Nucleus – Structure and Functions

🔹 Definition

The nucleus is the most prominent organelle of eukaryotic cells, serving as the information processing and administrative center.
It controls cell growth, protein synthesis, and cell division, while storing the cell’s hereditary material (DNA).

In animal cells: centrally placed.
In plant cells: pushed to the side due to the large central vacuole.
Occupies about 10% of a eukaryotic cell’s volume.

🔹 Discovery & Term

First observed: Robert Brown (1831) described the nucleus.
Term “nucleus”: Coined by Brown.
Modern understanding: Evolved with microscopy and molecular biology.

🔹 Literal Meaning

Nucleus (Latin): “kernel” or “inner core.”

🔹 Nucleoplasm (Nuclear Sap)

The semifluid matrix inside the nucleus.
Contains chromatin, nucleolus, and enzymes.
Provides medium for nuclear activities.

Nuclear Envelope and Nuclear Pores

The nuclear envelope is a double membrane surrounding the nucleus.
The perinuclear space (between membranes) is continuous with the rough ER.
The inner membrane is lined by the nuclear lamina (protein mesh) which supports chromatin.
During cell division, the envelope disassembles and reforms in daughter nuclei.

Nuclear Pores

Small openings that regulate traffic between cytoplasm and nucleus.
Permeable to small molecules (ions, nucleotides).
Larger proteins (e.g., histones) require active transport.

Made of nuclear pore complex (NPC) consisting of:

Annular subunits
Column subunits
Ring subunits
Lumenal subunits

Basket-like fibrils extend into nucleoplasm for selective transport.

Nucleolus

A darkly stained, spherical structure in nucleoplasm.
One or two nucleoli usually present.
Site of ribosome biogenesis.
Formed at specific chromosome sites called Nucleolus Organizer Regions (NORs).
DNA at NORs encodes rRNA.

Structure

Granular component → ribosomal subunits already formed.
Fibrillar component → rRNA + proteins (precursors of ribosomes).

Function

Synthesis of rRNA.
Assembly of ribosomal subunits, later exported via nuclear pores.

Chromatin and Chromosomes

Chromatin

Network of DNA + histone proteins in the nucleus.
Appears as tangled fibers in interphase nucleus.
Organized into nucleosomes (DNA wrapped around histone octamer).

Types of Chromatin:

Euchromatin → genetically active, transcribed into RNA, lightly stained.
Heterochromatin → genetically inactive, tightly packed, darkly stained.

Chromosomes

During cell division, chromatin condenses into chromosomes.
Each chromosome = two chromatids joined at centromere.
Humans: 46 chromosomes (23 pairs) in diploid somatic cells.

Examples:

Ants/roundworms → 2 chromosomes.
Ophioglossum reticulatum (fern) → ~1260 chromosomes.
Conclusion: Chromosome number does not reflect organism complexity.

DNA in Nucleus

Each human cell nucleus contains about 6 feet (2 meters) of DNA, packed into 46 chromosomes.
DNA encodes all proteins and regulates gene expression.