20 public questions tagged with this topic.
The poly-A tail is added to the 3' end of mRNA, not the 5' end.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
Post-transcriptional modifications in eukaryotes involve intron removal, exon splicing, 5' capping, and polyadenylation.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
The poly-A tail is added to the 3' end of mRNA, not the 5' end.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
Phosphorylation is a post-translational modification, not a post-transcriptional modification.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
The **poly-A tail is added to the 3' end** of mRNA, not the 5' end.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
Unlike siRNA and miRNA requiring Dicer RNase III and Drosha, piRNA biogenesis proceeds independently of those enzymes. Primary piRNAs derive from long single-stranded precursors transcribed from piRNA clusters, cleaved by mitochondrial endonuclease Zucchini PLD6 trimming 5' ends, followed by 3' trimming via exonuclease Nibbler and 2'-O-methylation by Hen1 stabilizing terminus. Amplification via ping-pong between PIWI proteins Aubergine and Ago3 slicing targets generates secondary piRNAs without Dicer. Hence standard RNAi enzymes dispensable, explaining answer none. This alternative biogenesis reflects dedicated germline pathway for transposon suppression rather than conventional dsRNA dicing mechanism conserved elsewhere across kingdoms significantly.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: piRNA Biogenesis Independent of Dicer and Drosha
MicroRNA biogenesis starts in nucleus where pri-miRNAs transcribed by Pol II are cropped by Microprocessor complex containing Drosha RNase III and DGCR8 cofactor. Drosha measures about eleven base pairs from basal junction cleaving hairpin releasing ~70 nucleotide pre-miRNA with two-nucleotide 3' overhang. Pre-miRNA exported via Exportin-5 Ran-GTP pathway to cytoplasm for Dicer processing to mature duplex. Nuclear localization separates processing from translation enabling quality control coupled to splicing. Some mirtrons bypass Drosha via splicing. Compartmentalized maturation ensures ordered steps, prevents premature RISC loading, and integrates miRNA production with transcriptional regulation and cellular signaling pathways efficiently.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7: Drosha Function in Nucleus for miRNA Biogenesis
Group II introns share mechanistic ancestry with spliceosomes, forming lariat structure rather than linear product. First step uses internal bulged adenosine within intron domain VI as nucleophile; its 2' OH attacks 5' splice site, generating branched intermediate where intron 5' end linked via 2'-5' phosphodiester to branch point, forming loop. Second step, 5' exon attacks 3' splice site, ligating exons and releasing lariat intron. This lariat pathway underpins evolution of spliceosomal snRNA-mediated splicing in eukaryotic nuclei. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8, Group II intron lariat formation via branch-point adenosine
Following two consecutive transesterifications, Group I intron is released as linear molecule carrying exogenous guanosine at its 5' terminus added during first step. The product is not branched because nucleophile is external, not internal branch adenosine. Linear intron may subsequently undergo circularization reactions. Ligated exons join via canonical 3'-5' phosphodiester bond to yield functional mRNA, rRNA or tRNA. Reaction underscores energy-independent splicing using RNA structure itself for catalysis, classic ribozyme chemistry characterized in Tetrahymena rRNA intron. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6, Linear excision product of Group I introns
Primary conserved function of RNase P across bacteria, archaea and eukarya is 5' maturation of precursor tRNAs. tRNA genes produce longer precursors with 5' leader sequences that must be removed to create mature cloverleaf with correct acceptor stem length for aminoacylation. RNase P introduces precise endonucleolytic cut at leader-tRNA junction, leaving 5' phosphate. It also processes polycistronic operons, some rRNA precursors, and tmRNA, but tRNA processing defines its essential housekeeping role; mitochondrial human RNase P evolved to protein-only form MRPP complex yet retains same cleavage specificity.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 32, RNase P function in 5' maturation of pre-tRNA
RNase P is classical example of ribozyme, catalytic RNA discovered by Sidney Altman. Bacterial holoenzyme includes M1 RNA of about 377 nucleotides in E. coli with catalytic activity and C5 protein cofactor enhancing substrate affinity. Archaeal and eukaryotic nuclear RNase P contain homologous RNA plus up to ten proteins, but RNA remains catalytic core capable of site-specific hydrolysis in vitro without protein at high salt. It uses two Mg2+ ions for transition state stabilization, cleaving phosphodiester bonds, demonstrating that RNA can act as true multiple-turnover enzyme, earning Nobel Prize 1989.
Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, RNase P as catalytic RNA ribozyme – Altman and Cech
RNase M5 and related maturation nucleases exemplify secondary trimming after primary endonucleolytic cleavage. While RNase III first excises individual pre-rRNAs from polycistronic transcripts, their ends remain extended by leader and trailer sequences. Secondary enzymes perform precise exonucleolytic and endonucleolytic trimming, removing surplus nucleotides, proofreading structures, and forming mature 5' and 3' ends. This staged processing couples with rRNA modification and ribosomal protein binding, ensures correct folding of functional centers, prevents degradation of imprecise transcripts, and guarantees productive ribosome biogenesis in rapidly growing bacteria.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8, Secondary trimming nucleases in rRNA maturation
