Chapter 3: Nucleus

Nucleus

  • Structures and components:
    • Nuclear envelope: double membrane surrounding the nucleus (outer membrane is continuous with the endoplasmic reticulum; inner membrane lines the nucleus).
    • Nuclear pores: gateways for selective traffic between nucleus and cytoplasm.
    • Nucleoplasm: the semi-fluid interior of the nucleus.
    • Nucleolus: substructure within the nucleus involved in ribosome production.
    • Chromatin: DNA packaged with proteins (e.g., histones) inside the nucleus; when condensed into visible chromosomes, cells are typically in mitosis/meiosis. During much of the cell cycle, chromosomes are dispersed as chromatin.
    • Ribosomes: large and small ribosomal subunits assembled via nucleolar processes; ribosomal proteins are imported from the cytoplasm.
  • Imaging notes (from the figures):
    • (a) TEM image at 20,000x magnification highlighting the nucleus, chromatin, nuclear envelope, and nuclear pores.
    • (b) Nucleolus and chromatin organization (TEM/SEM context).
    • (c) Outer and inner membranes of the nuclear envelope and nuclear pores (SEM 50,000x).
  • Functional overview:
    • The nucleus houses genetic material (DNA) and coordinates activities of the cell by regulating gene expression.
    • The nucleolus is the site of ribosomal RNA (rRNA) synthesis and the assembly of ribosomal subunits, which are later exported to the cytoplasm through nuclear pores.
    • Chromatin structure influences gene accessibility and transcriptional activity.

Chromosome Structure

  • DNA in the nucleus is associated with proteins (e.g., histones) to form chromosomes.
  • For most of the cell cycle, chromosomes are dispersed as chromatin (less condensed form).
  • Significance:
    • Chromatin organization regulates gene expression by controlling access to DNA.
    • Condensed chromosomes are visible during mitosis/meiosis, facilitating accurate segregation.

Production of Ribosomes

  • Location and components:
    • Nucleolus contains DNA (chromatin) and is the site of ribosomal RNA (rRNA) synthesis and initial ribosome assembly.
    • Ribosomal proteins are imported from the cytoplasm into the nucleolus.
  • Assembly process:
    • rRNA combines with ribosomal proteins to form small ribosomal subunits and large ribosomal subunits.
    • Subunits are assembled in the nucleolus and then exported to the cytoplasm via nuclear pores.
  • Role of transcription and translation in ribosome production:
    • DNA in chromatin is transcribed to produce rRNA components and ribosomal proteins are synthesized in the cytoplasm before import into the nucleus.
    • Translation of mRNA in the cytoplasm leads to synthesis of ribosomal proteins which are imported back to the nucleolus for assembly.
  • Related components:
    • mRNA is transported out to the cytoplasm where ribosomes translate it into proteins.

Endoplasmic Reticulum and Nuclear Envelope

  • Endoplasmic reticulum (ER): a network continuous with the outer membrane of the nuclear envelope.
    • Rough ER: studded with ribosomes; site of synthesis for proteins destined for secretion, membrane insertion, or use inside organelles.
    • Smooth ER: lacks ribosomes; involved in lipid synthesis and other metabolic processes.
  • Key structural elements:
    • Outer membrane of the nuclear envelope connects to the rough ER.
    • Nuclear pores regulate traffic between nucleus and cytoplasm.
    • Cisternae: flattened membrane-bound sacs of the ER.
  • TEM context: images captured at around 30,000x magnification to illustrate Rough ER and nuclear envelope features.

Golgi Apparatus

  • Structure: Flattened, membrane-bound sacs stacked on each other.
  • Functions:
    • Modification, packaging, and distribution of proteins and lipids.
    • Proteins and lipids are packaged into transport vesicles for delivery to secretion pathways or internal destinations.
  • Context: Golgi work flow receives cargo from ER, processes it, and sorts it for trafficking.

Function of the Golgi Apparatus (3-6)

  • Summary of roles:
    • Further modification of proteins and lipids (e.g., glycosylation, sulfation).
    • Sorting and targeting to appropriate destinations (secretory vesicles, lysosomes, plasma membrane).
    • Packaging into vesicles that bud off to transport cargo.

Lysosomes and Endocytosis (Action of Lysosomes)

  • Process overview (endocytosis and lysosomal digestion):
    • 1. Vesicle formation by endocytosis from the plasma membrane.
    • 2. Vesicle is taken into the cell.
    • 3. Vesicle fuses with a lysosome.
    • 4. contents are digested by lysosomal enzymes.
    • 5. Digested products are released for cytoplasmic use or exocytosis.
  • Context: Lysosomes contain hydrolytic enzymes that enable intracellular digestion of macromolecules, old organelles, and endocytosed material.

Mitochondria

  • Structure:
    • Outer membrane, intermembrane space, inner membrane, matrix.
    • Inner membrane folds into cristae to increase surface area for biochemical reactions.
  • Components:
    • Enzymes within the matrix and cristae drive ATP production via cellular respiration.
    • Mitochondrial DNA (mtDNA) is present and encoded genes contribute to mitochondrial functions.
  • Imaging notes:
    • TEM image at 34,000x magnification illustrates typical mitochondrial ultrastructure, including matrix and cristae.

Centrioles and Spindle Fibers

  • Content:
    • Centrioles (paired structures) participate in organizing the spindle apparatus during cell division.
    • Microtubule triplets form a basic unit of the centrosome-derived spindle.
  • Role:
    • Organize and separate chromosomes during mitosis and meiosis through spindle formation.

Structure of Cilia and Flagella

  • Core components:
    • Microtubules form the axoneme structure that powers motion.
    • Basal body anchors the cilium/flagellum to the plasma membrane.
    • Dynein arms generate bending movement for propulsion.
  • Context:
    • TEM image at 100,000x illustrates ciliary/flagellar ultrastructure and motor proteins in action.
  • Notable variants:
    • Cilia and flagella share a common axoneme organization, enabling movement and fluid flow across cell surfaces.

Microvilli

  • Structure:
    • Microvillus: a microscopic projection of the plasma membrane supported by actin filaments.
    • Actin filaments provide structural support and facilitate stretching of the membrane.
    • Cytoplasm and plasma membrane organization support increased surface area for absorption.
  • Imaging:
    • TEM image at 60,000x magnification shows microvilli distribution on the cell surface.

Genes and Gene Expression

  • Key components:
    • Nucleolus and nucleus: sites of transcriptional activity.
    • DNA strand: template for transcription.
    • mRNA strand: transcript produced by transcription; carries genetic information to the cytoplasm.
    • tRNA: delivers specific amino acids during translation.
    • Cytoplasm: site of translation and polypeptide synthesis.
  • Process flow:
    • Transcription:
      \text{DNA} \to \text{mRNA}
    • Translation: mRNA is read by ribosomes to assemble a polypeptide chain, using tRNA with amino acids from the amino acid pool.
    • Specific amino acids example shown: Arginine and Aspartic acid as components of the polypeptide.
    • Overall pathway:
    • DNA in nucleus is transcribed to mRNA in the nucleus/nucleolus,
    • mRNA is exported to the cytoplasm,
    • translation occurs at ribosomes to form a polypeptide chain,
    • amino acids are supplied by the amino acid pool and linked by peptide bonds.
  • Example sequence for gene expression:
    • DNA (gene) → transcription → mRNA → translation → polypeptide chain.
  • Practical notes:
    • Ribosomes are the molecular machines that read mRNA and assemble amino acids into polypeptides.
    • The process couples genetic information (DNA) to functional proteins required by the cell.