#RNA-seq

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

RNA-seq was first introduced in:

The concept of massively parallel cDNA sequencing for transcriptome profiling, now called RNA-seq, was demonstrated in pioneering studies published in 2008. Groups led by Wold, Grimmond and others used Illumina Genome Analyzer and 454 platforms to sequence yeast, mouse and human transcriptomes, showing superior dynamic range compared to microarrays for expression quantification and discovery of novel transcripts. The technology became feasible only after next-generation sequencers became robust. 2005 corresponds to early 454 sequencing, while 2010 and 2012 marked expansion into stranded, single-cell and long-read RNA-seq applications.

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 enzyme is required for reverse transcription in RNA-seq?

RNA-seq platforms sequence DNA, so RNA must be reverse transcribed. Reverse transcriptase, an RNA-dependent DNA polymerase derived from retroviruses such as M-MLV or AMV, synthesizes complementary DNA, cDNA, from RNA template using oligo-dT, random hexamer or gene-specific primers. RNase H or second-strand synthesis then generates double-stranded cDNA suitable for adapter ligation and PCR amplification. RNA polymerase synthesizes RNA from DNA, DNA polymerase requires DNA template, and ligase joins nicks. Only reverse transcriptase possesses RNA to DNA polymerase activity essential for initial conversion.

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 sequencing approach is best for small RNAs like miRNA?

Micro RNAs are approximately 22 nucleotide regulatory RNAs derived from hairpin precursors and lack poly-adenine tails. Whole transcriptome and messenger RNA sequencing protocols often include size selection steps that discard RNAs below 200 nucleotides or enrich poly-adenylated species, causing loss of micro RNAs. Small RNA sequencing employs specialized library preparation, preserving 18-35 nucleotide species, ligating adapters directly to 3 prime and 5 prime ends of total small RNA, reverse transcribing without fragmentation, and sequencing. This approach allows precise identification, quantification, discovery of novel micro RNAs and analysis of 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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Transcript analysis

Which of the following is false regarding RNA-seq?

RNA sequencing involves isolation of total RNA or poly-A selected RNA, fragmentation, conversion to complementary DNA by reverse transcription, adapter ligation, and high-throughput sequencing. It provides unbiased transcriptome coverage, enabling quantification of expression, discovery of novel transcripts, detection of alternative splicing isoforms, fusion genes, and allelic expression. Ribosomal RNA is typically depleted to avoid dominating reads, not exclusively sequenced. A library exclusively containing ribosomal RNA would defeat the purpose of profiling messenger RNAs and regulatory non-coding RNAs, hence the statement limiting RNA-seq to ribosomal RNA is fundamentally incorrect.

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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