Sequencing measurement · BEGINNER LESSON
Read depth
Read depth is the average number of times a target nucleotide is represented in sequencing reads. It is a measure of sequencing coverage, but a higher value does not automatically mean more precise or more reproducible results.
What it measures
Read depth measures how many sequencing reads cover a target nucleotide, on average. In deep sequencing, this is often called coverage or sequencing depth. A higher read depth usually gives more repeated observations of the same DNA or RNA sequence, which can help estimate things like allele frequency, but it is only one part of how reliable the data are. [0,1]
[1] 2. Results [2] Making sense of deep sequencing - PMC - NIH
How it affects results
If sequencing were error-free and the sampled population were large and representative, more read depth would make an estimated allele frequency more precise. In real data, however, sequencing error, uneven capture or amplification, and the biological mixture being sampled can all increase uncertainty. In virus studies, sequencing a small number of particles at very high read depth can repeatedly read the same original molecules, so the number of particles, not just the number of reads, can limit the useful information. For gene expression, read depth also interacts with the structure of the data: dominant transcriptional programs can often be recovered at low depth when genes vary together, but less stable signals need more reads. [0,2]
[1] 2. Results [3] Low-dimensionality in gene expression data enables the accurate extraction of transcriptional programs from shallow sequencing
Limits and interpretation
Read depth should not be treated as a guarantee of precision. The studies here show that a high absolute read depth can still fail to reproduce a population accurately if the sample is distorted, split unevenly, depleted, or amplified in a way that changes what is observed. A useful idea is effective depth: the depth of an idealized sequencing process that would create the same amount of variance as the real data. Effective depth cannot exceed absolute read depth and may be much lower when the sample preparation or biology adds extra noise. [0,1]
[1] 2. Results [2] Making sense of deep sequencing - PMC - NIH
SEE THE IDEA
From reads to useful information
DNA or RNA segment chosen for sequencing.
Many short sequence observations are collected.
Average number of times each target nucleotide is seen.
How much usable coverage remains after error and sampling limits.
More reads can raise precision, but sample composition and experimental noise can reduce the useful depth.
Read depth counts how often a target is read; effective depth asks how much of that counting is still informative after real-world limits.
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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.