DNA sequencers are medical technology devices that read the order of chemical bases in DNA. This order is a biological code that helps explain inherited traits, disease risk, infections, and how some cancers change over time. Modern sequencers can process millions or billions of DNA fragments in parallel, turning tiny chemical signals into digital data.
They matter because doctors and researchers use this information to diagnose disease, choose treatments, and track outbreaks.
Most sequencing machines begin by preparing DNA into short fragments and attaching them to a chip or flow cell. The machine then detects each base, A, T, C, or G, as the DNA is copied or passed through a tiny sensor. Optical sequencers read flashes of colored light, while nanopore sequencers measure changes in electric current as DNA moves through a pore.
Software converts these signals into base calls, checks their quality, and assembles the reads into useful genetic information.
Key Facts
- DNA bases are adenine, thymine, cytosine, and guanine, written as A, T, C, and G.
- Base pairing rules are A pairs with T and C pairs with G.
- A sequencing read is a short measured DNA sequence, often from about 50 to over 10,000 bases depending on the technology.
- Coverage = total bases sequenced / genome size.
- If 90,000,000 bases are sequenced from a 3,000,000 base bacterial genome, coverage = 30x.
- Accuracy is often reported with a quality score: Q = -10 log10(Perror).
Vocabulary
- DNA sequencer
- A machine that determines the order of bases in DNA and converts that information into digital data.
- Flow cell
- A small chip or channel system where DNA fragments are held and measured during sequencing.
- Base call
- The machine or software decision that a measured signal represents A, T, C, or G.
- Coverage
- The average number of times each base in a genome is read during a sequencing experiment.
- Variant
- A difference between a DNA sequence and a reference sequence, such as a substitution, insertion, or deletion.
Common Mistakes to Avoid
- Confusing sequencing with gene editing, because sequencing reads DNA but does not change the DNA sequence.
- Assuming one read equals a whole genome, because most machines read many short fragments that must be aligned or assembled.
- Ignoring quality scores, because a base call can be uncertain and low quality data can create false variants.
- Thinking higher coverage always solves every problem, because contamination, poor sample preparation, and biased amplification can still cause errors.
Practice Questions
- 1 A lab sequences 600,000,000 total bases from a human gene panel that contains 20,000,000 target bases. What is the average coverage?
- 2 A base call has an error probability of 0.001. Using Q = -10 log10(Perror), what is its quality score?
- 3 A patient sample and a virus reference sequence differ at several positions. Explain why a sequencer needs both accurate base calls and comparison to a reference sequence to identify those differences.