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 transcribed by Pol III in nucleoplasm imports. Ribosomal proteins, 33 for 60S and 21 for 40S plus assembly factors, imported from cytoplasm, combine hierarchically to pre-60S and pre-40S particles exported via CRM1 and RanGTP. Tripartite morphology fibrillar center containing Pol I, dense fibrillar component where processing occurs, granular component for assembly reflects this vectorial flow. Nucleolar stress with impaired rRNA synthesis stabilizes p53 via MDM2 sequestration. Rough ER, Golgi, mitochondria perform translation, glycosylation, and respiration, not ribosomal RNA synthesis.
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 eight S, twenty eight S RNAs. Granular component then hosts pre-ribosome assembly. In situ hybridization shows pre-rRNA rapidly moves from dense fibrillar component outward. Inhibition with five-fluorouridine labels dense fibrillar component preferentially, low actinomycin D disrupts its structure blocking processing while transcription persists transiently. DNA replication occurs at many foci throughout nucleoplasm S phase, protein folding ER, chromosome condensation at mitosis via condensin, distinguishing dense fibrillar component role specifically in ribosomal RNA maturation crucial for ribosome supply.
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 occur in dense fibrillar component, subsequent modifications include methylation by fibrillarin and pseudouridylation by dyskerin guided by snoRNAs. Pre-forty S and pre-sixty S assembled with imported ribosomal proteins one hundred copies transit to granular component before export. DNA replication initiates at many origins in nucleoplasm during S phase, ATP production mitochondrial oxidative phosphorylation, RNA modification widespread but concentrated for rRNA here. Actinomycin intercalates rDNA halting Pol I causing nucleolar disruption, confirming function as ribosomal RNA processing and assembly center coordinating growth with protein synthesis demand.
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, dyskerin pseudouridylation guided by snoRNAs. Five S rRNA transcribed by Polymerase III in nucleoplasm later imports. Rough ER does not transcribe, mitochondria transcribe its own small rRNA for mitoribosomes, Golgi processes glycoproteins. Low-dose actinomycin D selectively inhibits Pol I leading to nucleolar segregation and stress activating p53. High rate of Pol I transcription meets demand for ribosome production, explaining prominent nucleolus in growing cells and upregulation in cancer driven by Myc and mTOR pathways.
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 methylation and pseudouridylation occur guided by fibrillarin and dyskerin. Granular component then assembles pre-forty S and pre-sixty S particles with imported ribosomal proteins. Pre subunits export via CRM1 for final maturation. Endoplasmic reticulum and Golgi modify secretory proteins, peroxisomes detoxify lipids, none produce rRNA. Actinomycin D at low doses selectively inhibits Pol I disrupting nucleoli, proving synthetic role. Consequently nucleolar hypertrophy correlates with growth rate and is upregulated in transformed cells requiring high protein synthesis.
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
