Molecular genetics · BEGINNER LESSON
DNA sequence
DNA sequence is the order of DNA bases along a DNA molecule. Sequencing is the process of reading that order from many short or long pieces of DNA and comparing them to a reference or other genomes to identify differences.
What it is and what is measured
A DNA sequence is the ordered pattern of DNA bases in a stretch of genetic material. In sequencing, the measured output is the base order inferred from DNA fragments, often collected from blood or saliva. The goal is to reconstruct the sequence of a person’s genome or a region of interest and then compare it with a reference genome or with other genomes to find variants, meaning places where the sequence differs. [source 0,1,2]
[1] How scientists are rethinking genome sequencing to bolster ... [2] Harmonizing standards and resources for the medical genome [3] An information theory approach to quantifying the sequence ... - PMC
How sequencing works
Modern sequencing usually starts by purifying DNA, then breaking it into fragments. In short-read sequencing, many short fragments are read and aligned, or matched, to positions in a reference genome. The aligned reads are then examined position by position to call variants. In nanopore sequencing, a DNA strand moves through a tiny pore called a nanopore. An ion current is measured, and the current pattern depends on the DNA sequence in the pore; a motor enzyme helps control that motion, and its timing can also carry sequence information. [source 1,2]
[2] Harmonizing standards and resources for the medical genome [3] An information theory approach to quantifying the sequence ... - PMC
Why comparisons matter
A reference genome is a standard sequence used as a guide. It helps scientists rebuild an individual genome from many fragments and spot mutations, which are sequence changes that may be linked to disease. Some projects aim to improve this guide with a pangenome, a collection of real human genomes rather than a single reference. Some efforts also seek telomere-to-telomere, or T to T, sequences, meaning complete chromosome coverage. [source 0,1]
[1] How scientists are rethinking genome sequencing to bolster ... [2] Harmonizing standards and resources for the medical genome
Limitations
Short-read sequencing can miss hard-to-measure regions, especially long repeats that exceed the read length. Reference genomes are also incomplete and do not represent all human variation well, so rare but important variants can be missed. In nanopore sequencing, decoding depends on current patterns and enzyme behavior, so sequence calling still has error and benefits from additional models and quality checks such as Phred quality scores, which estimate confidence in each base call. [source 0,1,2]
[1] How scientists are rethinking genome sequencing to bolster ... [2] Harmonizing standards and resources for the medical genome [3] An information theory approach to quantifying the sequence ... - PMC
SEE THE IDEA
How a DNA sequence is obtained and checked
DNA is purified and broken into pieces.
Machines infer base order from short-read alignment or nanopore current signals.
The inferred sequence is compared with a guide or with other real genomes.
Differences such as mutations are identified.
Fragments are measured first, then assembled into sequence, then compared to detect differences.
Sequencing turns physical DNA fragments or signal traces into an inferred base order, which is then checked against a guide or other genomes.
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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.