#RNA sequencing

Sequencing

RNA-seq provides insight into:

DNA polymerase synthesizes DNA strictly in 5' to 3' direction by adding nucleotides to the free 3' hydroxyl group of the growing chain. In Sanger sequencing, labeled primer anneals to template and polymerase extends it; each termination product represents a strand made 5' to 3'. Consequently, when fragments are ordered by size from smallest to largest after electrophoresis, sequence read corresponds to 5' to 3' synthesis product, complementary to template strand oriented 3' to 5'. Synthesis never proceeds 3' to 5' enzymatically, and reading is directional not random.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which step in RNA-seq distinguishes novel transcripts?

Distinguishing novel transcripts from annotated ones depends heavily on post-sequencing alignment analysis. After sequencing cDNA libraries, reads are mapped with splice-aware aligners such as STAR or HISAT2 that can split alignments across introns. Reads spanning previously unannotated exon-exon junctions, retained introns or alternative splice sites emerge only during alignment. Reverse transcription and library preparation create cDNA fragments, sequencing depth improves sensitivity, but computational alignment reveals exon connectivity, fusion genes and novel isoforms. Hence alignment is step that identifies transcript novelty and structural variation.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

What kind of RNA is targeted in mRNA-seq?

mRNA-seq specifically enriches and sequences messenger RNAs that encode proteins. Total cellular RNA comprises predominantly ribosomal RNA and transfer RNA, with small nuclear RNA and other species. Messenger RNA typically contains poly-A tail and represents only 1-5 percent of total RNA but carries coding information. Protocols using oligo-dT beads selectively capture polyadenylated mRNA, allowing study of protein-coding expression levels, splice variants and untranslated regions. Sequencing rRNA, tRNA or snRNA would require distinct rRNA depletion or small RNA protocols, not standard mRNA-seq targeting coding transcripts.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

What is the primary goal of RNA sequencing?

RNA sequencing aims to capture the transcriptome, the complete set of RNA molecules transcribed from genome at a given time. After isolating RNA and converting to cDNA library, deep parallel sequencing provides quantitative and qualitative information: abundance of transcripts, alternative splicing isoforms, allele-specific expression, fusion transcripts and non-coding RNAs. This enables comparison of gene expression across tissues, developmental stages or disease states. It is not designed for protein purification, direct DNA mutation screening independent of expression, or removal of splice sites. Transcriptome analysis is its primary biological purpose.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcript analysis

Which method enables transcriptome-wide expression profiling?

Transcriptome-wide expression profiling demands technology capable of unbiased, high-throughput measurement across all transcripts present in cell. RNA-seq achieves this by massive parallel sequencing of complementary DNA libraries, yielding digital counts proportional to RNA abundance, enabling detection of differential expression, splicing, and novel isoforms genome-wide. EMSA and footprinting analyze protein-DNA binding at single locus level. RT-PCR quantifies few genes but lacks global scope. Only RNA-seq provides simultaneous interrogation of thousands of genes without requiring prior probe design, revolutionizing functional genomics and systems biology approaches.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Transcriptome analysis

What type of sequencing is used to target small RNAs like miRNA and siRNA?

Small RNA sequencing is a specialized RNA-seq workflow designed for non-coding RNAs of 18–40 nucleotides such as microRNAs and small interfering RNAs. After total RNA extraction, RNAs are size-selected by gel or beads to enrich the small fraction, followed by ligation of 5′ and 3′ adaptors that exploit the characteristic 5′ phosphate and 3′ hydroxyl ends. Reverse transcription and PCR generate libraries that preserve strand information. Unlike mRNA sequencing which depletes small RNAs through poly(A) selection and size exclusion, this approach accurately quantifies miRNA expression, isoforms, and modifications.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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