NGS FAQ

The differences between DNA and RNA sequencing.

DNA sequencing and RNA sequencing both use high-throughput sequencing technologies, but they answer different biological questions. DNA-seq examines the genome itself, while RNA-seq examines the RNA molecules produced from the genome under specific biological conditions.

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Quick answer

DNA sequencing reads DNA and is used to analyse the genome: inherited variants, somatic mutations, insertions and deletions, copy number changes, structural variants, genome assemblies, targeted panels, and sometimes DNA methylation depending on the protocol.

RNA sequencing reads RNA-derived libraries, usually after conversion of RNA to complementary DNA. It is used to analyse which genes are active, how strongly they are expressed, which transcript isoforms are present, whether splicing is altered, and whether fusion transcripts or RNA-level changes can be detected.

              In simple terms: DNA-seq asks “What is encoded in the genome?” RNA-seq asks “What is being expressed under this condition?”

DNA sequencing vs RNA sequencing

Feature	DNA sequencing	RNA sequencing
Main molecule	DNA	RNA, usually converted into cDNA before sequencing
Main question	What genomic sequence or genomic alteration is present?	Which genes and transcripts are expressed, and at what level?
Typical use	Variant calling, genome assembly, copy number, structural variants, targeted panels, WGS, WES	Differential expression, transcript isoforms, splicing, gene fusions, pathway activity, cell-state analysis
Biological stability	Usually stable across most cell types of the same organism, except mutations, rearrangements or mosaicism	Dynamic and strongly dependent on tissue, condition, time point, treatment and cell state
Sample concern	DNA quality, quantity, fragmentation, contamination and target coverage	RNA integrity, RNase contamination, degradation, rRNA content, library strandedness and transcript composition
Common file outputs	BAM/CRAM, VCF, coverage tables, CNV calls, structural-variant calls, annotation reports	BAM/CRAM, count matrices, TPM tables, differential-expression tables, isoform tables, splice-junction files, fusion calls
Interpretation focus	Genotype, variant consequence, genome structure, inherited or acquired alterations	Functional activity, gene regulation, molecular phenotype, biological response

Different biological questions

DNA-seq and RNA-seq differ first of all in the biological question they address. DNA-seq is usually selected when the goal is to identify, confirm or quantify changes in genomic DNA. RNA-seq is selected when the goal is to understand transcriptional activity and RNA-level regulation.

DNA-seq is appropriate when you ask:

Which variants are present?
Is there an inherited or somatic mutation?
Are there insertions, deletions or structural changes?
Is a region amplified, deleted or rearranged?
What is the genome sequence or targeted panel result?

RNA-seq is appropriate when you ask:

Which genes are upregulated or downregulated?
Which pathways are active?
Which transcript isoforms are expressed?
Is splicing altered?
Are fusion transcripts or RNA-level changes present?

Different sample input and quality requirements

DNA is generally more chemically stable than RNA. RNA is more sensitive to degradation and depends strongly on sample collection, storage, extraction and RNase control. For RNA-seq, RNA integrity and sample handling can strongly influence downstream results.

DNA-seq input considerations DNA quantity, DNA fragmentation, contamination, organism identity, tumour purity when relevant, capture efficiency, and target-region coverage.

RNA-seq input considerations RNA integrity, ribosomal RNA content, library selection method, strandedness, tissue heterogeneity, batch effects, and RNA degradation.

Different library-preparation logic

DNA-seq libraries are prepared from genomic DNA or enriched target regions. RNA-seq libraries usually begin with RNA, which is converted into cDNA. This reverse-transcription step is one of the main experimental differences between the two methods.

DNA-seq Extract DNA

Fragment / enrich Whole genome, exome, panel or amplicon

Sequence Generate reads from DNA library

Interpret Variants, coverage and genome changes

RNA-seq Extract RNA

Select RNA Poly(A), rRNA depletion, small RNA or total RNA

Reverse transcribe Convert RNA into cDNA

Interpret Expression, isoforms and RNA events

Different bioinformatics workflows

DNA-seq and RNA-seq workflows often begin similarly with FASTQ quality control, trimming if necessary, and read mapping or alignment. After that, they diverge substantially.

Typical DNA-seq analysis

FASTQ quality control and preprocessing.
Alignment to a reference genome.
Duplicate marking and alignment-quality assessment where appropriate.
Variant calling, copy-number analysis or structural-variant analysis depending on the project.
Variant filtering, annotation, prioritisation and reporting.

Typical RNA-seq analysis

FASTQ quality control and adapter trimming when required.
Alignment to genome or transcriptome, or pseudoalignment to transcript references.
Gene or transcript quantification.
Sample-level QC, normalisation and exploratory analysis.
Differential expression, isoform analysis, splice-junction analysis, fusion detection or pathway analysis depending on the design.

Different outputs and interpretation

DNA-seq and RNA-seq produce different result tables and figures. The interpretation also differs: DNA-seq is usually interpreted around genomic alterations, while RNA-seq is interpreted around expression states and functional consequences.

DNA-seq outputs

Alignment files: BAM or CRAM
Variant calls: VCF
Coverage and target-region metrics
SNV, indel, CNV and structural-variant tables
Variant annotation and prioritisation reports

RNA-seq outputs

Alignment or pseudoalignment outputs
Gene and transcript count matrices
Normalised expression tables
Differential-expression results
Pathway, splicing, isoform or fusion reports

Important limitations

Neither DNA-seq nor RNA-seq answers every molecular question on its own. Each method has specific blind spots.

DNA-seq limitations Standard DNA-seq does not directly measure whether a gene is expressed, which transcript isoform is used, whether a variant affects splicing, or whether a pathway is transcriptionally active.

RNA-seq limitations RNA-seq may miss variants in genes that are not expressed, expressed at low levels, affected by allele-specific expression, degraded, or poorly captured by the library strategy.

              RNA-seq can provide evidence that a DNA variant has functional consequences, but DNA-seq is usually more appropriate for complete variant discovery.
            

Which one should you choose?

The correct choice depends on the main endpoint of the project.

Project goal	Usually preferred method	Reason
Find inherited variants	DNA-seq	The relevant information is encoded in genomic DNA.
Find tumour mutations	DNA-seq	Somatic mutation calling usually requires direct genomic evidence.
Measure gene expression	RNA-seq	Expression is an RNA-level phenotype.
Study splicing	RNA-seq	Splicing is observed in mature or nascent RNA molecules.
Detect gene fusions	DNA-seq or RNA-seq	DNA-seq can detect rearrangements; RNA-seq can detect expressed fusion transcripts.
Connect variants with functional effects	DNA-seq + RNA-seq	DNA-seq identifies the alteration; RNA-seq shows expression, pathway or transcript consequences.

Why combine DNA-seq and RNA-seq?

In many research, translational and industrial projects, the strongest interpretation comes from combining the two methods. DNA-seq identifies the genomic alterations, while RNA-seq helps determine which alterations are expressed or associated with changes in biological state.

Examples of combined interpretation

A DNA variant is present and the affected gene is highly expressed.
A splice-site variant is accompanied by abnormal exon usage.
A structural rearrangement creates an expressed fusion transcript.
A copy-number amplification is associated with increased expression.

AI-assisted integration SciBerg can support workflows that connect DNA-seq, RNA-seq, metadata, literature and biological databases to organise interpretation, reporting and candidate prioritisation.

Frequently asked questions

What is the main difference between DNA sequencing and RNA sequencing?

DNA sequencing analyses DNA molecules and is usually used to study genome sequence, variants, structural changes, copy number changes, or methylation depending on the protocol. RNA sequencing analyses RNA-derived libraries and is usually used to study gene expression, transcript isoforms, splicing, fusion transcripts, RNA editing, and cell-state changes.

Can DNA sequencing measure gene expression?

Standard DNA sequencing does not measure gene expression because most cells contain largely the same genome regardless of which genes are active. Gene expression is usually measured with RNA sequencing or other transcriptomic methods.

Can RNA sequencing detect variants?

RNA sequencing can sometimes detect expressed variants, but it is not a complete replacement for DNA sequencing. Variants in genes or alleles that are not expressed, poorly covered, degraded, or affected by expression bias can be missed.

Which method should I choose for a disease or cancer project?

The best method depends on the biological question. DNA sequencing is usually preferred for inherited or somatic variant discovery, copy number changes, and structural genome alterations. RNA sequencing is preferred for expression signatures, pathway activity, splicing, gene fusions, and functional consequences of genomic alterations.

Why does RNA sequencing require special sample handling?

RNA is generally more labile than DNA and can degrade quickly through RNase activity or poor sample handling. RNA-seq projects therefore need careful extraction, integrity assessment, and library preparation.