Questions: Next Generation Sequencing Technologies and Platforms

Question 1

A student says: 'NGS is faster than Sanger sequencing because it reads each DNA fragment more quickly.' What is the actual source of NGS's overwhelming throughput advantage?

• A. The student is correct — NGS chemistry processes each base pair faster than Sanger's dideoxy chemistry
• B. NGS achieves massive parallelism: it sequences millions of DNA fragments simultaneously across a flow cell, while Sanger reads essentially one fragment at a time
• C. NGS requires no amplification step, eliminating the time needed for PCR before sequencing
• D. NGS reads much longer fragments, so fewer total reads are needed to cover a genome

Answer: (B)

The per-base chemistry of NGS is not necessarily faster than Sanger; each sequencing cycle still takes time. The transformative advantage is parallelism: a single Illumina run can generate billions of bases because millions of clusters are all being imaged simultaneously in each cycle. Sanger sequencing runs one capillary at a time (or a few dozen in a high-throughput setup). This parallelism is what dropped human genome sequencing cost from ~$100 million to under $1,000. Option C is incorrect — Illumina requires bridge amplification, a form of PCR.

Question 2

Why do short-read sequencing platforms like Illumina (producing ~150 bp reads) pose a challenge for assembling a complete genome without a reference sequence?

• A. Short reads have too many sequencing errors per base to be reliably aligned
• B. Repetitive genomic regions longer than ~150 bp cannot be uniquely spanned by any single read, making it impossible to determine how copies of a repeat connect to flanking unique sequence
• C. The bridge amplification step is incompatible with repetitive DNA, causing those regions to be underrepresented
• D. Short reads cannot be extended by the polymerase past their termination length, leaving gaps in coverage

Answer: (B)

If a repeat element appears thousands of times in the genome, a 150 bp read falling entirely within that repeat is identical regardless of which copy it came from. Without a unique sequence spanning across the repeat boundary, assembly algorithms cannot determine how to order the copies or what flanks them. Long-read platforms (PacBio, Oxford Nanopore) with reads of 10–100+ kb can span across most repeats, bridging them to unique flanking sequences and enabling true de novo assembly. This is why long reads are essential for building complete, reference-quality genomes.

Question 3

In Illumina sequencing, each cluster on the flow cell contains copies of many different original DNA fragments that were most amplified together in the same spot.

• T. True
• F. False

Answer: False

Each cluster contains approximately 1,000 identical copies of ONE original DNA fragment, generated by bridge amplification. A single adapted fragment binds to the flow cell surface, bends over to hybridize to an adjacent oligonucleotide, and is extended to form a double-stranded bridge. This is repeated in a localized PCR-like process until a dense colony of identical copies occupies a small spot. The homogeneity of each cluster is essential — it provides enough identical signal for the camera to reliably detect which fluorescent base was incorporated at each cycle.

Question 4

RNA-seq is an application of NGS technology that sequences the transcriptome, providing information about which genes are active and at what levels in a given cell type or condition.

• T. True
• F. False

Answer: True

RNA-seq works by isolating RNA from a sample, converting it to cDNA (reverse transcription), fragmenting and adapting the cDNA, and then sequencing millions of fragments using standard NGS. The number of reads mapping to each gene reflects its expression level. This has transformed transcriptomics — instead of measuring only pre-selected genes (as microarrays do), RNA-seq surveys the entire expressed genome at once, discovers novel transcripts, and detects alternative splicing events. It illustrates how NGS is a platform enabling many applications beyond genomic sequencing.

Question 5

Why does Illumina sequencing require bridge amplification before any base reading begins, and what specific problem does it solve?

Model answer: Bridge amplification solves the problem of signal detection sensitivity. A single DNA molecule is too dim to be reliably detected by the flow cell's camera when fluorescently labeled bases are incorporated one at a time. By amplifying each original fragment into a cluster of ~1,000 identical copies, the fluorescent signal from all copies in the cluster fires simultaneously during each imaging cycle, producing a signal strong enough to detect reliably. Without this clonal amplification step, the signal from a single molecule's incorporation event would be indistinguishable from background noise.

This is why NGS chemistry requires library preparation with adapter ligation before the flow cell step — the adapters provide the priming sites for bridge amplification on the flow cell surface. Single-molecule long-read platforms like PacBio and Oxford Nanopore use different signal detection approaches (fluorescence from individual polymerases in zero-mode waveguides, or ion current changes through a nanopore) that are sensitive enough to read individual molecules without clonal amplification — which is how they achieve much longer reads.