DNA sequencing is the process of determining the exact order of the bases A, T, C, and G in a DNA molecule. This order carries genetic information that helps cells build proteins, regulate traits, and pass instructions to the next generation. Sequencing matters because it lets scientists compare genes, identify mutations, track pathogens, and study evolution.
Modern biology, medicine, forensics, and agriculture all rely on accurate DNA sequence data.
In Sanger sequencing, DNA polymerase copies a template strand while occasional chain-terminating nucleotides stop growth at specific bases, creating fragments of different lengths. By separating these fragments and reading their fluorescent labels, a computer reconstructs the sequence. Next-generation sequencing uses millions of DNA fragments copied and read in parallel, producing many short reads that are aligned to a reference genome or assembled into longer sequences.
The final base-call output is a digital string of letters with quality scores that show how confident the instrument is in each base.
Understanding Biology: DNA Sequencing
A DNA molecule has two linked strands that run in opposite directions. Each strand can act as a template for making its partner because the bases fit by strict pairing rules. A sequencing machine usually reads newly made DNA, then software works out the sequence of the original template.
This direction matters. A result may need to be converted into its reverse complement before it can be compared with a gene or genome map.
Students often lose marks by reading a copied strand as though it were the template strand. Always check which strand is shown and which end is being read.
Before a sample reaches a machine, its DNA is broken into manageable pieces. Short known sequences called adapters are attached to the ends. These give the fragments places to bind and provide signals needed during reading.
Many methods first make many copies of each fragment. This step can introduce bias because some DNA regions copy more easily than others. Regions with many G and C bases can be difficult to separate and copy.
A sequence dataset is therefore not a perfectly even picture of the original sample. Scientists use controls and repeated measurements to notice technical errors.
Modern instruments produce many short pieces of sequence called reads. Software must decide where each read belongs. If a reference genome exists, reads are lined up against it.
This works well for finding small differences, such as one changed base. It is harder when DNA contains long repeated sections. A short read from one repeat may match several places equally well.
When no suitable reference exists, overlapping reads can be joined to build longer stretches. This is called assembly. Gaps, repeats, and mixed DNA from more than one organism can make assembly uncertain.
The letters in a final sequence are not equally reliable. Machines measure signals, and weak or overlapping signals can cause an incorrect base call. Quality scores give an estimate of confidence for every position.
A higher score means that an error is less likely, but it does not guarantee the call is correct. Coverage is important because reading the same position many times helps reveal whether a difference is real. If nearly all reads show one base and a few show another, the rare result may be an error.
In medicine, this careful checking matters when a genetic variant could affect diagnosis or treatment. In outbreak studies, comparing pathogen sequences can show which samples are closely related, though sequence similarity alone cannot prove direct person to person transmission.
Key Facts
- DNA sequence is written as a string using the bases A, T, C, and G.
- Base pairing rules are A pairs with T and C pairs with G.
- Sanger sequencing uses ddNTPs that stop DNA synthesis because they lack a 3 prime OH group.
- Fragment length in Sanger sequencing identifies the position of each base in the copied DNA strand.
- NGS coverage can be estimated as coverage = total bases sequenced / genome size.
- A Phred quality score is Q = -10 log10(P), where P is the probability of an incorrect base call.
Vocabulary
- DNA sequencing
- DNA sequencing is the process of determining the order of nucleotide bases in a DNA molecule.
- Sanger sequencing
- Sanger sequencing is a method that uses chain-terminating nucleotides to create labeled DNA fragments that reveal a sequence.
- Next-generation sequencing
- Next-generation sequencing is a set of high-throughput methods that read millions of DNA fragments at the same time.
- Base call
- A base call is the computer-assigned identification of a base as A, T, C, or G at a position in a sequencing read.
- Coverage
- Coverage is the average number of times each base in a DNA region is read during sequencing.
Common Mistakes to Avoid
- Confusing the template strand with the newly synthesized strand is wrong because sequencing usually reports the bases of the newly made strand or converts them to the complementary template sequence depending on the workflow.
- Forgetting that ddNTPs stop DNA synthesis is wrong because Sanger sequencing depends on termination at different positions to create readable fragment lengths.
- Assuming one short NGS read gives a complete genome is wrong because short reads must be aligned, assembled, and checked with coverage and quality scores.
- Ignoring quality scores is wrong because a base letter alone does not show how likely the base call is to be correct.
Practice Questions
- 1 A sequencing run produces 12,000,000 bases of data for a bacterial genome that is 4,000,000 bases long. What is the average coverage?
- 2 A Phred quality score is Q = 30. Using Q = -10 log10(P), what is the probability P that the base call is incorrect?
- 3 Explain why next-generation sequencing can detect rare mutations more effectively when the coverage is high.