10 public questions tagged with this topic.
Centrioles are generally absent in higher plant lls but play a crucial role in ll division in animal lls. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
Centrioles are found in animal lls but are usually absent in higher plant lls. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
Centrioles play a key role in organizing microtubules to form the spindle apparatus during ll division, especially in animal lls. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Cell Structure and Function, Cell Cycle and Biomolecules, Topic: Cell organelles and biomolecular structure.
Nucleolus is non-membrane-bound condensate within nucleus formed by liquid-liquid phase separation around nucleolar organizer regions on chromosomes 13,14,15,21,22 containing tandem arrays of 45S rRNA genes 400 copies in humans. RNA polymerase I, with transcription factors UBF and SL1, transcribes 47S pre-rRNA precursor containing 18S, 5.8S, 28S sequences separated by internal and external transcribed spacers. Co-transcriptional processing involves snoRNPs C/D box guiding 2'-O-methylation and H/ACA box guiding pseudouridylation, endonucleolytic cleavages generating mature rRNAs. 5S rRNA transc
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Nucleolus and rRNA Synthesis.
Nucleolar organization reflects sequential ribosome assembly. Fibrillar centers contain transcriptionally inactive rDNA, border to dense fibrillar component active transcription. Dense fibrillar component appears as tight fibrillar mesh containing fibrillarin methyltransferase, Nop56, Nop58 and U3 snoRNP required for early cleavages of forty seven S precursor at sites A0, A1 and A2 within external and internal transcribed spacers, plus two prime O methylation directed by box C/D snoRNAs and pseudouridylation by box H/ACA snoRNAs. This processing trims precursor into eighteen S, five point eigh
Ref: Boisvert Annu Rev Biochem; nucleolus DFC fibrillarin early cleavage methylation pseudouridylation.
Electron microscopy reveals nucleolus as darkly staining subnuclear domain due to high RNA protein density, tripartite with fibrillar centers containing inactive rDNA, dense fibrillar component where transcription and early processing occurs, granular component assembling pre-ribosomes. RNA polymerase I with transcription factor UBF drives high-rate synthesis of forty seven S precursor containing eighteen S, five point eight S, twenty eight S RNAs flanked by five prime external transcribed spacer and internal transcribed spacers ITS1 and ITS2. Early cleavages by U3 snoRNP at five prime ETS occ
Ref: Scheer Annu Rev Cell Biol; nucleolus FC DFC GC rRNA transcription processing assembly.
Eukaryotic cells separate transcription spaces using distinct RNA polymerases. Precursor rRNAs eighteen S, five point eight S and twenty eight S originate from single forty seven S transcription unit driven by RNA polymerase I complex concentrated in nucleolus, organized around nucleolar organizer regions containing hundreds tandem rDNA repeats on acrocentric chromosomes. Accessory factors UBF and SL1 recruit Pol I to promoters within fibrillar centers, nascent transcripts emerge into dense fibrillar component where U3 snoRNP performs early cleavages and fibrillarin mediates methylation, dyske
Ref: Granneman Annu Rev Biochem; Pol I 47S rRNA in nucleolus, snoRNP processing, 5S Pol III.
Ribosome biogenesis requires coordinated synthesis of forty seven S pre-ribosomal RNA containing eighteen S, five point eight S and twenty eight S sequences separated by internal transcribed spacers. This transcription occurs in nucleolus, a nonmembranous nuclear condensate nucleated by tandem repeats of ribosomal DNA located on nucleolar organizer regions of acrocentric chromosomes. RNA polymerase I machinery with upstream binding factor and selectivity factor SL1 concentrates in fibrillar centers, nascent transcripts enter dense fibrillar component where snoRNP-guided cleavages, two prime O
Ref: Hernandez-Verdun Trends Cell Biol; nucleolus Pol I 47S pre-rRNA ribosome factory, phase separated.
The nucleolus is not surrounded by a membrane. It is a spherical structure within the nucleoplasm and is a site of active ribosomal RNA synthesis.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cell Theory and Overview of Cell
Eukaryotic nucleolar rDNA tandem repeats are transcribed by RNA polymerase I as large 45S pre-rRNA, about 13 kb in humans, containing 5'ETS, 18S, ITS1, 5.8S, ITS2, 28S and 3'ETS. Non-transcribed spacers flank units. This single precursor ensures equimolar production of small and large subunit rRNAs. Co-transcriptional modification by H/ACA and C/D snoRNPs adds pseudouridines and 2'-O-methyl groups. Stepwise cleavage by endonucleases and exonucleases excises spacers, separating 18S for 40S and 5.8S-28S complex for 60S, while 5S is made by Pol III elsewhere.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6, Eukaryotic 45S pre-rRNA processing in nucleolus
