What is DNA Sequencing?
DNA sequencing is the process of determining the exact order of nucleotide bases—adenine (A), thymine (T), guanine (G) and cytosine (C)—in a strand of DNA. It underpins modern genetics, from diagnosing diseases to tracing evolutionary relationships.
DNA sequencing reads the precise order of A, T, G and C bases in a DNA molecule, most commonly using Sanger chain-termination or next-generation sequencing (NGS) technologies.
- 1↓Sample PreparationExtract and purify the DNA template to be sequenced.
- 2↓Chain-Termination ReactionMix DNA polymerase, primers, normal nucleotides, and fluorescently labeled ddNTPs that stop the strand when added.
- 3↓Fragment GenerationThe reaction produces DNA fragments of every possible length, each ending in a labeled ddNTP.
- 4↓Capillary ElectrophoresisFragments are separated by size as they migrate through a gel-filled capillary.
- 5Sequence Read-OutA laser detects the fluorescent tag on each fragment, and software reconstructs the base order.
Step-by-step worked examples
A Sanger sequencing reaction produces fragments ranging from 1 to 800 bases long. How many distinct fragment lengths are theoretically possible?
Fragments can end at any base position from 1 to 800 Number of distinct lengths = 800 − 1 + 1 = 800 Each length corresponds to one base call in the final read
A human genome has about 3.2 billion base pairs. If a sequencer reads 150 bases per run and covers the genome 30x, how many total bases must be sequenced?
Total bases needed = genome size × coverage Total bases = 3,200,000,000 × 30 = 96,000,000,000 bases At 150 bases per read: 96,000,000,000 ÷ 150 ≈ 640,000,000 reads needed
A researcher sequences a 1,200 base-pair gene using Sanger sequencing with a 700-base read length in both directions (forward and reverse). Is single-pass coverage enough?
Forward read covers bases 1–700 Reverse read covers bases 501–1,200 (reading backward from the end) Overlap region (501–700) confirms accuracy of 200 bases Combined, both reads span the full 1,200 bases with overlap for error-checking
Flashcards
Quick quiz
Q1.Which four bases does DNA sequencing determine the order of?
Q2.What stops DNA synthesis in Sanger sequencing?
Q3.What technology separates DNA fragments by size in Sanger sequencing?
Q4.Compared to Sanger sequencing, next-generation sequencing (NGS) is generally…
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Common mistakes
DNA sequencing and DNA replication are the same process. — Correct: Replication copies DNA inside cells; sequencing is a lab technique to read the base order.
Sanger sequencing and NGS produce reads the same way. — Correct: Sanger reads one fragment set via electrophoresis; NGS sequences millions of fragments in parallel with different chemistry.
Higher coverage always means a perfect genome assembly. — Correct: Coverage improves confidence but repetitive regions and errors can still cause gaps or misassembly.
Sequencing tells you gene function. — Correct: Sequencing only reveals the base order; function must be inferred from further analysis.
FAQ
What is DNA sequencing used for?
It's used to diagnose genetic diseases, study evolution, identify pathogens, and enable personalized medicine by reading an organism's genetic code.
What is the DNA sequencing formula for coverage?
Coverage = (Number of reads × Read length) ÷ Genome size, giving the average number of times each base is read.
What are examples of DNA sequencing methods?
Sanger (chain-termination) sequencing and next-generation sequencing (NGS) platforms like Illumina are the two most common examples.
How is DNA sequencing done step by step?
DNA is prepared, copied with labeled terminator bases, fragments are separated by size, and a detector reads the base order to reconstruct the sequence.