What Is NGS? A Beginner’s Guide to Next-Generation Sequencing

Next-generation sequencing (NGS), also known as high-throughput sequencing, is a technology that allows for the rapid sequencing of millions or billions of DNA or RNA fragments simultaneously.

This method has revolutionized genomics research by enabling large-scale analysis of genetic material, providing detailed information about genomes, genetic variations, and gene expression.

Sequencing DNA means determining the order of the four chemical building blocks - called "bases" - that make up the DNA molecule. The sequence tells scientists the kind of genetic information that is carried in a particular DNA segment. For example, scientists can use sequence information to determine which stretches of DNA contain genes and which stretches carry regulatory instructions, turning genes on or off. In addition, and importantly, sequence data can highlight changes in a gene that may cause disease.

In the DNA double helix, the four chemical bases always bond with the same partner to form "base pairs." Adenine (A) always pairs with thymine (T); cytosine (C) always pairs with guanine (G). This pairing is the basis for the mechanism by which DNA molecules are copied when cells divide, and the pairing also underlies the methods by which most DNA sequencing experiments are done. The human genome contains about 3 billion base pairs that spell out the instructions for making and maintaining a human being.

Next-Generation Sequencing (NGS) works by breaking DNA into small pieces, reading those pieces in parallel, and using software to reconstruct the full sequence. It’s like shredding a book into tiny slips of paper, scanning them all at once, and using a computer to put the story back together.

1. Sample Collection and DNA Extraction

Scientists start by collecting cells from bacteria, plants, or human samples and extract the DNA. This is the raw material to be sequenced.

Think of DNA as a long string of genetic instructions written in A, T, C, and G.

2. Fragmentation

The long strands of DNA are cut into smaller pieces. This is done using enzymes or sonication (sound waves) so they can fit into the sequencing machine.

Typical fragment sizes range from 150 to 600 base pairs.

3. Library Preparation

Each DNA fragment is given special adapters on both ends. These are short, known DNA sequences that:

1. Allow the DNA to stick to the flow cell (inside the sequencer)
2. Act as starting points for copying
3. Identify which sample the DNA came from (barcoding)

4. Amplification

The prepared fragments are copied many times using PCR (polymerase chain reaction). This ensures there’s enough DNA for the sequencer to detect.

This step boosts the signal, like turning up the brightness on a screen.

5. Sequencing

Here’s the magic:

1. DNA fragments are placed on a flow cell (a tiny glass slide).
2. A sequencing machine adds one fluorescently labeled base (A, T, C, or G) at a time.
3. A camera takes a picture after each base is added.
4. The color of the light tells the machine which base was added.
5. This repeats millions of times in parallel, reading all fragments.

This process is called sequencing by synthesis in most platforms like Illumina.

6. Data Analysis

1. Once all fragments are read, bioinformatics software assembles the sequences like a jigsaw puzzle:
2. It matches overlapping parts
3. Reconstructs the original DNA sequence
4. Identifies mutations, insertions, deletions, and gene expressions

This step uses algorithms to sort and align millions of reads accurately.

Clinical Genetics:

1. NGS detects more mutations than older methods like Sanger sequencing. It identifies small changes, deletions, and complex DNA rearrangements all in one test no need for separate assays like FISH or CGH arrays.
2. No prior knowledge needed: Unlike traditional methods that target known genes, NGS can scan the whole genome or exome to find completely new disease causing mutations. This helps diagnose unexplained genetic conditions, especially in children.
3. Higher sensitivity: NGS can detect rare or mosaic mutations that may be missed by older methods. This is useful for analyzing fetal DNA in maternal blood or tracking cancer cells in the bloodstream.

Microbiology:

1. NGS can identify pathogens by reading their entire genome, replacing older methods like staining or culturing. It helps trace infection sources and drug resistance.
2. Example: NGS uncovered an undetected MRSA outbreak in a UK hospital and traced it to one staff member.

Oncology:

1. Cancer is caused by genetic mutations. NGS allows full sequencing of tumors to find those mutations.
2. Benefits for patients include better diagnosis, prognosis, and the potential to match specific mutations with targeted treatments.
3. Pilot programs are already testing NGS in clinics for personalized cancer care.

1. High setup requirements: NGS needs powerful computers, large data storage, and skilled experts to interpret complex results.
2. Data management is challenging: The large volume of sequencing data must be carefully filtered to find clinically relevant information.
3. Cost efficiency depends on scale: While sequencing itself is cheap (millions of reads for ~£1000), it’s only cost-effective when run in large batches, often requiring centralized, regional labs.
4. Upfront investment: Setting up an NGS facility is expensive at first, but once established, it can serve nationwide needs and improve both efficiency and patient care.