RNA-seq

RNA-seq is a high-throughput sequencing technique that identifies and quantifies RNA sequences in a sample. It analyzes total RNA or specific populations like mRNA, tRNA, rRNA, and miRNA. Applications include transcriptome analysis, differential gene expression analysis, and RNA editing studies. RNA-seq provides higher coverage and throughput than earlier methods like microarray-based techniques.

Different RNA types require specific isolation methods. mRNAs are isolated using oligo dT probes complementary to poly A tails. MicroRNAs are isolated by size-based extraction methods since they are typically 15-30 nucleotides long. Contaminating ribosomal RNA is removed using oligonucleotides linked to magnetic beads, which are then separated using a magnet, leaving the RNA of interest.

Reverse transcriptase converts purified RNA into complementary DNA (cDNA), which serves as the template for sequencing. This conversion ensures the molecule's stability and easy handling, allowing the RNA to be integrated into next-generation sequencing workflows. The cDNA is then amplified using PCR to create a library suitable for sequencing.

Adapters are short oligonucleotide sequences ligated to cDNA fragments that serve as primer binding sites for PCR amplification. Barcode sequences are unique identifiers within adapters used to tag and identify individual cDNA strands. These sequences enable efficient amplification and allow researchers to track which sequences come from specific samples or sources.

After immobilization on a sequencing chip, single-stranded DNA undergoes bridge amplification to form clusters of identical sequences. This process ensures that all strands in a specific chip area originate from a single source and emit a uniform fluorescent signal during sequencing. Multiple million clusters of distinct cDNA fragments can be sequenced simultaneously using this approach.

Strand-specific protocols retain information about which DNA strand was transcribed by washing off the complementary strand before sequencing only the target strand. Non-strand-specific protocols sequence both strands without this discrimination. Strand-specific methods provide additional information about transcript orientation and are useful for analyzing overlapping genes or antisense transcripts.

After sequencing, fluorescently labeled nucleotides generate characteristic fluorescence signals read by detectors. The resulting data is aligned to a reference genome and assembled to produce an RNA sequence map for analysis. Different bioinformatic tools process the data depending on the analysis goal, such as quantifying expression levels or identifying alternatively spliced genes.