Eukaryotic Cell Ultrastructure: Nucleus and Endoplasmic Reticulum

Understanding the detailed ultrastructure of eukaryotic organelles is at the heart of OCR module 2.1.1 (e). This lesson examines the nucleus, the nucleolus, the nuclear envelope, and the endoplasmic reticulum (both rough and smooth), explaining how their structures are suited to their functions. Because A-Level depth demands precise terminology, you should learn the molecular-level features summarised here.

Key Definitions:

• Ultrastructure — the detailed three-dimensional organisation of a cell as revealed by electron microscopy.
• Organelle — a discrete subcellular structure with a specific function.
• Eukaryotic cell — a cell possessing a membrane-bound nucleus and membrane-bound organelles.

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The Nucleus

The nucleus is usually the largest organelle in a eukaryotic cell, typically 5–10 µm in diameter, and is easily visible under a light microscope. It contains almost all of the cell's genetic material in the form of chromatin — a complex of DNA wound around histone proteins.

Functions of the Nucleus

1. Stores genetic information as DNA in the form of chromosomes.
2. Controls cell activity by regulating which genes are transcribed to mRNA.
3. Site of DNA replication during S phase of the cell cycle.
4. Site of transcription — production of messenger RNA (mRNA) from DNA template.
5. Contains the nucleolus, which manufactures ribosomal RNA (rRNA) and assembles ribosomal subunits.

The Nuclear Envelope

The nucleus is enclosed by a double membrane known as the nuclear envelope:

• Both membranes are phospholipid bilayers, about 7 nm thick each, separated by a perinuclear space of around 20–40 nm.
• The outer nuclear membrane is continuous with the rough endoplasmic reticulum (RER) and is often studded with ribosomes.
• The inner nuclear membrane is lined by the nuclear lamina, a meshwork of intermediate-filament proteins (lamins) that provides structural support.
• The envelope is perforated by nuclear pores, protein-lined channels approximately 100 nm in diameter.

Nuclear Pores

Nuclear pores are complex structures composed of around 30 different nucleoporin proteins, collectively forming the nuclear pore complex (NPC). They regulate the passage of molecules between the nucleus and cytoplasm:

• Small molecules (ions, metabolites, up to ~40 kDa) diffuse freely.
• Large molecules (proteins, RNA, ribosomal subunits) are actively transported through the NPC, requiring signal sequences and energy.
• Nuclear pores allow mRNA to exit the nucleus for translation and allow proteins required in the nucleus (e.g., histones, transcription factors, DNA and RNA polymerases) to enter.

Chromatin and Chromosomes

Inside the nucleus, DNA is associated with histone proteins to form chromatin. During interphase, chromatin exists in two forms:

• Euchromatin — loosely packed, transcriptionally active.
• Heterochromatin — tightly packed, transcriptionally inactive; often seen as darker regions at the nuclear periphery under TEM.

At cell division, chromatin condenses into the visible rod-like structures known as chromosomes. Humans have 46 chromosomes (23 pairs) per somatic cell. Each chromosome consists of a single, linear DNA molecule wrapped around nucleosomes (histone octamers).

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The Nucleolus

Within the nucleus, a densely staining region known as the nucleolus is visible under both light and electron microscopy. A typical cell has one or two nucleoli, each approximately 1–3 µm across.

Functions of the Nucleolus

• Synthesises ribosomal RNA (rRNA) by transcription of rRNA genes on special chromosomal regions called nucleolar organiser regions (NORs).
• Assembles ribosomal subunits by combining rRNA with ribosomal proteins imported from the cytoplasm. The large (60S) and small (40S) subunits are then exported separately through nuclear pores to the cytoplasm, where they combine during translation to form functional 80S ribosomes.
• Involved in regulation of the cell cycle and cellular stress responses.

Key Point: Cells that secrete large amounts of protein (e.g., pancreatic acinar cells, plasma cells) have prominent nucleoli because they must produce huge numbers of ribosomes.

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Endoplasmic Reticulum (ER)

The endoplasmic reticulum is an extensive, continuous network of flattened membrane-bound sacs and tubules called cisternae. The lumen (interior) of the ER is distinct from the cytosol. There are two types of ER, distinguishable in electron micrographs by the presence or absence of ribosomes.

Rough Endoplasmic Reticulum (RER)

• Studded on its cytosolic face with 80S ribosomes, giving it a granular appearance under TEM.
• Continuous with the outer nuclear membrane.
• Arranged as flattened, stacked cisternae.

Function