- nucleus and ribosomes

Learning Outcomes

  • By the end of this lecture, you should be able to:

    • Define the role of the nucleus and ribosomes in the overall function and survival of a cell.

    • Describe the structural components of the nuclear envelope and the mechanism of nucleocytoplasmic transport.

    • Explain the process of ribosome biogenesis and the assembly of protein-synthesis machinery.

Overview of the Animal Cell

  • Structures within an animal cell include:

    • Microtubules & Actin filaments: Components of the cytoskeleton providing structural support and motility.

    • Centrosome: The microtubule-organizing center (MTOC) containing a pair of centrioles.

    • Chromatin: Complex of DNA and proteins (histones) residing in the nucleoplasm.

    • Nuclear pore: Regulated gateways in the nuclear envelope.

    • Extracellular matrix (ECM): Structural support outside the cell membrane.

    • Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.

    • Nucleus: The double-membrane bound storehouse of genetic information.

    • Peroxisome: Involved in lipid metabolism and chemical detoxification.

    • Plasma membrane: The selective lipid bilayer barrier.

    • Ribosomes: The multi-protein complexes responsible for translation.

    • Golgi apparatus: Responsible for modifying, sorting, and packaging proteins.

    • Cytosol: The aqueous component of the cytoplasm.

    • Vesicles: Transport sacs for moving lipids and proteins.

    • Mitochondria: The site of oxidative phosphorylation and ATP production.

    • Endoplasmic reticulum (ER): Rough ER (protein synthesis) and Smooth ER (lipid synthesis/calcium storage).

    • Lysosome: Contains digestive enzymes for breakdown of waste.

The Nucleus

Overview

  • The nucleus is the defining organelle of eukaryotic cells, enclosed by a double membrane.

  • Primary Functions:

    • Contain and Protect DNA: Prevents damage to the genome and separates transcription from translation.

    • Coordination of Cellular Activity: Regulation of gene expression and the cell cycle.

  • Physical Properties:

    • Anucleate Cells: Mature red blood cells (erythrocytes) lack a nucleus to maximize space for hemoglobin transport.

Nuclear Envelope and Pores

Structure of the Nuclear Envelope

  • The envelope consists of two distinct lipid bilayers:

    • Inner Nuclear Membrane (INM): Contains unique proteins that bind the nuclear lamina and chromosomes.

    • Outer Nuclear Membrane (ONM): Continuous with the Rough ER membrane and frequently studded with ribosomes.

Nuclear Pore Complexes (NPCs)

  • Architecture:

    • Composed of ≈30 different proteins called nucleoporins.

    • Each cell typically contains 3000 to 4000 pores.

Nucleoplasmic Transport

Passive Transport

  • Small molecules move freely through the water-filled channels of the NPC.

  • Permeable molecules: Water, ions, metabolites (ATP, nucleotides), and small proteins (<40kDa).

Active Transport

  • Larger molecules (>60kDa) require a signal-guided, energy-dependent process involving the Ran-GTPase cycle.

  • Signals:

    • Nuclear Localization Signal (NLS): A sequence of basic amino acids (e.g., Lysine, Arginine) that flags a protein for import via Importins.

    • Nuclear Export Signal (NES): A leucine-rich sequence that flags molecules for export via Exportins.

Nuclear Lamina

  • Description: A dense, cross-linked network of intermediate filaments (Lamin A, B, and C) located on the inner surface of the inner nuclear membrane.

  • Functions:

    • Maintains the spherical shape and mechanical integrity of the nucleus.

    • Regulates disassembly and reassembly of the nuclear envelope during mitosis (via phosphorylation).

    • Anchors the Nuclear Pore Complexes in position.

Chromosomes and Chromatin

  • Human cells contain roughly 2meters of DNA, which must be highly condensed to fit within a 5μm nucleus.

  • Levels of Packaging:

    1. Nucleosomes: DNA wrapped around eight histone proteins (11nm "beads on a string").

    2. Chromatin Fiber: Coiled nucleosomes (30nm fiber).

    3. Looped Domains: Attached to nuclear scaffolds.

  • Types of Chromatin:

    • Euchromatin: Less condensed, transcriptionally active.

    • Heterochromatin: Highly condensed, transcriptionally inactive.

Nucleolus

  • A non-membrane bound sub-compartment where ribosome production occurs.

  • Internal Regions:

    • Fibrillar Centre (FC): Transcription of ribosomal DNA (rDNA) by RNA Polymerase I.

    • Dense Fibrillar Component (DFC): Processing of the pre-rRNA transcript.

    • Granular Component (GC): Assembly of ribosomal subunits (rRNA + proteins) before export to the cytoplasm.

Ribosomes

Overview

  • Not True Organelles: Ribosomes lack a surrounding lipid membrane.

  • Function: Conduct Translation, the decoding of mRNA sequences into polypeptide chains.

Ribosomal RNA (rRNA)

  • Ribozyme Activity: The ribosome is essentially a ribozyme; it is the rRNA, not the proteins, that catalyzes the formation of peptide bonds (peptidyl transferase center).

Ribosome Structure and Classification

  • Bipartite Structure: Consists of a Large Subunit (LSU) and a Small Subunit (SSU).

    1. Small Unit: Responsible for mRNA binding and codon-anticodon matching.

    2. Large Unit: Responsible for peptide bond formation.

  • Svedberg Units (S): Measures the rate of sedimentation during centrifugation, indicating size and shape.

    • Prokaryotes (70S): 50S (Large) + 30S (Small).

    • Eukaryotes (80S): 60S (Large) + 40S (Small).

Translational Machinery

  • tRNA Binding Sites:

    • A (Aminoacyl) site: Receives the incoming tRNA carrying the next amino acid.

    • P (Peptidyl) site: Holds the tRNA attached to the growing polypeptide chain.

    • E (Exit) site: The deacylated tRNA stays here briefly before being ejected from the ribosome.

  • Free vs. Bound Ribosomes:

    • Free: Synthesize proteins destined for the cytosol or organelles like mitochondria.

    • Membrane-bound (Rough ER): Synthesize proteins for secretion or lysosomal insertion.