14 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
mRNA is not translated in the nucleus; it must first be processed and transported to the cytoplasm.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
snRNPs are components of the spliceosome that participate in removing introns during RNA splicing.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
The spliceosome removes introns from pre-mRNA and joins the exons together to form mature mRNA.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
RNA splicing removes non-coding introns from pre-mRNA and joins the exons to form mature mRNA.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Post-Transcriptional Modifications - Capping Splicing Polyadenylation
Transfer RNA splicing removes intervening introns typically located one nucleotide three prime to anticodon loop in eukaryotic pre-transfer RNAs. Unlike messenger RNA splicing mediated by spliceosomes or group I self-splicing, tRNA introns are excised by protein enzymes: heterotetrameric splicing endonuclease complex SEN comprises Sen2, Sen34 catalytic subunits, Sen15, Sen54 structural, cutting at both splice sites leaving two halves with 2',3' cyclic phosphate and five prime hydroxyl. Ligase complex containing Trl1 phosphorylates hydroxyl and joins ends via healing and sealing steps, adding phosphate from GTP before ligation, producing mature spliced tRNA ready for modifications.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: tRNA splicing enzyme mediated; Alberts 7th ed., Pre-tRNA processing and ligation
Most eukaryotic introns follow GT-AG rule with GU at five prime end and AG at three prime end recognized by major U2-dependent spliceosome comprising U1, U2, U4, U5, U6 small nuclear RNPs. Rare introns with AU at five prime and AC at three prime, as well as subset of GU-AG introns with distinct branch site consensus, are removed by minor or U12-dependent spliceosome comprising U11, U12, U4atac, U6atac, U5 snRNPs. U11-U12 di-snRNP functionally analogous to U1-U2. Minor introns occur in genes encoding ion channels, DNA repair proteins, and are highly conserved evolutionarily.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: AU-AC introns spliced by minor spliceosome; Alberts et al., U12-dependent splicing
During first step of splicing, formation of lariat structure arises because nucleophilic attack uses two prime hydroxyl of branch adenine instead of three prime hydroxyl standard for phosphodiester bond. Resulting linkage between two prime position of adenosine ribose and five prime phosphate of intron five prime guanosine creates unusual two prime to five prime phosphodiester bond within intron, alongside conventional three prime to five prime bonds elsewhere. This branched Y-shaped intermediate appears as loop with tail. Specific debranching enzyme DBR1 hydrolyzes two prime-five prime bond to linearize intron for exonucleolytic decay.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Lariat 2'OH and 5'P linkage; Alberts 7th ed., Lariat formation mechanism
Pre-messenger RNA splicing proceeds via two sequential transesterification reactions, chemically similar to group two self-splicing introns, with ester bonds exchanged without net ATP hydrolysis for chemistry. First reaction: two prime hydroxyl of branch adenosine attacks phosphodiester at five prime splice site, severing five prime exon from intron and generating free three prime hydroxyl on five prime exon plus lariat intermediate where intron five prime end linked to branch adenine. Second reaction: three prime hydroxyl of five prime exon attacks three prime splice site, ligating exons and releasing intron lariat later debranched.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Splicing involves two transesterifications; Lodish 9th ed., Chemistry of splicing
Branch site consensus in budding yeast Saccharomyces cerevisiae is invariant UACUAAC where central adenosine is nucleophile forming two prime-five prime linkage, highly conserved due to need for base pairing with U2 snRNA GUAGUA and recognition by splicing factors. Mammalian branch point consensus is more degenerate YNYYRAY, specifically CURAY motif where R is purine, Y is pyrimidine, containing bulged adenine. Sequence forms duplex with U2 snRNA positioning adenine for chemistry. Mutations in this motif cause exon skipping, inherited disease such as thalassemia, emphasizing sequence conservation importance.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Consensus branch site UACUAAC; Lodish 9th ed., Yeast vs mammalian branch point sequences
Branch point adenine, usually located eighteen to forty nucleotides upstream of three prime splice site within polypyrimidine tract, is recognized by U2 small nuclear RNP through RNA-RNA duplex formation between conserved GUAGUA region of U2 snRNA and branch site consensus, bulging adenine out of helix. Bulged adenine's two prime hydroxyl acts as nucleophile in first transesterification. Recognition involves SF1 protein initially binding branch site, then helicase Prp5 remodeling replacing SF1 with U2, stabilized by SF3B1 and SF3B3 proteins and U2AF65 interacting with polypyrimidine tract, ensuring accurate lariat formation.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Branch point A recognized by U2 snRNP; Lodish 9th ed., Branch point assembly
Early spliceosome assembly begins with E complex formation where U1 snRNP small nuclear ribonucleoprotein particle recognizes five prime splice site through complementary base pairing between five prime end of U1 snRNA CAGGUAAGU motif and pre-mRNA splice site sequence. This interaction stabilized by U1C protein and SR proteins binding exonic splicing enhancers. Recognition defines exon-intron border and recruits U2AF heterodimer to downstream polypyrimidine tract and AG at three prime site. Subsequent ATP-dependent rearrangements replace U1 with U6 snRNA forming catalytic center but initial fidelity depends primarily on U1.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: 5' splice site recognized by U1 snRNA; Alberts et al., Early spliceosome assembly E complex
