Biology

Replacement child A replacement child is one conceived or raised by bereaved parents to fill the place of an earlier deceased child; the term covers cultural practices, psychological effects, histor…

Replication: principles across science, biology, and computing Replication Overview of replication as copying or repeating in science, biology (DNA and self-replication), statistics, computing (data and system replication), and metallography, with uses, h…

Reproduction (biology): sexual and asexual processes and their significance Reproduction Overview of biological reproduction: sexual and asexual mechanisms, major forms, examples across life, evolutionary importance, and key distinctions.

Reproductive isolation: mechanisms and its role in speciation Reproductive isolation Reproductive isolation are biological barriers that reduce or prevent gene flow between populations. This article reviews types of isolating mechanisms, their evolution, measuremen…

Reptile — overview, characteristics, evolution, and major groups Reptile Reptiles are a diverse group of largely terrestrial, scaly, egg-laying vertebrates. This article explains their defining traits, evolutionary history, major living and extinct grou…

Reptiliomorpha — early tetrapods on the amniote stem Reptiliomorpha Reptiliomorpha are tetrapods more closely related to amniotes than to living amphibians. The group is variably defined and includes several terrestrial, egg‑laying lineages importa…

Cellular respiration: how organisms convert nutrients into usable energy Respiration Cellular respiration is the set of metabolic processes that convert organic molecules such as glucose into ATP. It includes aerobic and anaerobic pathways, occurs in mitochondria a…

Respiratory arrest: causes, signs, emergency care, and prognosis Respiratory arrest Respiratory arrest is the sudden cessation of breathing. This article outlines its causes, how it differs from respiratory failure and cardiac arrest, signs to watch for, emergency…

Respiratory droplet Small liquid particles expelled from the respiratory tract during breathing, speaking, coughing or sneezing; important for understanding infectious disease transmission and control…

Respiratory failure — causes, types, diagnosis and management Respiratory failure Respiratory failure is a life‑threatening condition in which the lungs cannot maintain adequate oxygenation or carbon dioxide removal. Overview, types, common causes, diagnosis, an…

Respiratory system The respiratory system moves oxygen into the body and removes carbon dioxide. This article explains its parts, how breathing and gas exchange work, links to cellular respiration, c…

Respiratory tract The respiratory tract comprises the airways and tissues that move air to the lungs and enable gas exchange, protecting and conditioning inhaled air and supporting vocalization and…

Resting potential: cellular membrane voltage at steady state Resting potential The resting potential is the steady-state electrical voltage across a cell's membrane, set by ion gradients, membrane permeabilities, and transport proteins. It underlies excitabil…

Restriction enzyme (restriction endonuclease) Restriction enzyme Enzymes that recognize and cut DNA at specific sequences. Found in bacteria and archaea, they form restriction–modification systems and are essential tools in molecular biology and…

Rete testis: anatomy, function and clinical significance Rete testis The rete testis is an intratesticular network of channels that conveys sperm from seminiferous tubules to efferent ductules. This article explains its structure, role, development…

Retina: structure, function, and clinical importance Retina The retina is the light-sensitive neural layer lining the back of the eye. This article explains its anatomy, photoreceptors, signal pathways, clinical conditions, imaging, and var…

Retrovirus A retrovirus is an RNA virus that replicates through a DNA intermediate via reverse transcription, integrating into host genomes; notable for high mutation rates and clinical impor…

Reverse transcriptase An enzyme that synthesizes DNA from an RNA template; central to retroviruses, telomerase biology and routine molecular methods such as cDNA synthesis and RT-PCR.

Rhinovirus: biology, symptoms, transmission and prevention Rhinovirus Rhinoviruses are the main cause of the common cold, infecting the upper respiratory tract. This article summarizes their biology, symptoms, spread, prevention, and clinical signifi…

Rhipidistia — lobe-finned fishes ancestral to tetrapods Rhipidistia Rhipidistia are a clade of lobe-finned fishes (Sarcopterygii) that includes lungfish and the tetrapodomorph lineage that gave rise to tetrapods, central to the fish-to-land transit…

Rhizaria: diverse amoeboid eukaryotes including foraminifera and radiolaria Rhizaria Rhizaria are a major eukaryotic supergroup of mostly single-celled organisms—many with filamentous pseudopods and mineral shells—important in marine ecosystems, paleontology and so…

Rhizobia: nitrogen‑fixing bacteria that form symbiotic nodules on legumes Rhizobia Rhizobia are soil bacteria that form symbiotic root nodules on legumes to fix atmospheric nitrogen into plant‑available forms; key organisms in ecology, agriculture and sustainable…

Rib (anatomy): structure, function, development and cultural notes Rib Ribs are paired curved bones forming the thoracic cage. This article explains rib anatomy, classes, development, functions, clinical relevance and the origin of the Adam-and-Eve ri…

Rib cage (thoracic cage) — structure, function, and clinical notes Rib cage The rib cage (thoracic cage) is the bony and cartilaginous framework that protects thoracic organs, supports respiration and the shoulder girdle, and varies across individuals and…

Riboflavin (Vitamin B2): function, sources, stability and uses Riboflavin Riboflavin (vitamin B2) is a water‑soluble vitamin essential for energy metabolism, forming cofactors FMN/FAD. This article covers its roles, food sources, stability, production, d…

Ribonuclease (RNase) — enzymes that degrade RNA Ribonuclease Ribonucleases (RNases) are diverse enzymes that cleave RNA molecules. They participate in RNA processing, turnover, defense, and are essential across all domains of life and in lab…

Ribose: structure, biological roles, and uses of the five‑carbon sugar Ribose Ribose is a five‑carbon monosaccharide central to RNA, nucleotides, and cellular energy metabolism. D‑ribose is used as a dietary supplement; biological, structural, and clinical r…

Ribosome: the cellular machine that builds proteins Ribosome Ribosomes are ribonucleoprotein complexes that translate mRNA into proteins. This article explains their structure, assembly, role in translation, variation across life, and signif…

Ribozyme: catalytic RNA molecules and their biological and experimental roles Ribozyme Ribozymes are RNA molecules that catalyze chemical reactions. This article covers their discovery, structures and mechanisms, biological functions, laboratory evolution, and biotec…

Ricin Ricin is a potent plant-derived protein toxin from castor seeds that blocks protein synthesis. This article explains its properties, mechanisms, history, routes of exposure, notabl…

Rickettsia: obligate intracellular bacteria and their medical significance Rickettsia Rickettsia is a genus of small, obligate intracellular bacteria transmitted by arthropods. They cause several human febrile illnesses and require living cells for growth; treatment…

RNA (ribonucleic acid): Structure, Types, and Biological Roles RNA Overview of RNA: chemical structure, main classes (mRNA, tRNA, rRNA, regulatory RNAs), functions in cells and viruses, biochemical differences from DNA, and key uses in medicine an…

RNA interference: cellular gene-silencing by small RNAs RNA interference RNA interference (RNAi) is a conserved cellular mechanism that uses small RNAs to regulate gene expression, defend against foreign nucleic acids, and enable research and therapeuti…

RNA polymerase: function, types, mechanism, and biological importance RNA polymerase RNA polymerase (RNAP) is the DNA-dependent enzyme that synthesizes RNA. This article explains its structure, types (bacterial, archaeal, eukaryotic), transcription cycle, regulatio…

RNA splicing: processing of pre-mRNA into mature messenger RNA RNA splicing RNA splicing is the cellular process that removes introns from pre-mRNA and joins exons to produce mature mRNA, enabling correct protein synthesis and generating transcript diversi…

Messenger RNA (mRNA) vaccines: principles, development, and uses RNA vaccine mRNA vaccines deliver synthetic messenger RNA to cells to produce viral antigens and trigger immunity. This article outlines how they work, composition, history, applications, and…

RNA virus A virus whose genome is ribonucleic acid (RNA). RNA viruses include positive‑sense, negative‑sense, double‑stranded and retroviruses and cause many human, animal and plant diseases…

Robert Brown (1773–1858): Scottish botanist and pioneer of plant microscopy Robert Brown Scottish botanist Robert Brown used the microscope to describe the cell nucleus and Brownian motion, advanced plant taxonomy and palynology, and made foundational collections durin…

Rod cell — the retina's low‑light photoreceptor Rod cell Rod cells are light-sensitive photoreceptors in the retina that enable vision in dim light and peripheral detection. They trade color and fine detail for high sensitivity and work…

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) RuBisCO RuBisCO is the central enzyme of photosynthetic carbon fixation. This article explains its function, structure, ecological importance, evolution, and efforts to improve its efficie…

Salinibacter ruber An extremely halophilic, red-pigmented bacterium found in salt-saturated waters; notable for its adaptations to high salinity, ecological role in salterns, and biotechnological pot…

Saliva: composition, production, functions, and clinical importance Saliva Saliva is the watery fluid produced in the mouth that begins digestion, protects oral tissues, aids swallowing and speech, and serves as a diagnostic medium for health and disease.

Salivary glands Salivary gland An overview of salivary glands: anatomy, function, composition, clinical importance, and distinctions between major and minor glands.

Salmonella: biology, diseases, transmission and prevention Salmonella Comprehensive overview of Salmonella bacteria: what they are, how they cause disease, common sources, diagnosis, treatment, prevention and public‑health significance.

Salmonella enterica: biology, disease, and public health Salmonella enterica Overview of Salmonella enterica, a Gram-negative bacterium responsible for typhoid fever and non-typhoidal salmonellosis, including characteristics, transmission, clinical features…

Saprophyte (saprotroph): organisms that decompose dead organic matter Saprophyte Saprophytes (saprotrophs) are organisms—mainly fungi and bacteria—that obtain energy from dead and decaying organic matter, driving decomposition, nutrient recycling and many indus…

Sarcoidosis Sarcoidosis is an inflammatory disease marked by noncaseating granulomas that most often affects the lungs but can involve many organs; course and treatment vary widely.

Satellite DNA: structure, roles, and biological significance Satellite DNA Satellite DNA comprises long arrays of tandemly repeated non-coding sequences found mainly in centromeric and pericentromeric heterochromatin; it contributes to chromosome structur…