Nucleus & Related Structures

Prominent organelle in all eukaryotic cells (central in animals, peripheral in plants)
Occupies \approx 10\% of total cell volume
Functions:
- Stores hereditary material \text{DNA}
- Coordinates cell activities (growth, protein synthesis, cell division)
Internal medium: nucleoplasm (semifluid matrix containing chromatin + nucleoli)

Double membrane; outer membrane continuous with rough ER
Inter-membrane gap = perinuclear space
Disassembles at mitosis onset; re-forms in daughter nuclei
Inner surface reinforced by nuclear lamina (protein meshwork binding chromatin)
Nuclear pores regulate exchange
- Freely pass small molecules; selectively transport larger proteins (e.g.
  histones) and RNA/protein complexes
Nuclear pore complex subunits:
- Annular (inner ring) • Column (wall) • Ring (outer) • Lumenal (anchor)
- Cytoplasmic & nuclear fibrils form a basket on nuclear side

Dark, spherical body; typically 1–2 per nucleus
Site of ribosomal subunit assembly
Forms at Nucleolus Organizer Regions (NORs) whose DNA encodes rRNA
Enlarges in cells with high protein synthesis demand
Disappears during mitosis; reappears after division
Structural zones:
- Fibrillar component: rRNA transcripts + associated proteins
- Granular component: partially assembled ribosomal subunits

DNA + histone proteins arranged as beads (nucleosomes)
Types
- Euchromatin – genetically active, RNA transcription
- Heterochromatin – genetically inactive
Interphase: dispersed tangle of fibres;
Mitosis onset: condenses into chromosomes

Each chromosome = 2 chromatids joined at a centromere; ends = telomeres
Human diploid cells: 46 chromosomes (total DNA (\approx 6\,\text{ft}))
Chromosome numbers are species-specific (range 2 to 1260+); not linked to organism complexity