Next-generation sequencing technologies

NGS applications

Illumina sequencing workflow

Overview

Short-read NGS

Long-read NGS

Sequencing by ligation

Sequencing by synthesis

Single-molecule approach

Synthetic approach

Illumina

General principles of short-read NGS

Construct a library of fragments

Generate clonal template populations

Massively parallel DNA sequencing reactions

Analyze data

Library preparation

• Prepares sample nucleic acids for sequencing Fragmentation

Generates double-stranded DNA flanked by Illumina adapters

Generates the same general template structure, but variables include Insert size Adapter type Index for multiplexing

Library preparation: OverviewPurified genomic DNA

Fragment DNA

Repair Ends

Fragments < 800bp

Add an “A” to the 3’ Ends

Blunt end fragments with 5’ phosphorylated ends

Ligate Paired-end adapters

Size-select on Gel

QC Library

300-600bp fragments

Amplified DNA with adapters

Genomic DNA Library

Library preparation: Fragmentation

The size of the target DNA fragments in the final library is a key parameter for NGS library construction.

Optimal library size is impacted by1. the process of cluster generation: Short products amplify more

efficiently than longer products. Longer library inserts generate larger, more diffuse clusters than short inserts.

2. the sequencing application: For example, 2×100 PE for exome sequencing since more than 80% of human exomes are under 200bp.

Library preparation: Fragmentation

Three approaches are available to fragment nucleic acids:

1. Physical: Acoustic shearing and sonication, main method for genomic DNA

2. Enzymatic: Non-specific endonucleases cocktails or Transposase tagmentation, a greater number of artifactual indels compared with the physical method, reduced sampling handling and preparation time

3. Chemical: Heat and divalent metal cation, reserved for mRNA

Library preparation: Repair Ends

Library preparation: A-tailing

To facilitate ligation to sequencing adapter To prevent self-ligation between blunt ended template molecules (concatermers), or between adapters (adapter dimers)

A PA P

Library preparation: Adapter ligation

Library preparation: Y-shaped adaptors

Library preparation: Y-shaped adapters

Y-shaped adapters Non Y-shaped adapters

Library preparation: Size-select on Gel

300bp area excised

600bp area excised

Library preparation: PCR

• Selectively enrich DNA fragments with adapters on both ends

• Amplify the amount of DNA in the library

Library preparation: PCR

Library preparation: QC LibraryQC by Agilent Bioanalyzer: gives size confirmation and visualizes unwanted products

Lower marker15bp

Upper marker1500bp

Analyze data

Cluster amplification: Flow cells

• Adapter-ligated library elements hybridize to complementary oligonucleotides on the surface of a flow cell. Each attached library fragment acted as a seed and is amplified to generate a clonal cluster containing thousands of identical fragments.

• Ideally, clusters are of similar size and spaced well apart from each other to achieve accurate resolution during imaging. In reality, DNA clusters are randomly distributed across the flow cell with many clusters in close proximity to neighboring clusters, if the sample is overloaded, making it difficult to discern individual clusters from each others and reducing the amount of information generated during the run.

Cluster amplification: Patterned flow cells

Cluster amplification: Patterned flow cells• Patterned flow cell technology provides even cluster spacing and uniform feature size to deliver extremely high cluster densities.

• Clusters can only form in the nanowells, allowing accurate resolution of clusters during imaging.

Cluster amplification

Cluster amplification: Hybridization and extension

Cluster amplification: Denaturation

Cluster amplification: Anchor the template to the surface

Cluster amplification: Bridge amplification

Cluster amplification: Denaturation

Cluster amplification: Bridge amplification

Cluster amplification: P5 Linearization

Cluster amplification: Blocking

Cluster amplification: Read1 sequencing

Analyze data

Sequencing by synthesis

Single read, paired-end and read lengths

• Program the system to sequence a specific number of bases (1-600 bases)

• Sequence the strands from both directions to achieve a total of e.g. 600 bases (2×300 bases)

Paired-end sequencing

Longer read lengths improve 1) the overall length of contiguous sequence that can be assembled, and 2) the certainty of short read alignments.

Several next-generation sequencers have offered increases in read length over time. Another improvement has resulted from paired-end sequencing, producing sequence data from both ends of each library fragment. Read pairs can be obtained by one of two mechanisms: 1) paired ends or 2) mate pairs.

Paired-end sequencing: P7 linearization

Paired-end sequencing(a) paired-end (b) mate-pair

Fragment length

< 1000 bp > 1000 bp

Advantage Higher accuracy of alignments than a single-end read of the same length

Providing a scaffold for de novo

sequencing by long-range order and orientation

Illumina: Summary

https://www.youtube.com/watch?v=fCd6B5HRaZ8

Illumina platforms: Benchtop sequencers

https://www.illumina.com/systems/sequencing-platforms.html

Illumina platforms: Production-scale sequencers

https://www.illumina.com/systems/sequencing-platforms.html

Choosing a library type

• Single read library• Unidirectional sequencing• Compatible with only single-read flow cells• Applications: ChIP-seq, mRNA-seq for quantification, low-coverage

resequencing

• Paired end library• Uni or Bidirectional sequencing• Compatible with both single-read and paired-end flow cells• Applications: the most common library type, de novo assembly,

structural variants detection, high-coverage resequencing

• Indexed libraries• Uni or bidirectional sequencing• Allows multiple libraries per lane• Single-indexed libraries: adds up to 48 unique 6-base index 1 (i7) se

quences to generate up to 48 uniquely tagged libraries.• Dual-indexed libraries: adds up to 24 unique 8-base index 1 (i7)

sequences and up to 16 unique 8-base index 2 (i5) sequences to generate up to 384 uniquely tagged libraries.

Single-indexed sequencing

The single-indexed sequencing workflow applies to all Illumina sequencing platforms.

Dual-indexed sequencing on a paired-end flow cellDual-indexed sequencing includes 2 index reads.

Dual-indexed adapters

Dual-indexed sequencing: Workflow A

7 dark-cycles

Dual-indexed sequencing: Workflow A

Dual-indexed sequencing: Workflow B

Reads and coverage

• The number of reads for a specific region is denoted “depth” or “coverage”

Next-generation sequencing technologies · 2017. 7. 14. · paired-end sequencing, producing...

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