Unit 4: Nucleus and Cytoskeleton

1. Structure of Nucleus

The nucleus is the large, membrane-bound organelle that acts as the "control center" of the eukaryotic cell. It contains the cell's genetic material (DNA) and controls all cellular activities by regulating gene expression.

Nuclear Envelope

• Structure: A double membrane (two phospholipid bilayers, an outer and an inner) that encloses the nucleus, separating its contents (nucleoplasm) from the cytoplasm.
• The outer membrane is continuous with the Endoplasmic Reticulum (ER).
• The space between the two membranes is the perinuclear space.

Nuclear Pore Complex (NPC)

• Structure: The nuclear envelope is perforated by thousands of large, complex protein structures called nuclear pores.
• Function: They act as regulated gateways, controlling all traffic into and out of the nucleus.
  • Outflow: mRNA, tRNA, and ribosomal subunits must be exported to the cytoplasm.
  • Inflow: Building blocks (like nucleotides) and proteins (like histones and polymerases) needed in the nucleus must be imported from the cytoplasm.

Nucleolus

• Structure: A dense, non-membranous region within the nucleoplasm. Cells can have one or more.
• Function: The primary site of ribosome synthesis.
  1. Ribosomal RNA (rRNA) is transcribed from DNA here.
  2. rRNA is combined with proteins (imported from the cytoplasm) to form the large and small ribosomal subunits.
  3. These subunits are then exported to the cytoplasm through nuclear pores.

2. Chromatin: Euchromatin and Heterochromatin

Chromatin is the complex of DNA and proteins (mainly histones) that forms the chromosomes within the nucleus of eukaryotic cells. During interphase (the non-dividing state), chromatin exists in two forms:

3. Chromatin: Packaging (Nucleosome)

A human cell's nucleus contains about 2 meters of DNA, which must be tightly packed into a nucleus only a few micrometers wide. This packing is achieved through multiple levels of coiling, starting with the nucleosome.

Nucleosome: The fundamental, repeating unit of chromatin packaging.

Structure of a Nucleosome

• Histone Octamer: A core "spool" made of 8 histone proteins (two each of H2A, H2B, H3, and H4). Histones are rich in basic amino acids (lysine, arginine), giving them a positive charge.
• DNA: The negatively-charged DNA (due to phosphate groups) wraps around this positively-charged histone octamer approximately 1.65 times.
• H1 Histone: A "linker" histone that binds to the DNA where it enters and exits the spool, helping to lock the nucleosome in place and pull them together into a more compact fiber.

Levels of Packaging:

1. Nucleosomes ("Beads-on-a-string"): The first level of compaction.
2. 30-nm Fiber (Solenoid): The nucleosomes are further coiled into a thicker fiber.
3. Looped Domains: The 30-nm fiber forms loops that attach to a protein scaffold.
4. Metaphase Chromosome: The most condensed form, visible during cell division.

4. Structure and Functions: Microtubules

The Cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates movement, and organizes the cytoplasm. It has three main components.

Microtubules

• Structure: The thickest of the filaments. They are hollow, unbranched tubes made of the globular protein tubulin (alpha-tubulin and beta-tubulin subunits).
• Dynamic Instability: They can rapidly assemble (polymerize) and disassemble (depolymerize), allowing the cell to change its shape or move components.
• Functions:
  1. Cell Shape & Support: Act as compression-resisting "girders" to maintain cell shape.
  2. Cell Motility: Form the core of cilia and flagella, the locomotive appendages of some cells.
  3. Chromosome Movement: Form the mitotic spindle (spindle fibers) that separates chromosomes during mitosis and meiosis.
  4. Organelle Transport: Act as "railway tracks" for motor proteins (like kinesin and dynein) to move vesicles and organelles around the cell.

5. Structure and Functions: Microfilaments

Microfilaments (or Actin Filaments)

• Structure: The thinnest filaments. They are solid, intertwined strands made of the globular protein actin.
• Functions:
  1. Muscle Contraction: Interact with the motor protein myosin to cause muscle cell contraction.
  2. Cell Motility: Enable amoeboid movement through the formation of pseudopods (false feet).
  3. Cell Shape: Form a network just inside the plasma membrane (the cortex) that helps support and shape the cell.
  4. Cytoplasmic Streaming: The circular flow of cytoplasm in plant cells, driven by actin-myosin interactions.
  5. Cytokinesis: Form the cleavage furrow that pinches an animal cell in two during cell division.

6. Structure and Functions: Intermediate Filaments

Intermediate Filaments

• Structure: Intermediate in size. They are tough, fibrous, rope-like proteins (e.g., keratin in skin, lamins in the nuclear envelope).
• Key Feature: Unlike the other two, they are not dynamic. They are very stable and permanent.
• Functions:
  1. Purely Structural: Their main role is to bear tension and provide mechanical strength.
  2. Anchoring Organelles: They anchor the nucleus and other organelles in place.
  3. Tissue Integrity: They are crucial components of desmosomes, linking cells together to form strong tissues.
  4. Nuclear Lamina: Form a meshwork inside the nuclear envelope that supports its shape.