeled bases reveal the sequence information at each position. Read length is approximately 700 bases. (B) Miniaturized Pyrosequencing. Highly parallel
and miniaturized Pyrosequencing reactions are achieved by rst performing a water-in-oil emulsion PCR that permits generation of hundreds of thousands of single-clone amplied
beads. The beads are then single tted into the wells of a PicoTiterPlate where individual Pyrosequencing reactions are taking place. A sequential addition of the four bases is
performed in a cyclic fashion and upon successful incorporation of each base, an enzyme cascade generates light which can be detected. Read lengths of 400 bases are now possible
allowing for a total of 500 Mbp from a PicoTiterPlate in each run. (C) Reverse Termination. The DNA fragments of choice are bridge-amplied using a solid-phase PCR on a surface
generating spatially separated colonies of approximately thousand fragments each. A cyclic sequence interrogation procedure is performed using uorescently labeled, reversibly
terminating nucleotides. All four bases are added in each cycle and following incorporation and stringent washing procedures, the color of each colony is detected. The dye is then
removed and the termination reversed allowing for interrogation of the following base in each colony. At 3035 bases, the error rate is becoming high thereby limiting the read-
length. (D) Sequencing-by-Ligation. Single-clone beads are amplied in an emulsion PCR and immobilized in a gel. By utilizing previously introduced general tag sequences, anchor
primers can be annealed next to unknown sequence regions. Hybridizing and ligating degenerated nonamers, where only one base and position in the primer is specic and un-
degenerated, reveals the base at the position in question since each primer and base is correlated to a particular color. The specicity of the ligase permi ts sequencing of six or seven
bases depending on the ligation direction (5′–3 and 3′–5 respectively). In the illustration, the third base (a lled circle) is interrogated at one bead and the color of ligated probe
indicates an A at the third position. Note that squares indicate degenerated bases). After each cycle, the ligated products are removed and a new round of ligation is performed by
shifting the position of the specic base in the nonamers.
107E. Pettersson et al. / Genomics 93 (2009) 105111
Page 3
454 Life Sciences is a competitor in the Archon X PRIZE and by
moving the parallel Pyrosequencing technology onto a microchip [23]
they believe the system will achieve the scalability it needs to win.
Reverse termination
The Illumina 1G Genome Analyzer is relying on clonal bridge
amplication on a ow cell surface generating 10 million single-
molecule clusters per square centimeter. Bridge amplication is
performed after immobilization of oligonucleotides complementary
to the adaptor sequences on a surface [15,43,44]. Sheared and adaptor-
ligated sample DNA fragments can be attached to the solid support
and due to the dense lawn of adaptor complementary sequences on
the surface, each will anneal to a nearby primer. A double stranded
bridge will form after elongation, and denaturing will free the two
strands, both now xed on the surface. Repeated cycles will form
colony like local clusters, each containing approximately 1000 copies
and with a diameter of about 1 µm (see Fig. 1C). Sequencing is then
carried out with uorescently labeled nucleotides that are also
reversible terminators. One base is incorporated and interrogated at
a time since further elongation of the chain is prevented [14]. When all
colonies are scanned at the end of a cycle and the base determined for
each colony, the uorophores are cleaved off and terminating bases
are activated, allowing another round of nucleotide incorporation (see
Fig. 1C). The presence of and competition among all four nucleotides is
claimed to reduce the chance of misincorporation. Incom plete
incorporation of nucleotides and insufcient removal of reverse
terminators or uorophores may be the explanation for the relatively
short read length of 35 bases. Although shorter read lengths than the
454 system, the throughput is much higher and, as of February 2008,
1.5 Gbp are generated in each run, which takes approximately 3 days.
The use of paired-end libraries will generate about 3 Gbp in a single
run. The raw accuracy is said to be at 98.5% and the consensus (3×
coverage) at 99.99%. The cost per base is approximately 1% of the cost
for Sanger sequencing [15,45]. A variant of Illumina's sequencing by
synthesis chemistr y was recently reported where a hybrid of
sequencing by synthesis and Sanger method promises longer reads
[46].
Ligating degenerated probes
Strategies for sequencing-by-ligation have been presented in the
form of Massively Parallel Signature Sequencing (MPSS) and Polony
sequencing [40,47]. MPSS was demonstrated as signature sequencing
of expression libraries of in vitro cloned microbeads, i.e. beads
carrying multiple copies of a single DNA sequence [48]. Signature
sequencing was carried out by restriction enzyme mediated exposure
of four nucleotides in each cycle followed by ligation of an interrogator
probe. This process was repeated for 45 cycles, i.e. querying 1620
bases in total. An overhang of four bases would require 256 different
complementary probes and just as many uorophores for immediate
recognition. Instead, the use of 16 (4×4) probes, each with a unique
decoder binding site, has enabled single dye detection.
Resequencing of a bacterial genome was used to demonstrate the
Polony sequencing method [40]. A mate-paired library was clonally
amplied with an emulsion PCR on 1 µm beads and subsequently
immobilized in a polyacrylamide gel. Each DNA-carrying bead
(polony) represented two 17
18 bp genomic sequences anked by
different universal sequences. Due to the nature of the mate-pair
construction, the two genomic sequences were separated by approxi-
mately 1 kb in the genome. Sequencing-by-ligation (see Fig. 1D) could
then be performed using degenerate nonamers, where each known
nucleotide was associated with one of four uorophores. By using four
different anchor primers, degenerate sequencing-by-ligation could be
performed from each end of the tags. 7 bases could be obtained when
sequencing in the 5 to 3 direction and 6 bases from 3 to 5. Ligated
primers were removed after each round rendering information of 26
bases from each amplicon in a pattern of: 7 bases, a gap of 45 bases, 6
bases, then a gap of approximately 1 kb (mate-paired constructed) and
then another 7 bases, a gap of 45 bases, followed nally by 6 bases.
These two methods have spawned the development of the
commercial SOLiD system (Sequencing by Oligonucleotide Ligation
and Detection) from Applied Biosystems where clonal amplicons on
1 µm beads are generated by an emulsion PCR, either from fragments
or mate-paired libraries. The beads are enriched, so that 80% of them
generate signals, and attached on a glass surface forming a very high-
density random array. Sequencing-by-ligation is performed by ligating
3-degenerated and 5-labeled probes to the amplicons and detecting
the color. Accuracy is improved by implementing a two-base encoding
system that leads to interrogation of each base twice. A sequencing
run takes 610 days and the output is high, approximately 36 Gbp
per run given a read length of 2535 bases per clonally amplied bead
[16].
An open source implementation [49] of the Polony sequencing
technology is the Danaher Motion Polonator G.007 where 200 such
modules will be used by a team competing in the Archon X PRIZE race.
They are hoping to reach the $10K per genome during 2008 by further
improvements and optimizations of the technology.
Future generation of DNA sequencing technologies
The initial sequencing and mapping of the human genome is
estimated to have cost about $3 billion [9,10]. The genome of Craig
Venter, determined a year ago [50], cost around $70 million [51].
Resequencing a human genome with the Sanger sequencing method
would today cost approximately $10 million [14,50] while the 454
system enables a 10-fold reduction in cost and abou t 20-fold
reduction in time [52]. Illumina claims to be able to sequence a
human genome with the 1G Analyzer for approximately $100,000
[45]. Neither 454 nor Illumina has shown data describing the exact
workload and reagent cost but this important information will
hopefully be revealed to the scientic community soon. Although
the progresses in the last few years have shown a signicant reduction
in sequencing cost, it is still too early and too expensive to use these
platforms to routinely sequence human genomes at a larger scale. The
realization of the $1000 genome requires novel approaches and there
is an immense activity in the eld.
As mentioned above, in 2004 NHGRI initiated the Advanced
Sequencing Technology Development Projects where grants were
approved for some 20 novel ideas and approaches to develop cutting-
edge, low cost sequencing for the future. Today around 35 projects in
industry and academia have been granted a total of $56 million for
technology development in the quest for the $1000 genome [53].A
key feature among most contenders is to look at single molecules.
Although it is challenging to sequence single DNA fragments, there are
advantages such as improved read length, since molecules are not
getting out of phase, and a signicant drop in reagent cost. A number
of routes to the future are pursued, such as sequencing-by-synthesis
approaches like 454 and Solexa, without the prior amplication step,
and indirect approaches using physical recognition of the DNA strand
and the investigation of bases using nano pores or equivalents.
Single-molecule sequencing
The concept of sequencing-by-synthesis without a prior amplica-
tion step i.e. single-molecule sequencing is currently pursued by a
number of companies. Helicos Biosciences [54] has an instrument, the
HeliScope, with a claimed throughput of 1.1 Gpb per day (as of
October 2008 [54]). Single fragments are labeled with Cy3 for
localization of template strands on an array and a predened, Cy5-
labeled nucleotide (for instance A) are incorporated, detected by a
uorescent microscope and cleaved off in each cycle [55,56]. Four
108 E. Pettersson et al. / Genomics 93 (2009) 105111
Page 4
cycles, one for each nucleotide, constitutes a quad and multiple quad
runs are claimed to produce read lengths of up to ~55 bases (see Fig.
2A). At 20 bases or longer, 86% of the strands are available and at 30
bases, around 50%. The rst order of a HeliScope instrument was
announced in the beginning of February 2008 and the company claims
its machine to be able to sequence a human genome for $72,000.
A different, although very promising, approach is taken by Pacic
Biosciences [57]. The technology, denoted Single-molecule Real Time
Sequencing-by-synthesis (SMRT), has in a proof-of-concept study
shown read lengths of single DNA fragments of over 1500 bases in
3000 parallel reactions. The heart of the technology is so called
zero-mode waveguides (ZMW) [58] which essentially are nanometer
scale wells with a diameter of 70 nm (see Fig. 2B). Light bulges
inward at the opening, permitting illumination of a detection
volume of 20 zl (10
21
) where a single DNA polymerase is
immobilized. Nucleotides, uorescently labeled at the terminal
phosphate, are incorporated by the p olymerase and thereby
exposing its base-specic uorophore for a few milliseconds
which is enough for detection. Benets are long read lengths of
thousands of bases in one stretch and high speed (10 bases per
second and molecule). It is still at the proof-of-concept stage and no
commercial instrument is ready. Thousands of ZMWs in parallel
may in a future instrument (no sooner than 2010) generate 100
gigabases per hour. A second generation instrument capable of
sequencing a human genome for $1000 is an additional number of
years in the future. The Menlo Park based company has received
grants from the NHGRI but is not signed up for the X PRIZE race so
far.
Unlike Pacic Biosciences, a contender in the X PRIZE is Visigen
Biotechnologies [59], Houston, TX, which platform consists of an
engineered polymerase and modied nucleotides for single-molecule
detection. An immobilized polymerase on a surface, modied with a
uorescence resonance energy transfer (FRET) donor incorporates
nucleotides modied with different acceptors, allowing base-specic
and real time detection of incorporation events (see Fig. 2C). A
theoretical throughput of 1 million bases per instrument second has
Fig. 2. (A) Helicos Biosciences. True single-molecule sequencing (tSMS) is achieved by initially adding a poly A sequence to the 3-end of each fragment, which allows hybridization
to complementary poly T sequences in a ow cell. After hybridization, the poly T sequence is extended and a complementary sequence is generated. In addition, the template is
uorescently labeled at the 3-end and thus, illumination of the surface reveals the location of each hybridized template. This process allows generation of a map of the single-
molecule landscape before the labeled template is removed. Fluorescently labeled nucleotides are added, one in each cycle, followed by imaging. A cleavage step removes the
uorophore and permits nucleotide incorporation in the next round. (B) Pacic Biosciences. A zero-mode waveguide contains a single polymerase macromolecule immobilized at the
bottom (hexagon), nucleotides (circles) that are uorescently labeled at the triphosphates (colored triangles) and a DNA strand which permits single-molecule real time (SMRT)
sequencing. Single incorporation events are possible to detect with this design since an excitation beam penetrates the lower 2030 nm of the waveguide, i.e. approximately a volume
of 20 zl. This volume is sufcient to detect the incorporated nucleotide while avoiding excitation of unincorporated nucleotides, thereby reducing the noise. (C) Visigen
Biotechnologies. A slightly different approach for single-molecule real time sequencing is to immobilize a modied polymerase (hexagon) on a glass surface. The polymerase is
engineered to carry a uorescent donor molecule and by coding the four different nucleotides with different acceptors, base-specic FRET emission upon incorporation will reveal the
sequence. (D) Nano Pores and Nano-Knife Edge Probes. Nano pores and nano-knife edge probes are two approaches for physical and direct recognition of bases. Bases in a DNA strand
can be recognized either by threading through a nano pore (left), measuring a change in conductivity, or using an array of nano-knife edge probes (right) tuned to recognize each base
in a stretched, immobilized DNA strand by detecting the unique electron tunneling characteristics for each base.
109E. Pettersson et al. / Genomics 93 (2009) 105111
Page 5
been given, although no proof-of-concept study has been presented.
Applied Biosystems has completed an equity investment as of
December 2005.
Intelligent Biosystems [60] is pursuing an array-based sequencing-
by-synthesis approach [61] similar to the Illumina 1G system and
claims launch of an instrument by the end of 2008 that might reduce
the cost of a genome to $10,000. The company received a grant in 2006
by NHGRI and is not competing for the X PRIZE.
Further in the future may lie sequencing approaches that utilize
physical recognition of nucleic bases. One alternative is nano pores,
where the aim is to sequence a DNA strand that is pulled
electrophoretically through a synthetic or natural pore, only 1.5 nm
wide, measuring changes in conductivity (See Fig. 1D). A common
issue with nano pores is the sensitivity of detection. By utilizing
conversion of single bases to longer Design Polymers by LingVitae [62]
such problems may be circumvented.
Reveo [63], an X PRIZE contender, is developing a Personal Genome
Sequencer (PGS) based on nano-knife edges permitting non-destruc-
tive detection of bases in single DNA strands by measuring electron
tunneling characteristics for each base [23] (see Fig. 2D).
Remarks
Several X PRIZE attempts can be anticipated within a year and the
$1000 genome may be as little as three years away.
One's destiny is a result of genes, environment, life style, behavior
and luck. As we are entering an era of low-cost sequencing and
preventive medicine one can discover a new meaning to the ancient
inscription at the temple of Apollo. Γνωϑɛις ɛαυτον Know thyself.
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  • Source
    • "The reduced cost makes it feasible for individual investigators to undertake genome sequencing projects, but it carries decreases in read length and accuracy. Sanger sequencing produces 400–900 bp sequencing reads with a per-base accuracy of 99.9% compared to 50–300 bp sequencing-by-synthesis reads (although long reads are possible) (Petterson et al., 2009; Quail et al., 2012 ). Nextgeneration sequencing has an average per-base accuracy of 99% but this decreases systematically with high and low GC bias (Dohm et al., 2008) and results in reduced sequencing of these regions (Kozarewa et al., 2009; Chen et al., 2013). "
    [Show abstract] [Hide abstract] ABSTRACT: Citation: Fierst JL (2015) Using linkage maps to correct and scaffold de novo genome assemblies: methods, challenges, and computational tools. Front. Genet. 6:220. Modern high-throughput DNA sequencing has made it possible to inexpensively produce genome sequences, but in practice many of these draft genomes are fragmented and incomplete. Genetic linkage maps based on recombination rates between physical markers have been used in biology for over 100 years and a linkage map, when paired with a de novo sequencing project, can resolve mis-assemblies and anchor chromosome-scale sequences. Here, I summarize the methodology behind integrating de novo assemblies and genetic linkage maps, outline the current challenges, review the available software tools, and discuss new mapping technologies.
    Full-text · Article · Jul 2015 · Frontiers in Genetics
  • Source
    • "However, Sanger sequencing, referred to as a " first generation " sequencing method, is expensive and impractical for large sequencing projects, and in recent times, this method has partly been replaced by " next-generation " sequencing (NGS) methods (Metzker 2010). NGS methods are high-throughput DNA sequencing technologies permitting the sequencing of millions of DNA strands in parallel generating large volumes of sequence data in a relatively short period of time (Pettersson et al. 2009). There are currently a number of methods in use (Table 1), but for the purposes of this review, we will briefly compare four of the most commercially viable methods, namely Roche 454-pyrosequencing, Illumina-Solexa, Life Sciences Ion-Torrent and Pacific Biosciences Single-molecule real-time sequencing (SMRT). "
    [Show abstract] [Hide abstract] ABSTRACT: Lactococcal and streptococcal starter strains are crucial ingredients to manufacture fermented dairy products. As commercial starter culture suppliers and dairy producers attempt to overcome issues of phage sensitivity and develop new product ranges, there is an ever increasing need to improve technologies for the rational selection of novel starter culture blends. Whole genome sequencing, spurred on by recent advances in next-generation sequencing platforms, is a promising approach to facilitate rapid identification and selection of such strains based on gene-trait matching. This review provides a comprehensive overview of the available methodologies to analyse the technological potential of candidate starter strains and highlights recent advances in the area of dairy starter genomics.
    Full-text · Article · Apr 2015 · Dairy Science & Technology
  • Source
    • "generation_sequencing/). Although NGS platforms are improving at a very quick rate, thereby reducing costs by a factor of two to three each year, the cost is still too high for routine largescale sequencing of whole genomes for scientific research [19]. At this point, next generation platforms are usually used as complementary to microarray analysis. "
    [Show abstract] [Hide abstract] ABSTRACT: Over the recent years, next generation sequencing and microarray technologies have revolutionized scientific research with their applications to high-throughput analysis of biological systems. Isolation of high quantities of pure, intact, double stranded, highly concentrated, not contaminated genomic DNA is prerequisite for successful and reliable large scale genotyping analysis. High quantities of pure DNA are also required for the creation of DNA-banks. In the present study, eleven different DNA extraction procedures, including phenol-chloroform, silica and magnetic beads based extractions, were examined to ascertain their relative effectiveness for extracting DNA from ovine blood samples. The quality and quantity of the differentially extracted DNA was subsequently assessed by spectrophotometric measurements, Qubit measurements, real-time PCR amplifications and gel electrophoresis. Processing time, intensity of labor and cost for each method were also evaluated. Results revealed significant differences among the eleven procedures and only four of the methods yielded satisfactory outputs. These four methods, comprising three modified silica based commercial kits (Modified Blood, Modified Tissue, Modified Dx kits) and an in-house developed magnetic beads based protocol, were most appropriate for extracting high quality and quantity DNA suitable for large-scale microarray genotyping and also for long-term DNA storage as demonstrated by their successful application to 600 individuals.
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