The cell nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of their DNA, organized into chromosomes, and provides a specialized environment for maintaining and using genetic information. The nucleus separates the nuclear genome from the cytoplasm, enabling distinctive mechanisms of gene expression and molecular transport. It is not simply a storage compartment: its internal organization helps coordinate DNA copying, RNA production, and the assembly of components needed for protein synthesis. (genome.gov)
Nuclear envelope and transport
The nuclear envelope consists of inner and outer membranes separated by a narrow space. The outer membrane is continuous with the endoplasmic reticulum, and the space between the nuclear membranes connects with that organelle’s internal lumen. The two membranes have different protein compositions, reflecting their different interactions with nuclear and cytoplasmic components. Ribosomes can attach to the cytoplasmic surface of the outer membrane. (ncbi.nlm.nih.gov)
In animal cells, a network called the nuclear lamina lies beneath the inner membrane. Its principal components, lamins, provide mechanical support and contribute to interactions between the envelope and chromatin. This lamin-based structure is not universal: plants and many unicellular eukaryotes lack conventional lamins, although other proteins can perform related structural functions. (ncbi.nlm.nih.gov)
Nuclear pore complexes span the envelope and regulate exchange between the nucleus and cytoplasm. Small molecules can pass through these channels by diffusion, whereas many large proteins and RNA-containing complexes require selective transport mechanisms. Nuclear proteins are generally synthesized outside the nucleus and subsequently imported. Conversely, RNA molecules and developing ribosomal subunits must be exported to perform their cytoplasmic functions. The envelope therefore maintains compartmentalization without isolating the nucleus from the rest of the cell. (ncbi.nlm.nih.gov)
Chromatin and spatial organization
Nuclear DNA is associated with proteins in a complex called chromatin. Much of its packaging depends on histones, around which DNA wraps to form repeating units called nucleosomes. This organization compacts long DNA molecules while allowing particular regions to become accessible when they are needed. Packaging is consequently both a structural requirement and a means of regulating access to genetic information. (genome.gov)
Between divisions, chromatin varies in its degree of compaction. Euchromatin is generally less condensed and more accessible to transcription machinery; heterochromatin is generally more compact and often associated with reduced transcriptional activity. These are broad organizational categories rather than an absolute division between active and inactive DNA. Much heterochromatin occurs near the nuclear periphery or around nucleoli. (ncbi.nlm.nih.gov)
Individual chromosomes occupy preferential regions known as chromosome territories rather than being randomly mixed throughout the nucleus. The nuclear interior also contains domains enriched in particular molecular activities. Its organization is dynamic: chromatin structure, chromosome positioning, and the distribution of processing factors help create different local environments for genome function. The surrounding internal medium, the nucleoplasm, contains soluble molecules as well as these organized structures. (ncbi.nlm.nih.gov)
Transcription and RNA processing
The nucleus is the principal site of transcription of the nuclear genome. During transcription, RNA polymerases copy selected DNA sequences into RNA. For a protein-coding gene, the initial transcript typically undergoes processing before functioning as messenger RNA. Processing commonly includes addition of a protective cap, modification of the RNA’s end, and splicing, which removes introns and joins the retained sequences. (ncbi.nlm.nih.gov)
Processed messenger RNA is exported to the cytoplasm, where translation produces proteins. Separating transcription from translation allows the cell to regulate RNA maturation and export before protein synthesis begins. It also permits different mature transcripts to arise from one initial RNA through alternative processing. Another regulatory mechanism involves controlling the entry of a transcription factor into the nucleus, thereby controlling its access to DNA. (ncbi.nlm.nih.gov)
Nucleolus and other nuclear bodies
The nucleolus is a prominent nuclear structure associated with production and processing of ribosomal RNA and assembly of ribosomal subunits. Unlike the nucleus itself, it has no surrounding membrane. It forms around chromosomal regions containing ribosomal RNA genes and concentrates RNA, processing factors, and imported ribosomal proteins. Developing subunits subsequently leave the nucleus for further maturation and participation in protein synthesis. (ncbi.nlm.nih.gov)
Other membrane-free structures include nuclear speckles and Cajal bodies. These concentrate molecules involved in RNA processing or the assembly and modification of RNA–protein complexes. Their components can exchange with the surrounding nucleoplasm; they are not sealed miniature compartments. Nuclear bodies illustrate how a cell can organize biochemical processes through local molecular associations without enclosing every functional domain in a membrane. (ncbi.nlm.nih.gov)
Replication and nuclear division
Before a proliferating cell divides, DNA replication duplicates its nuclear chromosomes. Replication takes place at numerous sites within the nucleus, with its machinery organized into spatially distributed clusters. Subsequent chromosome segregation ensures that the resulting nuclei receive the appropriate genetic material. (ncbi.nlm.nih.gov)
During mitosis in animal cells, chromosomes condense and the nuclear envelope disassembles, allowing spindle microtubules to interact with them. After the chromosomes separate, envelopes re-form around the daughter chromosome groups, nuclear transport resumes, and chromatin decondenses. This “open” mitosis contrasts with “closed” mitosis in organisms such as many yeasts, where chromosome segregation occurs within a largely intact nuclear envelope. Nuclear division and division of the cell are related but distinct events. (ncbi.nlm.nih.gov)
Variation among cells
Not every eukaryotic cell has exactly one nucleus. Mature mammalian red blood cells expel their nuclei during development. Skeletal muscle fibers, by contrast, contain many nuclei because they form through the fusion of precursor cells. These specialized exceptions do not alter the distinction between eukaryotes and bacteria or archaea, which lack a membrane-enclosed nucleus. Nuclear DNA must also be distinguished from the separate DNA retained in mitochondria; the nucleus does not contain all genetic material in every eukaryotic cell. (openstax.org)