1. Nucleolus — Structure and Function
Nucleolus is a dense, non-membrane bound organelle inside the nucleus. Discovered by Fontana (1781). Functions: (1) Transcription of ribosomal RNA (rRNA) genes — rDNA (tandemly repeated copies). (2) Processing of pre-rRNA into mature 18S, 5.8S, 28S rRNA. (3) Assembly of ribosomal subunits (with ribosomal proteins imported from cytoplasm). Disappears during cell division (mitosis prophase), reappears in telophase. Cells with high metabolic/protein synthesis activity have large nucleoli (e.g., liver cells, secretory cells). Nucleolus contains: fibrillar centre, dense fibrillar component, granular component.
2. Ribosome Structure
Ribosomes are ribonucleoprotein complexes (rRNA + proteins). Prokaryotic: 70S (50S + 30S subunits). 50S = 23S + 5S rRNA + 34 proteins. 30S = 16S rRNA + 21 proteins. Eukaryotic: 80S (60S + 40S subunits). 60S = 28S + 5.8S + 5S rRNA + ~49 proteins. 40S = 18S rRNA + ~33 proteins. Mitochondrial ribosomes: 55S (55S, similar to prokaryotic — supports endosymbiont theory). rRNA in nucleolus: 28S, 18S, 5.8S synthesised in nucleolus. 5S rRNA synthesised outside nucleolus by RNA Pol III.
3. Nucleus — Components
Nucleus contains: nuclear envelope (double membrane with nuclear pores), nuclear lamina (supports nuclear envelope), nucleoplasm (nuclear sap), chromatin (DNA+histone proteins), and nucleolus. Nuclear pores: allow transport of mRNA, tRNA out, and proteins (histones, RNA polymerase) in. Pore complexes are 120nm diameter — largest in cell. Human nucleus: 6 μm diameter. Largest human nucleus: oocyte nucleus (germinal vesicle).
4. rRNA Genes — Location
rRNA genes (rDNA) are located on specific chromosomes at Nucleolar Organiser Regions (NOR). In humans: NOR on chromosomes 13, 14, 15, 21, 22 (five pairs = 10 chromosomes). Tandemly repeated — hundreds of copies. High copy number ensures rapid rRNA production. 5S rRNA gene: located on chromosome 1 in humans (NOT in NOR). Multiple nucleoli from different NORs may fuse into one large nucleolus during interphase.
5. Cell Organelles — Summary
Mitochondria: ATP synthesis, double membrane. Chloroplast: photosynthesis, triple membrane system. Endoplasmic Reticulum (rough/smooth): protein/lipid synthesis. Golgi apparatus: processing and packaging of proteins. Lysosome: intracellular digestion (suicide bag). Vacuole: storage, turgor pressure in plants. Peroxisome: fatty acid oxidation, H₂O₂ breakdown. Centrosome: spindle formation in animal cells. Nucleolus: rRNA synthesis. Ribosome: protein synthesis. Centrioles: basal bodies for cilia/flagella.
6. Differences — Prokaryote vs Eukaryote Nucleus
Prokaryotes: No membrane-bound nucleus. Genetic material (nucleoid) is circular DNA in cytoplasm. No histone proteins (but histone-like proteins). No nuclear envelope. Ribosomes 70S. No membrane-bound organelles. No nucleolus. Eukaryotes: Membrane-bound nucleus with nuclear envelope. Linear DNA with histones. 80S ribosomes (cytoplasmic). Membrane-bound organelles. True nucleolus for rRNA synthesis.
7. RNA Polymerases in Eukaryotes
Three main RNA polymerases: RNA Pol I: in nucleolus → transcribes rRNA genes (28S, 18S, 5.8S rRNA). RNA Pol II: in nucleoplasm → transcribes mRNA precursors (hnRNA) and some snRNA. RNA Pol III: in nucleoplasm → transcribes tRNA, 5S rRNA, snRNA. Remember: Pol I = rRNA (in nucleolus), Pol II = mRNA, Pol III = tRNA + 5S rRNA. Alpha-amanitin (from Amanita mushroom) inhibits RNA Pol II at low concentration, Pol III at high concentration. Rifampicin inhibits prokaryotic RNA polymerase (used as antibiotic for TB).
8. Centrosome vs Kinetochore — Distinction
Centrosome: Organelle present in animal cells (absent in plant cells — plant cells use other mechanisms). Contains two centrioles arranged perpendicularly. Organises microtubules → forms the mitotic spindle. Duplicates before cell division. Kinetochore: Protein complex assembled at the centromere of each chromosome during cell division. Site where spindle microtubules attach to chromosomes. Helps chromosomes move to poles during anaphase. Each chromosome has two kinetochores (one on each sister chromatid).
Frequently Asked Questions
1. Why is nucleolus called 'factory of ribosomes'? ⌄
Nucleolus is not membrane-bound but is a functional organelle. It has all the machinery for: (1) Transcribing rRNA genes (using RNA Pol I). (2) Processing the pre-rRNA transcript (45S → 28S + 18S + 5.8S). (3) Assembling ribosomal proteins (imported from cytoplasm) with rRNA into pre-ribosomal subunits. The 60S and 40S pre-subunits are then exported through nuclear pores to the cytoplasm. Final assembly of 80S ribosome occurs in cytoplasm when 60S + 40S join on mRNA.
2. How many nucleoli does a cell have? ⌄
Varies by cell type. Human cells: 1 large nucleolus during interphase (formed by fusion of multiple NOR-associated nucleoli). During prophase: nucleolus disappears (transcription stops, chromatin condenses). During telophase: nucleolus reappears as nucleolar organiser regions become active again. Rapidly dividing cells may have multiple smaller nucleoli. Metabolically active cells (liver, pancreatic acinar cells) have large prominent nucleoli. Cancer cells often show enlarged, abnormal nucleoli — used in cancer diagnosis.
3. What happens to the nucleolus during cell division? ⌄
Prophase: nucleolus gradually disappears as rRNA genes are silenced and chromosomes condense. Metaphase: no nucleolus visible — rDNA on chromosomes (NOR regions visible as secondary constrictions). Anaphase: no nucleolus. Telophase: nucleolus reappears in daughter cells as chromosomes decondense and rRNA gene transcription resumes. The disappearance and reappearance of nucleolus is a useful marker for prophase and telophase stages.
4. What is the role of chromatin in gene expression? ⌄
Chromatin = DNA + histone proteins + non-histone proteins. Heterochromatin: densely packed, transcriptionally inactive (constitutive or facultative). Euchromatin: loosely packed, transcriptionally active. Histone modification: acetylation of histones (by HATs) → loosens chromatin → promotes transcription. Deacetylation (by HDACs) → tightens chromatin → represses transcription. DNA methylation: methylation of cytosine (at CpG sites) → represses transcription. Together these epigenetic mechanisms regulate gene expression without changing DNA sequence.
5. What are the differences between 70S and 80S ribosomes? ⌄
70S (prokaryotic): total = 50S + 30S. 50S large subunit: 23S rRNA + 5S rRNA + 34 proteins. 30S small subunit: 16S rRNA + 21 proteins. 80S (eukaryotic cytoplasmic): total = 60S + 40S. 60S large subunit: 28S + 5.8S + 5S rRNA + ~49 proteins. 40S small subunit: 18S rRNA + ~33 proteins. Chloroplast/mitochondrial ribosomes: ~55S (70S-like, supports endosymbiont theory). Antibiotics targeting 70S (not 80S): streptomycin (30S), chloramphenicol (50S), erythromycin (50S), tetracycline (30S) — this is why these antibiotics kill bacteria but not eukaryotic cells.
6. Why do actively secreting cells have large nucleoli? ⌄
Secretory cells (e.g., pancreatic acinar cells secreting digestive enzymes, plasma cells secreting antibodies) need massive amounts of protein synthesis. More proteins → more ribosomes needed → more rRNA needed → more nucleolar activity. Large nucleolus = actively transcribing rRNA → producing many ribosomes → high protein synthesis capacity. Conversely, non-dividing cells with low metabolic activity (e.g., muscle cells, neurons) have small or inconspicuous nucleoli. In rapidly dividing cancer cells, nucleoli are often abnormally large.
7. What is the difference between nucleolus and nucleus? ⌄
Nucleus: membrane-bound organelle (nuclear envelope = double membrane + nuclear pores). Contains all genetic material (chromosomes). Present in all eukaryotic cells (absent in prokaryotes, RBCs, platelets). The 'control centre' of the cell. Nucleolus: non-membrane-bound sub-organelle INSIDE the nucleus. Not a separate organelle but a specialised nuclear body. Specifically involved in rRNA synthesis and ribosome assembly. A nucleus can have one or more nucleoli. When the question asks 'site for rRNA synthesis,' the answer is nucleolus (a structure within the nucleus, not the nucleus itself).
8. What is euchromatin vs heterochromatin? ⌄
Euchromatin: loosely condensed chromatin, genetically active (transcribed), visible as lightly staining regions in nucleus. Most housekeeping genes in euchromatin. Heterochromatin: densely condensed, genetically inactive, darkly staining. Two types: Constitutive heterochromatin — always condensed, contains repetitive sequences, e.g., centromeric DNA. Facultative heterochromatin — can switch between active and inactive states, e.g., inactivated X chromosome (Barr body) in female mammals. Barr body = condensed inactive X in female somatic cells. Number of Barr bodies = n-1 (where n = number of X chromosomes).