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- DNA is a nucleic acid whose sequence of bases carries genetic information. In the usual double helix, sugar-phosphate backbones support bases paired A with T and C with G. DNA sequence can specify protein or functional RNA products.
- Gene expression is the use of a gene's information to produce a functional product. Transcription produces RNA; for protein-coding genes, messenger RNA is translated into an amino-acid sequence. Some genes produce functional RNA that is not translated into protein.
Cellular expression uses genetic sequence information to produce RNA or protein.
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- In semiconservative replication of double-stranded DNA, each parental strand serves as a template for a new complementary strand. Each resulting double helix contains one parental strand and one newly synthesized strand.
- DNA is a nucleic acid whose sequence of bases carries genetic information. In the usual double helix, sugar-phosphate backbones support bases paired A with T and C with G. DNA sequence can specify protein or functional RNA products.
Complementary DNA strands serve as templates in semiconservative copying.
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- In semiconservative replication of double-stranded DNA, each parental strand serves as a template for a new complementary strand. Each resulting double helix contains one parental strand and one newly synthesized strand.
- In a typical mitotic eukaryotic cell cycle, nuclear DNA is replicated during S phase before mitosis separates sister chromatids into daughter nuclei. Cytokinesis usually divides the cytoplasm into daughter cells. Mitosis and cytokinesis are distinct processes.
Nuclear DNA replication precedes mitosis in the typical mitotic eukaryotic cell cycle.
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- In typical diploid meiosis, one round of DNA replication is followed by two nuclear divisions that reduce two chromosome sets to one. Homologous chromosomes separate in meiosis I, and sister chromatids separate in meiosis II. Crossing over and independent assortment can reshuffle inherited variants.
- In ordinary Mendelian segregation, the two alleles at a locus in a diploid individual separate during gamete formation, so each gamete receives one of them. Fertilization combines parental alleles. This discrete inheritance differs from permanent blending of parental hereditary factors.
Meiotic chromosome segregation provides the cellular context for ordinary Mendelian segregation.
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- In ordinary Mendelian segregation, the two alleles at a locus in a diploid individual separate during gamete formation, so each gamete receives one of them. Fertilization combines parental alleles. This discrete inheritance differs from permanent blending of parental hereditary factors.
- Biological evolution is descent with inherited modification. At the population level, a change in allele frequencies across generations is one way to describe evolutionary change; an individual's growth or temporary acclimation is not, by itself, such a change.
Population genetics connects inherited alleles with changes in their frequencies.
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- Natural selection occurs when heritable differences affect organisms' contributions to later generations. Variants associated with greater reproductive contribution in the relevant environment can become more frequent, producing adaptation under suitable conditions.
- Biological evolution is descent with inherited modification. At the population level, a change in allele frequencies across generations is one way to describe evolutionary change; an individual's growth or temporary acclimation is not, by itself, such a change.
Selection is one mechanism that can change inherited population composition.
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- Genetic drift is random change in allele frequencies due to chance in which genetic variants are transmitted to later generations. It can cause a variant to be lost or fixed without that outcome being caused by a reproductive advantage of the variant.
- Biological evolution is descent with inherited modification. At the population level, a change in allele frequencies across generations is one way to describe evolutionary change; an individual's growth or temporary acclimation is not, by itself, such a change.
Chance transmission is an evolutionary mechanism, not an adaptive advantage.
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- Understanding Evolution: An introduction to evolution ↗
- Understanding Evolution: The history of life — looking at the patterns ↗
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- Different species can descend from a shared ancestral lineage. Evolutionary trees represent these branching relationships and the inheritance of features from common ancestors, rather than a ladder ranking present-day species.
- Biological evolution is descent with inherited modification. At the population level, a change in allele frequencies across generations is one way to describe evolutionary change; an individual's growth or temporary acclimation is not, by itself, such a change.
Descent with modification connects lineage change with shared ancestors.
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- Metabolism includes chemical pathways that break down substances and build biological materials. Catabolic pathways break complex molecules into simpler ones, often releasing usable energy; anabolic pathways build complex molecules and require energy input. Enzymes catalyse many of these reactions.
- Carbon cycles among the atmosphere, organisms, soils, oceans and rocks. Biological processes such as photosynthesis, respiration and decomposition exchange carbon, while weathering, sediment formation and tectonic processes participate in slower geological transfers.
Photosynthesis, respiration and decomposition transfer carbon through biological processes.
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- Oxygenic photosynthesis uses captured light energy to support the conversion of carbon dioxide into organic compounds. Water supplies electrons and oxygen is released. This form occurs in photosynthetic plants, algae and cyanobacteria.
- Metabolism includes chemical pathways that break down substances and build biological materials. Catabolic pathways break complex molecules into simpler ones, often releasing usable energy; anabolic pathways build complex molecules and require energy input. Enzymes catalyse many of these reactions.
Light-energy capture and carbon fixation supply chemical intermediates and organic compounds in photosynthetic metabolism.
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- In aerobic glucose breakdown, glycolysis and subsequent oxidation pathways transfer energy and electrons through carriers. Carbon dioxide is released during pyruvate oxidation and the citric acid cycle. The respiratory electron-transport chain ultimately reduces oxygen to water, and a proton gradient supports ATP synthesis.
- Metabolism includes chemical pathways that break down substances and build biological materials. Catabolic pathways break complex molecules into simpler ones, often releasing usable energy; anabolic pathways build complex molecules and require energy input. Enzymes catalyse many of these reactions.
Aerobic nutrient breakdown transfers energy through electron transport to ATP production.
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- Cell membranes are selectively permeable. Passive transport allows net movement down the relevant gradient, through the membrane or transport proteins. Active transport couples movement against a gradient to an energy source, directly or through another gradient. Osmosis is passive net water movement across a selectively permeable membrane.
- Metabolism includes chemical pathways that break down substances and build biological materials. Catabolic pathways break complex molecules into simpler ones, often releasing usable energy; anabolic pathways build complex molecules and require energy input. Enzymes catalyse many of these reactions.
Cell metabolism can supply energy for active membrane transport.
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- Carbon cycles among the atmosphere, organisms, soils, oceans and rocks. Biological processes such as photosynthesis, respiration and decomposition exchange carbon, while weathering, sediment formation and tectonic processes participate in slower geological transfers.
- Greenhouse gases absorb and emit thermal infrared radiation. This affects outgoing heat loss and allows Earth's surface and lower atmosphere to be warmer than they would be without those gases under comparable solar input.
Atmospheric carbon dioxide links carbon reservoirs with the greenhouse mechanism; not all carbon is atmospheric gas.
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- Protein primary structure is amino-acid sequence; secondary structure describes recurring local conformations; tertiary structure describes a polypeptide's three-dimensional organization; quaternary structure describes assembly of multiple polypeptides. Molecular interactions and environment influence folding and biological function.
Proteins are amino-acid polymers with several levels of structural description.
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- For the simple mechanism E + S ⇌ ES → E + P, a quasi-steady-state approximation gives a saturating initial-rate relation. The Michaelis constant is the substrate concentration at half the model's maximum rate; it includes binding and turnover rate constants.
The rate model is derived from a specific enzyme-binding and turnover mechanism.
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- Cells couple ATP hydrolysis to otherwise energetically unfavourable processes through shared reaction steps or conformational changes. The Gibbs-energy change belongs to the overall reaction under cellular conditions; breaking an ATP bond alone is not the source of released free energy.
The source's ATP-coupled ion pump is a membrane transport example.
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- A mutation is a change in DNA sequence. It can arise during replication or from other processes that alter DNA. Whether a mutation enters subsequent generations depends on the affected cell lineage and the organism's route of reproduction.
Replication can introduce DNA-sequence changes; it is not the sole mutation source.
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- Gene flow transfers genetic variants among populations through reproductive contribution of migrants or transferred genetic material, such as pollen. It can alter allele frequencies and connect populations that would otherwise evolve separately.
Genetic contributions between populations can change variant frequencies.
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- Incomplete dominance produces an intermediate heterozygote phenotype, whereas codominance expresses distinguishable contributions from both alleles. A population can contain more than two alleles at a locus. Such patterns extend simple dominant/recessive models without necessarily changing allele segregation.
Phenotype dominance can differ while Mendelian segregation assumptions still hold.
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- Regulatory proteins can activate or repress transcription through interactions with DNA and transcription machinery. Bacterial operons provide examples of coordinated regulation of several genes. Gene-expression control also occurs after transcription, so transcript abundance alone does not determine active protein abundance.
Transcription is one of several regulated expression stages.
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- Chemical modifications of DNA and associated proteins can affect how cells use genetic information without changing the underlying DNA sequence. Some regulatory states persist through cell division; inheritance across organismal generations is a separate, context-dependent question.
DNA-associated regulatory marks can affect expression without a sequence change.
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- Speciation can involve divergence of populations and development of reproductive barriers. Prezygotic barriers reduce mating or fertilization, while postzygotic barriers reduce hybrid viability or fertility. Geographic separation is one route; isolation can also evolve without complete geographic separation.
Reproductive barriers are interpreted within a stated species concept.
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- Phylogenetic inference compares morphological or molecular characters to propose evolutionary relationships. Shared derived characters help identify groups. Parsimony is one inference strategy that favours fewer character changes under its assumptions; additional data can change the inferred tree.
Character ancestry must be assessed when proposing a phylogeny.
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- Xylem transports water and dissolved minerals; transpiration and cohesion–tension help drive upward water movement. Phloem transports photosynthates from sources to sinks through pressure-driven flow. A plant part can switch between source and sink as development or season changes.
Photosynthates move from sources to sinks in vascular plants.
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- In many neurons, voltage-dependent ion conductances generate action potentials that propagate along excitable membranes. At chemical synapses, released neurotransmitter acts on receptors of a receiving cell. Signal integration and channel dynamics determine whether further electrical activity occurs.
Ion conductances across membranes generate electrical activity.
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- Animal endocrine signals affect target cells through receptors. Some hormones act through cell-surface receptors and intracellular signalling pathways; others act through intracellular receptors that influence gene expression. A hormone's presence does not imply that every cell responds in the same way.
Some intracellular hormone receptors influence transcription.
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- In mammalian kidneys, glomerular filtration forms a filtrate that renal tubules modify through reabsorption and secretion. Regulated transport of water and solutes contributes to extracellular-fluid composition and waste excretion. Other animal groups use different osmoregulatory structures.
Tubular transport regulates water and solute reabsorption and secretion.
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- Gross primary production is the rate at which primary producers incorporate energy or carbon. Net primary production subtracts their respiration from that gross production. Transfers between trophic levels depend on consumption, assimilation and production; no one efficiency applies to all ecosystems.
Photosynthetic primary production introduces energy into food webs.
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- A simple two-sample mark–recapture estimator uses the fraction of marked individuals in a later sample to estimate total population size. Its interpretation depends on mixing, capture probabilities and persistence of marks, rather than only arithmetic on capture counts.
Population size is estimated using specified ecological sampling assumptions.
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- Asexual reproduction can produce offspring without gamete fusion; sexual eukaryotic cycles combine meiosis and fertilization. Animals commonly have diploid-dominant cycles, whereas plants alternate multicellular haploid and diploid generations. Other lineages have different distributions of these stages.
The placement of meiosis differs among sexual life cycles.
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- Multicellular development coordinates cell division, differentiation and spatial organization. Cell differentiation involves changes in gene-expression programmes and specialized function. In many animal embryos, cleavage and gastrulation organize early development; stem-cell populations can contribute specialized descendants.
Differential expression helps explain specialized cell function.
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- Geological history is organized into nested eons, eras, periods, epochs and ages. Rock successions and fossil correlation established relative ordering; numerical dating supplies additional age constraints. The International Commission on Stratigraphy maintains a revised global framework whose boundary definitions and numerical estimates must be distinguished.
Fossil succession supports relative correlation.
- Biological oxygen production and geological sinksPhotosynthetic electron flow and photophosphorylationRelated to
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- Oxygenic photosynthesis supplied oxygen while chemical reactions with reduced material consumed it. Atmospheric accumulation depended on production, consumption and burial rather than oxygen production alone. Geological redox evidence constrains changing conditions through Earth's history; oxygenation was not one instantaneous switch to the modern atmosphere.
Water oxidation in oxygenic photosynthesis supplies molecular oxygen.
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- Oxygenic photosynthesis supplied oxygen while chemical reactions with reduced material consumed it. Atmospheric accumulation depended on production, consumption and burial rather than oxygen production alone. Geological redox evidence constrains changing conditions through Earth's history; oxygenation was not one instantaneous switch to the modern atmosphere.
Production and burial couple carbon and oxygen budgets.
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- Oxygenic photosynthesis supplied oxygen while chemical reactions with reduced material consumed it. Atmospheric accumulation depended on production, consumption and burial rather than oxygen production alone. Geological redox evidence constrains changing conditions through Earth's history; oxygenation was not one instantaneous switch to the modern atmosphere.
Oxidation alters minerals and records redox conditions.
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- An ion's equilibrium potential balances its chemical and electrical driving terms. With multiple permeant ions, membrane voltage need not equal any one equilibrium potential. A conductance model separates ionic currents from capacitive charging.
Electrochemical equilibrium is applied to one permeant ion.
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- An ion's equilibrium potential balances its chemical and electrical driving terms. With multiple permeant ions, membrane voltage need not equal any one equilibrium potential. A conductance model separates ionic currents from capacitive charging.
Changing conductance changes membrane dynamics.
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- Eukaryotic protein abundance depends on synthesis and degradation. Ubiquitin activation, conjugation and substrate-selective ligases can target suitable proteins for proteasomal degradation. Lysosomal pathways process other material, including cargo delivered through autophagy. Regulated turnover helps change cell state and remove damaged components.
Regulated cyclin turnover contributes to cycle transitions.
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- Eukaryotic protein abundance depends on synthesis and degradation. Ubiquitin activation, conjugation and substrate-selective ligases can target suitable proteins for proteasomal degradation. Lysosomal pathways process other material, including cargo delivered through autophagy. Regulated turnover helps change cell state and remove damaged components.
Lysosomal delivery uses compartment trafficking.
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- Eukaryotic protein abundance depends on synthesis and degradation. Ubiquitin activation, conjugation and substrate-selective ligases can target suitable proteins for proteasomal degradation. Lysosomal pathways process other material, including cargo delivered through autophagy. Regulated turnover helps change cell state and remove damaged components.
Protein synthesis and turnover jointly determine abundance.
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- In representative animal apoptosis pathways, activated initiator caspases promote an execution cascade that cleaves cellular targets. Death-receptor signalling can recruit activation complexes; mitochondrial cytochrome-c release can promote adaptor-dependent activation. Bcl-2-family proteins regulate mitochondrial involvement with pro-survival and pro-death members.
This supplies specified molecular pathways behind the controlled-removal overview.
- Caspases and mitochondrial/death-receptor apoptosisReceptors, second messengers and signal transductionRelated to
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- In representative animal apoptosis pathways, activated initiator caspases promote an execution cascade that cleaves cellular targets. Death-receptor signalling can recruit activation complexes; mitochondrial cytochrome-c release can promote adaptor-dependent activation. Bcl-2-family proteins regulate mitochondrial involvement with pro-survival and pro-death members.
Extrinsic signals can initiate intracellular activation.
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- In representative animal apoptosis pathways, activated initiator caspases promote an execution cascade that cleaves cellular targets. Death-receptor signalling can recruit activation complexes; mitochondrial cytochrome-c release can promote adaptor-dependent activation. Bcl-2-family proteins regulate mitochondrial involvement with pro-survival and pro-death members.
Cytochrome c has a separate respiratory-chain role.
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- Base-excision repair begins when a glycosylase removes a recognized damaged base, leaving a site that is processed and restored by DNA synthesis and ligation. Nucleotide-excision repair instead removes a DNA segment containing a recognized lesion. Both use the complementary strand as an informational template when that template remains available.
This separates two excision mechanisms in the repair overview.
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- Base-excision repair begins when a glycosylase removes a recognized damaged base, leaving a site that is processed and restored by DNA synthesis and ligation. Nucleotide-excision repair instead removes a DNA segment containing a recognized lesion. Both use the complementary strand as an informational template when that template remains available.
Complementarity provides an intact repair template.
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- End joining can restore continuity of a double-strand-broken DNA molecule without using a long homologous donor template. Processing and ligation can preserve or change sequence at the junction. This differs from homology-directed repair, whose strand invasion uses another DNA molecule as a template.
Template-directed repair has a distinct mechanism.
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- End joining can restore continuity of a double-strand-broken DNA molecule without using a long homologous donor template. Processing and ligation can preserve or change sequence at the junction. This differs from homology-directed repair, whose strand invasion uses another DNA molecule as a template.
A changed junction can be a mutational outcome.
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- Sensory receptors convert appropriate physical or chemical stimuli into changes in cellular electrical signalling. Receptor potentials can vary with stimulus strength and influence action-potential output or transmitter release. Mechanically gated channels provide one example; different sensory modalities use different transduction mechanisms.
Graded input and regenerative spiking are distinct signalling stages.
- Sensory transduction and graded receptor potentialsReceptors, second messengers and signal transductionRelated to
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- Sensory receptors convert appropriate physical or chemical stimuli into changes in cellular electrical signalling. Receptor potentials can vary with stimulus strength and influence action-potential output or transmitter release. Mechanically gated channels provide one example; different sensory modalities use different transduction mechanisms.
Chemical and physical stimuli can alter channel behaviour.
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- Experimental Design Assistant, Experimental Unit ↗
- Key Elements of a Well-Designed Experiment, randomization and blinding ↗
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- The experimental unit is the entity independently assigned to a treatment in a specified design. Multiple observations from one treated unit need an appropriate within-unit analysis and do not automatically increase the number of independent treatment replicates. Defining units, randomization and masking helps relate biological variability and measurement variability to the intended inference.
Controls and independent treatment units serve different purposes.