Cell Biology Terms Starting With N
Cell Biology Glossary: N
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Necrosis
/ neh-KROH-sis / · Greek: nekros (dead body) + -osis (process)
Necrosis is uncontrolled cell death triggered by severe injury, infection, or loss of blood supply, in which the dying cell swells, loses membrane integrity, and releases its contents into surrounding tissue.
Necrosis results from acute cellular injury such as severe hypoxia, trauma, or toxin exposure that overwhelms cellular defense mechanisms and triggers uncontrolled lysis. Dying necrotic cells lose plasma membrane integrity, releasing intracellular contents including lactate dehydrogenase, potassium ions, and damage-associated molecular patterns that activate local inflammation. Unlike apoptosis, which produces intact membrane-bound fragments cleared by phagocytes, necrotic cells swell as osmotic balance fails and organelles rupture, spilling enzymes and proteins into surrounding tissue.
The inflammatory response to necrosis can damage neighboring healthy cells, potentially spreading tissue injury, as seen in myocardial infarction where oxygen-deprived heart muscle dies and triggers extensive immune cell infiltration.
Researchers identified a regulated form of necrosis called necroptosis in the early 2000s, showing that some necrotic-looking cell death is actually controlled by specific signaling proteins, including receptor-interacting protein kinase 3 (RIPK3), and can be blocked by targeted inhibitors.
All cell death is uncontrolled and damaging to surrounding tissue. Apoptosis is a genetically programmed form of cell death that produces membrane-bound fragments, avoids triggering inflammation, and leaves neighboring cells unharmed.
Cell Death →After a heart attack, oxygen-starved cardiac muscle cells undergo necrosis within 20 to 40 minutes of blood flow loss. The ruptured cells release troponin and other proteins into the bloodstream, which clinicians measure as diagnostic markers of myocardial damage.
Nuclear Envelope
/ NOO-klee-er EN-veh-lohp / · Latin: nucleus + French: enveloppe
Nuclear envelope is the double membrane surrounding the nucleus of eukaryotic cells, separating the genetic material from the cytoplasm and regulating molecular traffic between these compartments.
The nuclear envelope consists of two concentric phospholipid bilayers separated by a 20 to 40 nanometer perinuclear space that connects directly to the lumen of the rough endoplasmic reticulum. Approximately 30 nuclear pore complexes per square micrometer perforate both membranes, each built from about 30 different nucleoporin proteins that form a selective transport barrier. During cell division in most animal cells, the nuclear envelope disassembles into vesicles when cyclin-dependent kinase activity peaks, then reassembles around separated chromosome sets during telophase.
Transport through the envelope depends on importin and exportin proteins that carry cargo molecules bearing specific nuclear localization or nuclear export signals, ensuring that only appropriate molecules cross in each direction.
Some organisms never fully disassemble their nuclear envelope during cell division. Fungi such as budding yeast (Saccharomyces cerevisiae) undergo closed mitosis, in which the spindle forms inside an intact nucleus and the envelope pinches apart only after chromosomes segregate, a strategy that differs fundamentally from the open mitosis seen in most animal cells.
The nuclear envelope is a solid, impermeable barrier around the nucleus. Two distinct membrane layers form the envelope, and hundreds of large protein channels called nuclear pore complexes span both layers to regulate the selective passage of molecules in both directions.
In cells of the African clawed frog (Xenopus laevis), the nuclear envelope reassembles around chromosomes at the end of mitosis by recruiting membrane vesicles derived from the endoplasmic reticulum. This reassembly process takes roughly 30 minutes and requires the sequential recruitment of more than 1000 nucleoporin molecules per nucleus.
Differences Between Plant and Animal Cells →Nuclear Pore
/ NOO-klee-er por / · Latin: nucleus + porus (passage)
Nuclear pore is a large protein channel that spans the nuclear envelope and controls the selective movement of molecules between the nucleus and the cytoplasm.
Each nuclear pore complex weighs approximately 125 megadaltons and contains about 30 different nucleoporin proteins arranged in an eightfold symmetric cylindrical scaffold spanning both membranes of the nuclear envelope. Transport through nuclear pores requires cargo binding to importin or exportin proteins that recognize specific amino acid sequences, called nuclear localization signals or nuclear export signals, on target molecules. The selective barrier permits passive diffusion of molecules smaller than roughly 5000 daltons but actively transports larger cargos, including histones, ribosomal proteins, and messenger RNA, at rates up to 1000 transport events per minute per pore.
Ran GTPase powers directional transport by maintaining a steep concentration gradient across the envelope, with RanGTP concentrated in the nucleus and RanGDP concentrated in the cytoplasm.
A single human cell contains roughly 2000 to 5000 nuclear pore complexes, but the number scales with transcriptional activity. Highly active cells such as Xenopus laevis oocytes can accumulate more than 50 million nuclear pore complexes in a single nucleus to support the massive RNA export needed for early embryonic development.
Anything small can freely pass through nuclear pores. Many larger cargos require specific transport signals and dedicated carrier proteins, and even some small molecules are actively excluded or concentrated on one side of the envelope by the Ran GTPase gradient.
In human cells, mature messenger RNA exits the nucleus through nuclear pore complexes after 5-prime capping, splicing, and 3-prime polyadenylation are complete. Each mRNA molecule is escorted by export factors that thread it through the approximately 9-nanometer central channel of the pore before releasing it to cytoplasmic ribosomes.
Translation Biology →Nucleolus
/ nyoo-KLEE-oh-lus / · Latin nucleolus, small kernel
Nucleolus is a dense, membrane-free region inside the eukaryotic cell nucleus where ribosomal RNA genes are transcribed and ribosomal subunits begin assembly.
The nucleolus transcribes three of the four ribosomal RNA molecules using RNA polymerase I, drawing on tandemly repeated ribosomal DNA genes located on chromosomes 13, 14, 15, 21, and 22 in humans. Ribosomal proteins imported from the cytoplasm combine with nascent rRNA to form preribosomal particles that undergo sequential modification and cleavage before export. Approximately 60 percent of all transcription in a growing eukaryotic cell occurs in the nucleolus, as dozens of RNA polymerase I complexes simultaneously transcribe overlapping ribosomal RNA genes on each active chromosome.
During mitosis, the nucleolus dissolves as chromosomes condense and reforms around nucleolar organizing regions during telophase, with its size reflecting the cell’s current demand for new ribosomes.
Beyond ribosome production, the nucleolus coordinates the cellular stress response. When DNA damage or other stress signals accumulate, the tumor suppressor protein p53 is stabilized partly through nucleolar sequestration of MDM2, the protein that normally targets p53 for degradation, linking ribosome biogenesis directly to cell cycle arrest.
The nucleolus is a smaller, separate nucleus inside the cell. The nucleolus is a specialized region within the nucleus, not a separate organelle, and it lacks its own membrane; its boundaries form through liquid-liquid phase separation of specific RNA-binding proteins.
In the giant amoeba Chaos carolinense, the nucleolus can reach diameters of several micrometers and remains visible under a standard light microscope without staining. This cell maintains hundreds of nuclei simultaneously, each with its own nucleolus, reflecting the enormous ribosome demand needed to sustain its large cytoplasmic volume.
Nucleus
/ NOO-klee-us / · Latin: nucleus (kernel, nut)
Nucleus is a membrane-bound organelle found in eukaryotic cells that houses most of the cell's DNA and directs cellular activity by controlling gene expression.
The human nucleus contains approximately 3 billion base pairs of DNA organized into 46 chromosomes, protecting genetic material from cytoplasmic enzymes and reactive metabolites. Around 2000 nuclear pore complexes perforate the double-membrane nuclear envelope, selectively passing messenger RNA molecules and regulatory proteins while restricting free diffusion of large macromolecules. During mitosis, the nuclear envelope breaks down in most animal cells, allowing spindle fibers to attach to chromosomes, then reforms during telophase to re-establish compartmentalization.
Gene transcription occurs within the nucleus, where RNA polymerase II synthesizes pre-messenger RNA from DNA templates before the transcript is capped, spliced, and exported to cytoplasmic ribosomes.
The nucleus was first described in plant cells by Scottish botanist Robert Brown in 1831, but it took several more decades before biologists recognized it as a universal feature of eukaryotic cells. Brown initially thought the structure was unique to orchid epidermal cells, not anticipating that it would prove to be the defining feature separating eukaryotes from prokaryotes.
Difference Between Prokaryotic and Eukaryotic Cells →Every cell has a nucleus. Mature human red blood cells eject their nucleus during development to maximize space for hemoglobin, and all prokaryotic cells, including bacteria and archaea, lack a nucleus entirely, storing their DNA in an unenclosed region called the nucleoid.
In human cheek epithelial cells stained with methylene blue, the nucleus appears as a dense blue-purple oval roughly 5 to 10 micrometers in diameter. At this scale it occupies approximately 10 percent of the total cell volume, a proportion typical of most mammalian somatic cells.