Biology

Nerve impulse (action potential) Nerve impulse A nerve impulse is a rapid electrical signal that travels along neurons, allowing the nervous system to transmit information. Covers mechanism, conduction, history, examples, and c…

Nervous system: structure, function, development, and clinical importance Nervous system Overview of the nervous system, its main divisions, cellular makeup, evolution, functions, and relevance to health and behaviour.

Nervous tissue: structure, cells, function, and organization Nervous tissue Nervous tissue is the cellular network that senses, conducts, and integrates signals in the brain, spinal cord and peripheral nerves. It comprises neurons and supporting glial cell…

Neurodegeneration: causes, features, examples and current approaches Neurodegeneration Neurodegeneration: progressive loss of neuronal structure or function leading to cognitive, motor, and autonomic deficits. Overview of mechanisms, major diseases, and research dire…

Neuroethology: the neural basis of natural animal behavior Neuroethology Neuroethology studies how nervous systems produce natural behaviors. It combines field observation, electrophysiology, imaging and comparative approaches to link stimuli, neural ci…

Neuron: structure, function, development and significance Neuron A neuron is a specialized nerve cell that transmits electrical and chemical signals in the nervous system, enabling sensation, movement, reflexes and higher cognition.

Neuroscience — the scientific study of the nervous system and brain function Neuroscience Neuroscience examines the structure, function, development and disorders of the nervous system, integrating biology, psychology, medicine and computational approaches to understand…

Neurosecretion: how neurons produce and release hormones Neurosecretion Neurosecretion is the process by which certain neurons synthesize, store and release hormones (neurohormones) to regulate bodily functions from water balance to stress responses.

Neurotoxin: Definition, Mechanisms, Sources, Effects and Uses Neurotoxin A neurotoxin is a substance that damages or disrupts the function of nerve cells. This article explains how neurotoxins work, common natural sources, typical effects, uses, and imp…

Neutrophil: frontline white blood cell in innate immunity Neutrophil Neutrophils are the most abundant white blood cells. This article explains their structure, functions, lifespan, role in infections and inflammation, clinical significance, and dis…

Niacin (Vitamin B3): Roles, Sources, Uses, and Safety Niacin Niacin (vitamin B3) is a water-soluble nutrient required for energy metabolism and NAD/NADP production. This article explains its functions, dietary sources, deficiency signs, ther…

NADP and NADPH: cellular reducing coenzymes in biosynthesis and antioxidant defense Nicotinamide adenine dinucleotide phosphate Overview of NADP and NADPH, cellular redox cofactors that donate electrons for biosynthesis and antioxidant defense, their structure, generation (pentose phosphate pathway, photosy…

Nidovirales: biology, families, and significance Nidovirales Nidovirales is an order of enveloped positive-sense RNA viruses that produce nested subgenomic mRNAs; it includes coronaviruses and related families that infect animals, humans, an…

Nipple (mammary nipple) Nipple Overview of the nipple: anatomy, development, role in lactation, variations, clinical considerations and cultural context in humans and other mammals.

Nitrogen cycle: processes, organisms, and environmental significance Nitrogen cycle A clear overview of the nitrogen cycle: its stages, the organisms and chemistry involved, human impacts such as fertilizer use and pollution, and management approaches.

Nomascus — the crested gibbons Nomascus Nomascus is a genus of crested gibbons native to Southeast Asia. This article summarizes their taxonomy, appearance, behavior, distribution, and conservation status, including nota…

Nomen dubium (doubtful name) Nomen dubium A nomen dubium is a scientific name of uncertain application, often because its type material is missing or non‑diagnostic; nomenclatural codes provide procedures, such as neotype…

Non-coding DNA: functions, types and scientific significance Non-coding DNA Overview of DNA sequences that do not specify proteins, their categories, roles in regulation and genome structure, historical views and modern research including key debates and m…

Non-coding RNA (ncRNA): types, roles and history Non-coding RNA Non-coding RNAs are RNA molecules not used as templates for proteins but that perform structural, regulatory and catalytic functions in cells. Overview, types, discovery and biolog…

Nose: structure, function, health, and evolutionary context Nose Comprehensive overview of the nose: anatomy, how smell and breathing work, common disorders and clinical importance, variations across animals, and further reading links.

Nostril Anatomy and function of the nostrils (nares): external openings of the nose involved in breathing, smell, air conditioning, and reflexes; comparative and clinical notes.

Notochord: embryonic axial structure and evolutionary role Notochord The notochord is a flexible, rod-like axial structure present in all chordate embryos. It provides mechanical support, directs nearby tissue patterning through signaling, and is va…

Novolac (phenol–formaldehyde resin) Novolac Novolac is an acid‑catalyzed phenol–formaldehyde resin used as a thermosetting binder and as a base for positive photoresists; valued for heat resistance, mechanical strength and e…

Nuclear DNA: structure, inheritance, and role in eukaryotic cells Nuclear DNA Nuclear DNA is the genetic material housed in the eukaryotic cell nucleus, organized into chromosomes and governed by replication, recombination and regulation to direct cell funct…

Nuclear membrane (nuclear envelope) Nuclear membrane The nuclear membrane, or nuclear envelope, is the double membrane that encloses the eukaryotic nucleus, controls molecular traffic via nuclear pores, and helps organize chromosomes…

Nuclease (enzyme that cleaves nucleic acids) Nuclease A nuclease is an enzyme that cleaves phosphodiester bonds in nucleic acids. Types include endonucleases, exonucleases, restriction enzymes, DNases and RNases; they are essential in…

Nucleic acids: structure, function, and biological significance Nucleic acid Nucleic acids (DNA and RNA) are biological polymers that store, transmit and help express genetic information. This article explains composition, differences, cellular roles, histo…

Nucleobase — fundamental bases of DNA and RNA Nucleobase Nucleobases are the molecular components of DNA and RNA that encode genetic information, form base pairs, and influence structure, mutation and molecular recognition in biology.

Nucleoid (prokaryotic genetic region) Nucleoid The nucleoid is the irregular, non-membrane-bound region in prokaryotic cells that contains the chromosome(s) and associated proteins, organizing DNA for replication, transcription…

Nucleolus: structure, function and role in ribosome production Nucleolus The nucleolus is a membrane‑less nuclear body where ribosomal RNA is transcribed and ribosomes are assembled. This article describes its structure, formation, functions and medical…

Nucleosome A nucleosome is the basic repeating unit of chromatin: DNA wrapped around histone proteins that compacts chromosomes and helps regulate gene activity, replication and repair.

Nucleotide (basic unit of nucleic acids) Nucleotide A nucleotide is a molecule composed of a nitrogenous base, a five‑carbon sugar and phosphate; it is the monomer of DNA and RNA and also participates in energy transfer, signaling a…

Nucleus: central structures in cells, atoms, comets, and brains Nucleus A nucleus is a central core or cluster occurring in different sciences: the cell nucleus contains genetic material; the atomic nucleus holds protons and neutrons; comet nuclei are…

Nucleus (brain structure) Nucleus (brain) A nucleus is a discrete cluster of neurons in the central nervous system. This article explains their form, examples, connectivity, development, and clinical importance.

Nutrient (biology) Nutrient A nutrient is a chemical element or compound used by organisms for growth, maintenance and metabolism; in animals it includes macronutrients, micronutrients, and water, with roles,…

Nutrition: Principles, Major Nutrients, Health Effects and Practical Guidance Nutrition Overview of nutrition covering major nutrients, digestion, history, malnutrition, professional roles, dietary recommendations and distinctions between macronutrients and micronutri…

Obazoa: a proposed eukaryote clade linking opisthokonts, apusomonads and breviates Obazoa Obazoa is a proposed grouping of unikont eukaryotes that joins Opisthokonta with Apusomonadida and Breviatea. The group's limits and internal relationships remain under study using…

Occipital bone — anatomy, function, and clinical relevance Occipital bone The occipital bone forms the back and base of the skull, surrounding the foramen magnum and providing muscle attachments, articulation with the atlas, and protection for posterior…

Occipital lobe The occipital lobe is the posterior region of the cerebral cortex that primarily processes visual information, containing the primary visual cortex and several visual association a…

Octocorallia (Octocorals): soft corals, sea fans and sea pens Octocorallia Octocorallia are eight‑armed colonial anthozoans (soft corals, sea fans, sea pens) distinguished by eight-fold symmetry, sclerites or gorgonin support, diverse habitats, and ecolog…

Oestradiol (E2): roles, biology, and clinical significance Oestradiol Oestradiol (E2) is the principal female sex steroid that regulates reproductive development, menstrual cycling, bone health, and has measurable roles in males; commonly measured in…

Oestrogen (estrogen): roles, types, production and clinical significance Oestrogen Comprehensive overview of oestrogen: major forms, sources, biological roles in development and reproduction, mechanisms, clinical uses and risks, and distinctions from related horm…

Oestrone Oestrone is a naturally occurring estrogenic steroid hormone, produced mainly by ovaries and adipose tissue. It is biologically weaker than estradiol and has clinical and diagnosti…

Oestrous cycle The oestrous cycle is the recurring reproductive cycle in most female mammals, controlled by hormones and expressed as periods of sexual receptivity (heat). It differs from the men…

Olfactory bulb — structure, function and significance Olfactory bulb The olfactory bulb is a forebrain structure that receives input from smell receptors, transforms odor signals, and relays them to cortical and limbic areas. It supports odor discri…

Olfactory system: structure, function, and role in vertebrate behavior Olfactory system Overview of the olfactory system, its anatomy and physiology, accessory pathways, evolutionary differences, clinical relevance, and key discoveries in chemosensory biology.

Oligotroph: organisms adapted to nutrient-poor environments Oligotroph Oligotrophs are organisms adapted to survive and reproduce where available nutrients are scarce; they show slow growth, efficient uptake strategies, and play key roles in low-produ…

Omnivore — animals with mixed diets Omnivore An omnivore is an organism that consumes both plant and animal matter. This article covers defining features, adaptations, life-stage variation, ecological roles, examples, and dis…