Nucleus and Ribosomes

Core Explanation

The flow of genetic information in cells follows the central dogma of molecular biology: DNA → RNA → protein. This chapter examines the two cellular structures most intimately involved in this process — the nucleus, which stores and organizes the DNA and transcribes it into RNA, and the ribosomes, which translate that RNA into polypeptide chains.

The Nucleus

The nucleus is the most conspicuous organelle in a eukaryotic cell, typically occupying about 10% of the cell's volume. It houses the vast majority of the cell's genetic material and serves as the command center, directing protein synthesis and cellular activity through differential gene expression.

Nuclear Envelope

The nucleus is enclosed by the nuclear envelope, a double membrane that separates the nucleoplasm from the cytoplasm. This structure is continuous with the rough endoplasmic reticulum (rough ER) and consists of:

• Outer nuclear membrane: Studded with ribosomes on its cytoplasmic face; continuous with the rough ER membrane.
• Inner nuclear membrane: Lined by the nuclear lamina, a meshwork of intermediate filament proteins (lamins) that provides mechanical support, maintains nuclear shape, and anchors chromatin.
• Perinuclear space: The ∼20–40 nm gap between the two membranes, continuous with the ER lumen.

The nuclear envelope is not a passive barrier. Its double-membrane design physically separates transcription (nucleus) from translation (cytoplasm), enabling extensive RNA processing — capping, splicing, polyadenylation — before the transcript ever encounters a ribosome. This is a fundamental distinction from prokaryotes, where transcription and translation are coupled in space and time.

Nuclear Pores

Molecular traffic between the nucleus and cytoplasm occurs exclusively through nuclear pore complexes (NPCs). Each NPC is an enormous protein assembly — approximately 125 MDa (million Daltons) in vertebrates, composed of about 30 different proteins called nucleoporins present in multiple copies, totaling roughly 500–1,000 polypeptide chains per pore. A typical mammalian cell contains 3,000–4,000 NPCs.

The NPC functions as a selective gate:

• Small molecules and ions (below ∼40 kDa): Pass freely by passive diffusion.
• Large macromolecules (proteins, RNA–protein complexes): Require active, signal-mediated transport. This is accomplished through nuclear transport receptors (importins and exportins) that recognize specific nuclear localization signals (NLS) or nuclear export signals (NES) on cargo molecules.

The NPC thus tightly controls which macromolecules enter the nucleus (e.g., histones, transcription factors, polymerases) and which leave (e.g., mRNA, tRNA, ribosomal subunits). This selectivity maintains the distinct protein compositions of the nucleoplasm and cytoplasm.

Chromatin and Chromosomes

The cell's DNA is not a naked double helix floating free; it is organized with proteins into a dynamic, regulated complex called chromatin.

Levels of DNA organization:

1. DNA double helix (2 nm diameter): The fundamental molecule.
2. Nucleosome ("beads on a string") (10 nm fiber): ∼147 base pairs of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, H4). Histone H1 binds the linker DNA between nucleosomes.
3. 30 nm fiber (solenoid or zigzag model): Nucleosomes coil or stack into a thicker fiber.
4. Loop domains: The 30 nm fiber forms loops anchored to a protein scaffold.
5. Metaphase chromosome (700 nm diameter): Maximum compaction, achieved only during cell division.

This hierarchical packaging solves a profound physical problem: the human diploid genome contains approximately 2 meters of DNA that must fit inside a nucleus roughly 5–10 µm in diameter. The packing ratio — the length of DNA divided by the length of the chromosome — reaches roughly 10,000:1 in a fully condensed mitotic chromosome.

Chromatin exists in two functionally distinct states:

• Euchromatin: Less condensed, transcriptionally active ("open"). Appears lighter in electron micrographs. Genes in euchromatin are accessible to transcription factors and RNA polymerase.
• Heterochromatin: Highly condensed, largely transcriptionally inactive ("closed"). Includes constitutive heterochromatin (permanently silenced, e.g., centromeres and telomeres) and facultative heterochromatin (conditionally silenced, e.g., the inactive X chromosome in female mammals — the Barr body).

The distinction is not fixed — chromatin undergoes dynamic remodeling (acetylation, methylation, phosphorylation of histones) that alters the accessibility of specific genomic regions, regulating gene expression without altering the DNA sequence itself. This is the basis of epigenetics.

The Nucleolus

The nucleolus is the most prominent substructure within the nucleus, visible under light microscopy as a dense, dark-staining body. It is not a membrane-bound organelle; rather, it is a dynamic region where the cell's ribosomal RNA (rRNA) genes are clustered, transcribed, and assembled with ribosomal proteins into ribosomal subunits.

The nucleolus forms around nucleolar organizer regions (NORs) — specific chromosomal loci containing tandem repeats of rRNA genes. In humans, NORs are located on the short arms of the five acrocentric chromosomes (13, 14, 15, 21, and 22).

The nucleolus has three ultrastructural zones visible under electron microscopy:

• Fibrillar center (FC): Contains the rRNA genes themselves; site where transcription is initiated.
• Dense fibrillar component (DFC): Surrounds the FC; active rRNA transcription and early processing occur here.
• Granular component (GC): Outermost region where ribosomal subunits are assembled from rRNA and ribosomal proteins imported from the cytoplasm.

Cells that are actively synthesizing large amounts of protein (e.g., secretory cells, rapidly dividing cells) have prominent, enlarged nucleoli, reflecting the demand for ribosome production. Cancer cells frequently exhibit enlarged and irregular nucleoli — a feature used diagnostically by pathologists.

Ribosomes

Ribosomes are the molecular machines that translate the genetic code in mRNA into the amino acid sequence of proteins. They are not membrane-bound organelles; they are ribonucleoprotein complexes — roughly 60% RNA and 40% protein by mass.

Composition and Structure

Every ribosome consists of two subunits, one large and one small. The ribosome is measured in Svedberg units (S), which reflect the rate of sedimentation during ultracentrifugation and are not directly additive (the complete ribosome sediments more slowly than the sum of its parts because of shape effects).

Eukaryotic ribosome (80S):

The 28S, 5.8S, and 18S rRNAs are transcribed by RNA polymerase I in the nucleolus as a single large precursor (45S pre-rRNA), which is then cleaved and chemically modified. The 5S rRNA is transcribed separately by RNA polymerase III in the nucleoplasm and imported into the nucleolus.

Within the ribosome, the rRNAs are not merely structural scaffolding — they are catalytic. The peptidyl transferase center (the active site that forms peptide bonds) resides in the 28S rRNA of the large subunit and is a ribozyme (an RNA enzyme). Ribosomal proteins primarily stabilize the rRNA scaffold and fine-tune its structure.

The functional sites of the assembled ribosome include:

• A site (aminoacyl): Accepts incoming aminoacyl-tRNA.
• P site (peptidyl): Holds the tRNA carrying the growing polypeptide chain.
• E site (exit): Where deacylated tRNA exits the ribosome.

Prokaryotic ribosome (70S): For comparison, the bacterial ribosome has a 50S large subunit (23S + 5S rRNA) and a 30S small subunit (16S rRNA). This structural difference is the basis for many clinically important antibiotics (aminoglycosides, tetracyclines, macrolides) that selectively target the bacterial ribosome without inhibiting human ribosomes.

Free vs. Bound Ribosomes

Ribosomes in a eukaryotic cell exist in two populations that are structurally identical — the same 80S ribosome participates in both pools. The distinction is purely a matter of location and the type of protein being synthesized.

Free ribosomes:

• Suspended in the cytosol, not associated with any membrane.
• Synthesize proteins destined to remain in the cytosol or to be imported into mitochondria, chloroplasts, peroxisomes, or the nucleus.
• Many of these proteins contain specific targeting sequences recognized by import receptors on the target organelle.

Bound ribosomes:

• Attached to the cytoplasmic face of the rough endoplasmic reticulum (rough ER).
• Synthesize proteins destined for secretion, incorporation into the plasma membrane, residence within the endomembrane system (ER, Golgi, lysosomes), or export from the cell.

The critical mechanistic insight — and a common source of confusion — is that free and bound ribosomes are the same ribosomes. They are not two distinct classes of ribosomes with different structures. The very same ribosome can exist as a free ribosome one moment and become bound to the ER the next. The decision to bind depends entirely on the mRNA being translated:

1. All protein synthesis begins on free ribosomes in the cytosol.
2. If the nascent polypeptide contains an N-terminal signal peptide (a sequence of roughly 20 hydrophobic amino acids), it is recognized by a signal recognition particle (SRP) as it emerges from the ribosome exit tunnel.
3. The SRP–ribosome complex docks at an SRP receptor on the rough ER membrane.
4. The ribosome becomes bound, and the growing polypeptide is cotranslationally translocated across the ER membrane into the ER lumen through a protein-conducting channel called the translocon (Sec61 complex).
5. Once translation is complete, the ribosome dissociates from the mRNA, releases from the ER, and returns to the cytosolic pool, where it may initiate translation of another mRNA — with or without a signal peptide.

Why are they structurally identical but functionally associated with different destinations? The answer lies in the signal peptide, not in the ribosome itself. The ribosome is a universal translator — it reads the codon sequence and assembles the specified amino acids regardless of where the protein will end up. The protein's final destination is determined by:

• Targeting sequences encoded in the protein's own amino acid sequence (e.g., N-terminal signal peptide → ER; N-terminal presequence → mitochondria; C-terminal SKL motif → peroxisomes; internal basic sequences → nucleus).
• Cellular sorting machinery (SRP, translocons, import receptors, translocases) that recognize those signals and direct the ribosome–nascent chain complex to the correct membrane.

In short: the ribosome does not know or care where a protein will end up. It translates whatever mRNA it binds. The protein itself carries the address, and the cell's sorting machinery reads that address and directs the ribosome (and thus the nascent protein) to the appropriate destination.

A table summarizing protein destinations:

How It Works

Signal Recognition and Cotranslational Translocation (Step-by-Step)

1. Translation initiates on a free ribosome in the cytosol.
2. The first ∼20–30 amino acids emerge from the ribosome exit tunnel.
3. If this sequence is a hydrophobic signal peptide, it is recognized and bound by the signal recognition particle (SRP) — a cytoplasmic ribonucleoprotein.
4. SRP binding causes a pause in translation (elongation arrest), preventing the polypeptide from folding in the cytosol.
5. The SRP–ribosome complex diffuses to the rough ER and binds to the SRP receptor, a transmembrane protein embedded in the ER membrane.
6. SRP is released (GTP hydrolysis), and the ribosome docks onto a Sec61 translocon, a protein-conducting channel in the ER membrane.
7. Translation resumes, and the growing polypeptide is threaded directly through the translocon into the ER lumen — this is cotranslational translocation.
8. Signal peptidase, an enzyme in the ER lumen, cleaves the signal peptide.
9. Chaperone proteins (e.g., BiP) in the ER lumen assist proper folding.
10. When translation terminates, the ribosome dissociates into its subunits, which return to the cytosolic pool.

This elegant mechanism ensures that secretory and membrane proteins never enter the cytosol in a folded state, which would make translocation across the membrane energetically impossible.

Nuclear Import (Step-by-Step)

1. A newly synthesized protein destined for the nucleus (e.g., a transcription factor) contains a nuclear localization signal (NLS) — typically a short stretch rich in basic amino acids (lysine and arginine).
2. In the cytosol, the NLS is recognized by an importin (a nuclear transport receptor).
3. The importin–cargo complex diffuses to the NPC and enters the central channel.
4. The complex interacts with nucleoporins (specifically FG-repeat domains) and translocates through the pore.
5. Inside the nucleus, Ran-GTP binds to importin, causing a conformational change that releases the cargo protein.
6. The importin–Ran-GTP complex is recycled to the cytoplasm, where Ran-GTP is hydrolyzed to Ran-GDP, releasing the importin for another cycle.

The directionality of transport is driven by the Ran GTPase gradient: Ran-GTP is high in the nucleus (where Ran's GEF is localized) and low in the cytoplasm (where Ran's GAP is localized). This gradient is maintained by the nuclear envelope's separation of GEF and GAP activities.

Biological / Medical Relevance

• Cancer diagnosis: Enlarged, irregular nucleoli are a hallmark of many cancers and are used by pathologists as a morphological indicator of malignancy (e.g., in Pap smears and histological grading of tumors).
• Laminopathies: Mutations in lamin genes cause a spectrum of diseases (e.g., Hutchinson–Gilford progeria syndrome — premature aging; Emery–Dreifuss muscular dystrophy; dilated cardiomyopathy). These conditions highlight the structural role of the nuclear lamina.
• Antibiotics: Many antibiotics exploit the structural difference between 70S (bacterial) and 80S (human) ribosomes. Tetracyclines bind the 30S subunit; macrolides (erythromycin) bind the 50S subunit; aminoglycosides cause translational misreading. Understanding ribosome structure underpins selective toxicity.
• Ribosomopathies: Diseases caused by defects in ribosome biogenesis, such as Diamond–Blackfan anemia (mutations in ribosomal protein genes) and Treacher Collins syndrome (defects in rRNA transcription), illustrate that ribosome dysfunction has tissue-specific consequences — paradoxically, the craniofacial skeleton and hematopoietic system are most affected.
• Viral exploitation: Many viruses (e.g., influenza virus, HIV) hijack the NPC to import their genomes or export viral mRNA; understanding nuclear transport mechanisms enables development of antiviral strategies.
• Neurodegeneration: Defects in nucleocytoplasmic transport are implicated in ALS and frontotemporal dementia — accumulation of TDP-43 protein aggregates disrupts NPC function.

Common Misconceptions and Exam Traps

• Misconception: The nucleus has a single membrane. Reality: The nuclear envelope is a double membrane. The outer membrane is continuous with and essentially part of the rough ER.
• Misconception: The nucleolus is a membrane-bound organelle inside the nucleus. Reality: The nucleolus is a phase-separated, non-membrane-bound region where rRNA genes cluster and ribosomal subunits assemble.
• Exam trap: Confusing chromatin and chromosomes as fundamentally different substances. They are the same material (DNA + histones) in different states of condensation. Chromatin is the interphase form; chromosomes are the maximally condensed form visible during mitosis.
• Misconception: Free and bound ribosomes are permanently distinct populations. Reality: They are the same ribosomes cycling between the cytosolic pool and the ER membrane; the decision to bind is made anew with each round of translation.
• Exam trap: Stating that proteins destined for mitochondria enter the ER. Mitochondrial proteins are synthesized on free ribosomes, imported post-translationally — not through the ER.
• Misconception: Ribosomal proteins catalyze peptide bond formation. Reality: The 28S rRNA is the catalyst (a ribozyme). Ribosomal proteins provide structural stabilization and fine-tuning.
• Exam trap: Confusing the Svedberg values. 80S ≠ 60S + 40S (they are not arithmetically additive because S values depend on shape, not mass alone).
• Misconception: All proteins begin synthesis on bound ribosomes. Reality: All protein synthesis begins on free ribosomes; binding to the ER occurs only if the nascent protein carries a signal peptide recognized by SRP.