Nucleus: Structure, Functions, and Discovery

The nucleus is the control center of the cell, the place where its DNA is stored, protected, and read. Almost everything a cell does traces back to instructions held in the nucleus, so understanding this organelle well is a foundation for nearly every later topic in cell biology and genetics: how genes are switched on and off, how cells divide, and what separates a bacterium from a human cell. This article works through the nucleus part by part, covers what each part does, and clears up the points students most often confuse.

The nucleus is a cell organelle that carries the genetic material (DNA, deoxyribonucleic acid) of a cell. Its commonly known jobs are to control and regulate cellular activities such as growth and metabolism, and to carry the genes that hold the cell's genetic information.

What is the nucleus?

The nucleus is a roughly spherical, membrane-bound organelle that contains the cell's DNA. It is bounded by a thin covering called the nuclear envelope (or karyotheca). Inside is a fluid called the nucleoplasm (or karyolymph), which holds fine threads of chromatin (the DNA and its associated proteins) and a dense body called the nucleolus.

The nucleus, eukaryotes, and prokaryotes

The presence of a true, membrane-bound nucleus is the single feature that defines a eukaryotic cell (plants, animals, fungi, protists). Prokaryotic cells (bacteria and archaea) have no nuclear membrane. Their DNA sits directly in the cytoplasm in a region called the nucleoid, not enclosed by an envelope. This one difference, DNA wrapped in a membrane or not, is the basis of the whole prokaryote-eukaryote divide that runs through all of cell biology and microbiology.

A few eukaryotic cells are exceptions worth remembering:

• Mature red blood cells lose their nucleus as they develop, which leaves more room for hemoglobin. Without a nucleus they cannot divide or repair themselves, which limits their lifespan to about 120 days.
• Skeletal muscle cells are multinucleate: they contain many nuclei, because they form by the fusion of many cells.

Discovery of the nucleus

The nucleus was one of the first cell structures to be seen under a microscope. The Dutch microscopist Antonie van Leeuwenhoek observed it in the blood cells of fish, amphibians, and birds, reporting it as a centrally placed clear area. Fontana later described it as an ovoid structure in the epidermal cells of eel skin in 1781.

However, in 1831 Robert Brown was the first to use the term "nucleus," describing a prominent body in orchid cells and recognizing it as a regular feature of cells. This began the concept of the nucleated cell. The word nucleus comes from the Latin nux, meaning "nut"; the sense is kernel or center part.

Structure of the nucleus

The nucleus has five structural components: the nuclear envelope, the nucleoplasm, the nuclear matrix, the chromatin, and the nucleolus. The nucleus is bounded by the thin nuclear envelope; inside it, the nuclear matrix, the chromatin, and the more spherical nucleolus are suspended in the nucleoplasm.

Nuclear envelope

The nuclear envelope (also called the karyotheca) is the outermost part of the nucleus and separates it from the cytoplasm. It has two membranes, an inner and an outer one. Each membrane is about 75 Å thick and, like the plasma membrane, has a trilaminar lipoprotein structure. A gap called the perinuclear space, about 250 Å wide, separates the two membranes.

In places, the outer membrane is studded with ribosomes and is continuous with the rough endoplasmic reticulum (RER), so the nuclear envelope is physically connected to the cell's protein-making network.

The inner membrane is lined by a dense protein layer called the nuclear lamina, a network of filaments made up of proteins called lamins A, B, and C. The lamina gives shape and support to the inner membrane, anchors the chromatin at the nuclear periphery, and takes part in the breakdown and reformation of the nuclear envelope during cell division.

Nuclear pores and their function

The nuclear envelope is not sealed. It is pierced by thousands of nuclear pores, each formed where the inner and outer membranes fuse. A typical nucleus has between 1,000 and 10,000 pores, and each is filled with a ring of proteins called the nuclear pore complex.

The function of nuclear pores is to control what moves between the nucleus and the cytoplasm. They act as selective gateways:

• Small molecules and ions (water, sugars, amino acids) pass through freely.
• Large molecules, especially RNA leaving the nucleus and proteins entering it, cannot pass freely. They are moved through the pore by an active, regulated transport process that recognizes specific "address" signals on the molecule.

This selective control is essential. It lets the cell keep DNA safely inside the nucleus while allowing the messenger RNA and ribosomal subunits made there to reach the cytoplasm, where proteins are built. The nuclear pore is, in effect, the security checkpoint between the cell's control room and the rest of the cell.

Functions of the nuclear envelope:

• It maintains the shape of the nucleus.
• It acts as a barrier between the cytoplasm and the nucleoplasm.
• It controls the flow of materials in and out of the nucleus.

Nucleoplasm

The nucleoplasm (also called karyolymph) is the transparent fluid that fills the nucleus. It houses the nucleolus and the chromatin, and it contains the nucleotides, polymerase enzymes, and metal ions (such as Mn²⁺ and Mg²⁺) needed to synthesize DNA and RNA. It also holds the proteins required to assemble ribosomal subunits. The finished ribosomal subunits and RNA molecules then leave the nucleus through the nuclear pores.

Functions of the nucleoplasm:

• It is the site where RNA and ribosomal subunits are synthesized.
• It houses the nuclear matrix, chromatin, and nucleolus.
• It maintains the internal pressure (turgidity) of the nucleus.

Nuclear matrix

The nuclear matrix is a network of thin, criss-crossed, protein fibrils connected to the nuclear envelope. It acts as a nuclear skeleton (nucleoskeleton) and stays intact even when the DNA and chromatin are removed.

Functions of the nuclear matrix:

• It maintains the shape of the nucleus.
• It anchors the chromatin.
• It provides a structural framework for transcription and replication.
• It helps process newly made RNA and transport it out of the nucleus.

Chromatin

Chromatin was named by Flemming in 1879. During interphase (the non-dividing stage), it appears as fine threads, called chromatin fibers, arranged in a criss-cross network sometimes referred to as the chromatin reticulum. Chromatin occupies most of the nucleus and has a fiber diameter of about 250 Å. During cell division, these fibers condense into short, thick rods, the chromosomes, and return to the extended chromatin form after division ends. In other words, chromatin and chromosomes are the same material in two different states.

Chromatin exists in two forms that matter greatly for how genes are used:

• Euchromatin is loosely packed and open. Because the DNA is accessible, this is the active chromatin, where genes can be switched on and transcribed into RNA.
• Heterochromatin is tightly packed and condensed. Because the DNA is compacted and hard to reach, this is largely inactive chromatin, where genes are switched off. It occurs in two kinds, constitutive and facultative.

The same DNA can shift between these states, and this packing-and-unpacking is one of the main ways a cell controls which genes are active. That idea, that access to DNA controls gene expression, is a foundation for much of molecular biology and genetics you will meet later.

Nucleosome. According to the model of Kornberg and Thomas (1974), chromatin is built from repeating subunits called nucleosomes. Each nucleosome is a core of eight histone proteins with a length of DNA (about 140 nucleotides) wrapped around it. A short DNA linker (about 60 nucleotides) connects one nucleosome to the next, so a nucleosome plus its linker (about 200 nucleotides) is called a chromatosome. The overall structure looks like beads on a string, where each bead is a nucleosome. Prokaryotes lack nucleosomes.

Function of chromatin: Chromatin packages the long DNA molecule so it fits inside the nucleus, controls which genes are accessible for transcription, and condenses into chromosomes during cell division.

Nucleolus

Do not confuse the nucleolus with the nucleus. The nucleus is the whole organelle. The nucleolus is a small, dense structure inside the nucleus whose job is to build ribosomes. One nucleus can contain one or more nucleoli.

The nucleolus was described by F. Fontana in 1781, in the slime of eel skin. Most cells have a nucleolus, though mature muscle cells and male reproductive cells are exceptions. It is usually spherical, has no membrane, and disappears during cell division, reforming afterward at specific chromosome sites called nucleolar organizer regions (NORs). It has four regions:

• Fibrillar region (nucleolonema): fine fibrils about 50 to 100 Å across, representing the long rRNA precursor before it is trimmed.
• Granular region: dense granules about 150 to 200 Å across, which are ribosomal subunits nearly ready for transport to the cytoplasm.
• Amorphous region: a structureless protein matrix in which the fibrillar and granular regions sit.
• Nucleolar chromatin: chromatin fibers carrying many copies of the DNA that directs rRNA synthesis.

Functions of the nucleolus:

• It synthesizes and stores rRNA (ribosomal RNA).
• It stores ribosomal proteins.
• It assembles ribosomal subunits by wrapping rRNA with ribosomal proteins.
• It plays a role in cell division.

Functions of the nucleus

The nucleus holds the DNA, and its functions all follow from that:

• It directs the synthesis of proteins that maintain the structure of the cell.
• It regulates the cell's metabolism through the enzymes it directs the cell to make.
• It stores the genetic information needed for the organism's reproduction, development, and behavior.
• It is where DNA is replicated before cell division.
• It is where ribosomal subunits are formed (in the nucleolus).
• It is the source of the genetic variation that drives evolution.
• It regulates which genes are switched on, which allows cells to specialize (differentiation).

How to Remember

The nucleus is the control room. It holds the DNA (the master instructions), and everything it does follows from that: it stores the genes, copies them, and sends out the RNA messages that tell the cell which proteins to build.

Envelope, pores, plasm, chromatin, nucleolus. Five parts: the double-membrane envelope (the wall), the pores (the doors), the nucleoplasm (the fluid inside), the chromatin (the DNA), and the nucleolus (the ribosome factory).

Eu- is easy access; hetero- is hidden. Euchromatin is open and active (genes can be read); heterochromatin is condensed and inactive (genes hidden away).

Nucleolus makes ribosomes. The little body inside the nucleus is the ribosome factory. No membrane, made of rRNA and protein, disappears during cell division.

No nucleus, no division. Mature red blood cells have no nucleus, so they cannot divide or repair. Bacteria have no true nucleus at all (just a nucleoid).

Key exam facts

Where Students Get Confused

The nucleus and the nucleolus are the same thing. They are not. The nucleus is the whole organelle that holds the DNA. The nucleolus is a small structure inside the nucleus that makes ribosomes.

Heterochromatin has more RNA than euchromatin. No. Heterochromatin is condensed and inactive, so little transcription happens there. Euchromatin is open and active, and that is where genes are read into RNA.

All cells have a nucleus. Not all. Bacteria (prokaryotes) have no true nucleus, only a nucleoid. Among eukaryotic cells, mature red blood cells have lost their nucleus, and skeletal muscle cells have many.

Chromatin and chromosomes are different substances. They are the same material in different states. Chromatin is the loose, extended form seen between divisions; during cell division it condenses into the compact rods we call chromosomes.

Anything can pass through the nuclear envelope freely. No. Small molecules pass freely through the pores, but large molecules such as RNA and proteins are moved by a controlled, selective transport process.

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. New York: W. W. Norton; 2022.
2. Iwasa J, Marshall W. Karp's Cell and Molecular Biology. 8th ed. Hoboken: Wiley; 2016.
3. Reece JB, Urry LA, Cain ML, et al. Campbell Biology. 12th ed. New York: Pearson; 2021.