Molecular biology · BEGINNER LESSON
Sequencing
Sequencing is the process of determining the exact order of bases in a DNA strand. It measures the order of the DNA letters A, T, C, and G, which carries biological information.
What sequencing is
DNA, or deoxyribonucleic acid, is the molecule that stores biological instructions. A genome is an organism’s complete set of DNA, and sequencing means finding the exact order of the DNA bases A, T, C, and G in a strand. Because DNA bases pair in fixed ways, the base on one strand predicts its partner on the other strand. That makes it possible to read the sequence without separately reporting both bases of a pair.
[1] A Brief Guide to Genomics [3] A Brief Guide to Genomics (Archive)
How sequencing is done
A common modern method is sequencing by synthesis. In this approach, DNA polymerase, the enzyme that makes DNA, builds a new strand from a strand of interest. Each added nucleotide carries a fluorescent tag, so when a base is incorporated, a light signal identifies which nucleotide was added. The detected signal reveals the order of bases in the template strand.
[1] A Brief Guide to Genomics [3] A Brief Guide to Genomics (Archive)
Why many short reads are needed
Sequencers often produce reads, which are short stretches of sequence. To reconstruct a larger DNA region, researchers sequence overlapping segments and assemble them like pieces of a linear jigsaw puzzle. This overlap helps confirm the sequence because each base is usually read several times. A practical challenge is that the original genome is broken into many fragments, so the exact source position of a read is not always obvious at first.
[1] A Brief Guide to Genomics [2] 14 DNA sequencing: a fundamental tool for study- ing biology.
Limits of sequencing data
All DNA sequencing has some errors, so accuracy depends on repeated reading and careful assembly. Short-read methods are very efficient, but the reads are limited in length. Long-read methods can read much longer stretches, but the supplied sources say they are generally lower throughput and more error-prone than short-read Illumina sequencing. In other words, different sequencing technologies trade off speed, read length, and error rate.
[2] 14 DNA sequencing: a fundamental tool for study- ing biology.
What sequencing is used for
Researchers use sequencing to find genetic variation and mutations, including single-base changes and larger deletions. It is also used for genome resequencing, de novo genome assembly, and molecular counting. Large projects such as the Human Genome Project created a high-quality representative human genome sequence that can support many kinds of biomedical studies.
[1] A Brief Guide to Genomics [2] 14 DNA sequencing: a fundamental tool for study- ing biology. [3] A Brief Guide to Genomics (Archive)
SEE THE IDEA
How DNA sequencing turns a molecule into readable information
A molecule made of ordered A, T, C, and G bases.
DNA polymerase adds tagged nucleotides one by one.
Each added base gives a fluorescent signal that identifies it.
Overlapping reads are combined to reconstruct a longer sequence.
The DNA is chemically copied, the added bases are detected by light, and the short reads are assembled into a longer sequence.
Sequencing reads the order of DNA bases through tagged chemistry and then reconstructs longer regions from overlapping pieces.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.