Demand has never been greater for revolutionary technologies that deliver fast, inexpensive and accurate genome information. This challenge has catalysed the development of next-generation sequencing (NGS) technologies. The inexpensive production of large volumes of sequence data is the primary advantage over conventional methods. Here, I present a technical review of template preparation, sequencing and imaging, genome alignment and assembly approaches, and recent advances in current and near-term commercially available NGS instruments. I also outline the broad range of applications for NGS technologies, in addition to providing guidelines for platform selection to address biological questions of interest.

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Sequencing technologies-the next generation

We describe a flexible system for gene expression profiling using arrays of tens of thousands of oligonucleotides synthesized in situ by an ink-jet printing method employing standard phosphoramidite chemistry. We have characterized the dependence of hybridization specificity and sensitivity on parameters including oligonucleotide length, hybridization stringency, sequence identity, sample abundance, and sample preparation method. We find that 60-mer oligonucleotides reliably detect transcript ratios at one copy per cell in complex biological samples, and that ink-jet arrays are compatible with several different sample amplification and labeling techniques. Furthermore, results using only a single carefully selected oligonucleotide per gene correlate closely with those obtained using complementary DNA (cDNA) arrays. Most of the genes for which measurements differ are members of gene families that can only be distinguished by oligonucleotides. Because different oligonucleotide sequences can be specified for each array, we anticipate that ink-jet oligonucleotide array technology will be useful in a wide variety of DNA microarray applications.

Expression profiling using microarrays fabricated by an ink-jet oligonucleotide synthesizer.

We describe the draft genome of the microcrustacean Daphnia pulex, which is only 200 megabases and contains at least 30,907 genes. The high gene count is a consequence of an elevated rate of gene duplication resulting in tandem gene clusters. More than a third of Daphnia's genes have no detectable homologs in any other available proteome, and the most amplified gene families are specific to the Daphnia lineage. The coexpansion of gene families interacting within metabolic pathways suggests that the maintenance of duplicated genes is not random, and the analysis of gene expression under different environmental conditions reveals that numerous paralogs acquire divergent expression patterns soon after duplication. Daphnia-specific genes, including many additional loci within sequenced regions that are otherwise devoid of annotations, are the most responsive genes to ecological challenges.

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The ecoresponsive genome of Daphnia pulex

Physical interactions between genetic elements located throughout the genome play important roles in gene regulation and can be identified with the Chromosome Conformation Capture (3C) methodology. 3C converts physical chromatin interactions into specific ligation products, which are quantified individually by PCR. Here we present a high-throughput 3C approach, 3C-Carbon Copy (5C), that employs microarrays or quantitative DNA sequencing using 454-technology as detection methods. We applied 5C to analyze a 400-kb region containing the human beta-globin locus and a 100-kb conserved gene desert region. We validated 5C by detection of several previously identified looping interactions in the beta-globin locus. We also identified a new looping interaction in K562 cells between the beta-globin Locus Control Region and the gamma-beta-globin intergenic region. Interestingly, this region has been implicated in the control of developmental globin gene switching. 5C should be widely applicable for large-scale mapping of cis- and trans- interaction networks of genomic elements and for the study of higher-order chromosome structure.

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Chromosome Conformation Capture Carbon Copy (5C): A massively parallel solution for mapping interactions between genomic elements

Laboratoire de Génie Enzymatique et Biomoléculaire, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, 43 Boulevard du 11 Novembre 1918, Villeurbanne F-69622, France, UMR5246, Centre National de La Recherche Scientifque, Villeurbanne F-69622, France, Université de Lyon, Lyon F-69622, France, Université Lyon 1, Lyon F-69622, France, Institut National des Sciences Appliquées de Lyon, EÄ cole d’Ingénieurs, Villeurbanne F-69621, France, and EÄ cole Supérieure Chimie Physique EÄ lectronique de Lyon, Villeurbanne F-69616, France

DNA biosensors and microarrays.

Oligonucleotide microarrays, also called "DNA chips," are currently made by a light-directed chemistry that requires a large number of photolithographic masks for each chip. Here we describe a maskless array synthesizer (MAS) that replaces the chrome masks with virtual masks generated on a computer, which are relayed to a digital micromirror array. A 1:1 reflective imaging system forms an ultraviolet image of the virtual mask on the active surface of the glass substrate, which is mounted in a flow cell reaction chamber connected to a DNA synthesizer. Programmed chemical coupling cycles follow light exposure, and these steps are repeated with different virtual masks to grow desired oligonucleotides in a selected pattern. This instrument has been used to synthesize oligonucleotide microarrays containing more than 76,000 features measuring 16 microm 2. The oligonucleotides were synthesized at high repetitive yield and, after hybridization, could readily discriminate single-base pair mismatches. The MAS is adaptable to the fabrication of DNA chips containing probes for thousands of genes, as well as any other solid-phase combinatorial chemistry to be performed in high-density microarrays.

Maskless fabrication of light-directed oligonucleotide microarrays using a digital micromirror array.

The synthesis of arrays of DNA probes sequences, polypeptides, and the like is carried out using a patterning process on an active surface of a substrate. An image is projected onto the active surface of the substrate utilizing an image former that includes a light source that provides light to a micromirror device comprising an array of electronically addressable micromirrors, each of which can be selectively tilted between one of at least two positions. Projection optics receives the light reflected from the micromirrors along an optical axis and precisely images the micromirrors onto the active surface of the substrate, which may be used to activate the surface of the substrate. The first level of bases may then be applied to the substrate, followed by development steps, and subsequent exposure of the substrate utilizing a different pattern of micromirrors, with further repeats until the elements of a two dimensional array on the substrate surface have an appropriate base bound thereto. The micromirror array can be controlled in conjunction with a DNA synthesizer supplying appropriate reagents to a flow cell containing the active substrate to control the sequencing of images presented by the micromirror array in coordination of the reagents provided to the substrate.

Micromirror apparatus for synthesis of arrays of DNA probes

A flow cell which can be used in the synthesis of DNA probes on an active surface of the substrate includes a base having a central window opening and a registration surface against which the substrate may be mounted. A gasket having a central opening defining an active area is mounted on the active surface of the substrate and has inlet and output extension openings which extend away from the central opening. A press block is engaged against the gasket to fully enclose an active volume between the press block, the peripheral walls of the central opening in the gasket, and the active surface of the substrate. A press is mounted to the base to selectively press the press block against the gasket and hold it in position. Channels in the press block extend to the extension openings in the gasket to allow flow of reagent into and out of the active volume, which is confined to the thickness of the gasket and can thus be minimized. The press may include a press screw which can be turned to engage against the press block to hold it into position, and which can be turned to release the press block from the gasket, allowing rapid and easy replacement of substrates.

Flow cell for synthesis of arrays of dna probes and the like

Biological lithography: development of a maskless microarray synthesizer for DNA chips

The synthesis of arrays of DNA probes sequences, polypeptides, and the like is carried out using a patterning process on an active surface of a substrate. An image is projected onto the active surface of the substrate utilizing reflective projection optics. The projection optics project a light image onto the active surface of the substrate to deprotect linker molecules thereon. A first level of bases may then be applied to the substrate, followed by development steps, and subsequent exposure of the substrate utilizing a different light image, with further repeats until the elements of a two dimensional array on the substrate surface have an appropriate base bound thereto.

Sangeet Singh-Gasson

Papers

Maskless fabrication of light-directed oligonucleotide microarrays using a digital micromirror array.

Micromirror apparatus for synthesis of arrays of DNA probes

Flow cell for synthesis of arrays of dna probes and the like

Biological lithography: development of a maskless microarray synthesizer for DNA chips

Methods for the synthesis of arrays of DNA probes