Biology terms in Unity and diversity

The origin of living cells from non-living matter through a series of chemical stages, thought to have happened on the early Earth.

How close a measurement or the mean of several measurements is to the true value of the quantity being measured.

A change in an organism's body or behaviour that develops during its lifetime through the environment or use, and is not inherited because it does not alter the genes in its gametes.

The rapid diversification of one ancestral species into many new species, each adapted to a different niche, often after colonising a new habitat or after other species became extinct.

A purine base (A) found in DNA and RNA, which pairs with thymine in DNA and with uracil in RNA by two hydrogen bonds.

The attraction between water molecules and molecules of a different substance, caused by hydrogen bonds or other attractions to polar or charged surfaces.

One of the different forms of a gene, differing from other forms of the same gene by small differences in base sequence.

The formation of new species from populations that are separated geographically, so that gene flow between them is prevented by a physical barrier.

Polyploidy in which the chromosome sets come from two different species after hybridisation; doubling of the chromosome number makes the otherwise sterile hybrid fertile.

A structure that performs a similar function in different species but has a different evolutionary origin and basic structure, because it evolved independently.

Caused or produced by human activity, such as anthropogenic extinction or anthropogenic climate change.

Gradual change in the surface antigens of a virus, such as influenza haemagglutinin and neuraminidase, caused by the accumulation of point mutations in their genes.

A sudden major change in the surface antigens of influenza virus, caused by reassortment of RNA segments when two different strains infect the same cell.

Running side by side in opposite directions; the two strands of DNA are antiparallel, so the 5′ end of one lies opposite the 3′ end of the other.

Treatment of HIV infection with a combination of drugs, usually three, that act on different viral targets, so that resistance to all of them at once is very unlikely to evolve.

The scientific study of the origin, evolution and possible distribution of life in the universe, including the search for liquid water beyond Earth.

Polyploidy in which all the sets of chromosomes come from a single species, usually after unreduced gametes form or the chromosome number doubles.

Any chromosome that is not a sex chromosome; humans have 22 pairs of autosomes.

The normal, low rate at which species become extinct between mass extinctions, estimated from the fossil record.

The domain of prokaryotes with peptidoglycan cell walls and ester-linked membrane lipids, including organisms such as Escherichia coli and cyanobacteria.

A virus that infects bacteria, such as bacteriophage lambda, which infects Escherichia coli; often shortened to phage.

Two complementary bases on opposite strands joined by hydrogen bonds (A–T or C–G); DNA length is measured in base pairs (bp).

Reproductive isolation caused by differences in courtship behaviour, songs, signals or mate preferences, so that members of two populations do not mate with each other.

The division of a prokaryote cell into two genetically identical daughter cells after its circular DNA has been copied; mitochondria and chloroplasts divide in a similar way.

The international system of naming each species with two words: the genus name followed by the specific name, for example Homo sapiens.

The variety of life in all its forms, levels and combinations, including ecosystem diversity, species diversity and genetic diversity.

The current rapid loss of biodiversity worldwide at the genetic, species and ecosystem levels, at rates far above the background rate, caused mainly by human activity.

The use of computers to store, retrieve and analyse large amounts of biological data, such as base sequences of genes and amino acid sequences of proteins.

The definition of a species as a group of organisms that can interbreed and produce fertile offspring, and that are reproductively isolated from other such groups.

The liquid part of blood, about 90% water, which carries dissolved substances such as glucose, amino acids, ions, urea, hormones and carbon dioxide.

The upward force on an object in a fluid, equal to the weight of fluid it displaces; water gives far more buoyancy than air because it is much denser.

The movement of water along narrow spaces, even against gravity, caused by adhesion of water to the walls together with cohesion between water molecules.

The protein coat of a virus, made of many repeating protein subunits, which encloses and protects the viral genome.

Breeding animals of a threatened species in zoos or other controlled conditions, choosing mates to maintain genetic diversity, often to reintroduce them to the wild.

A type of stony meteorite from water-rich asteroids, containing water in clay minerals and carbon compounds; its D/H ratio is close to that of Earth's oceans.

The theory that all living organisms are composed of one or more cells, that the cell is the smallest unit of life, and that cells arise only from pre-existing cells.

A strong, freely permeable layer outside the plasma membrane that gives shape and support and stops the cell bursting; made of peptidoglycan in bacteria, cellulose in plants and chitin in fungi.

A short cylinder of microtubules found in pairs near the nucleus of animal cells, which organises microtubules and forms the base of cilia and flagella.

The region of a chromosome where the two sister chromatids are held together and where spindle microtubules attach during cell division.

In double-stranded DNA the amount of adenine equals thymine and guanine equals cytosine, so purines equal pyrimidines, while the proportions of A + T and G + C vary between species.

The formation of increasingly complex carbon compounds from simple inorganic molecules without living organisms, using energy such as lightning, UV or heat.

A tough nitrogen-containing polysaccharide that forms the cell walls of fungi and the exoskeletons of arthropods.

A double-membrane organelle of plants and algae in which photosynthesis takes place; it contains thylakoids stacked into grana in a fluid stroma, with 70S ribosomes and circular DNA.

A DNA molecule carrying genes; in eukaryotes chromosomes are linear and bound to histone proteins, while the main prokaryote chromosome is a naked circular loop.

A short hair-like projection from the surface of a eukaryote cell, with a core of microtubules, that beats to move the cell or to move fluid over the cell surface; cilia are usually many and short.

The collection of scientific data by members of the public, usually volunteers following a standard method, in projects designed and analysed by scientists.

A group of organisms that consists of a common ancestor and all of its descendants, which share characteristics inherited from that ancestor.

A method of classification that places organisms into clades based on shared characteristics inherited from a common ancestor, so that groups reflect evolutionary relationships.

A tree diagram showing the most probable sequence of divergence in a group of organisms, based on shared derived characteristics; each branch point represents a common ancestor.

The attraction between molecules of the same substance; in water it is caused by hydrogen bonds between water molecules.

Descent of different species from a shared ancestor; the near-universal genetic code is evidence that all living organisms share a common ancestor.

A phloem cell with a nucleus and many mitochondria that lies beside a sieve tube element, is connected to it by plasmodesmata and keeps it alive.

The specific pairing of bases by hydrogen bonds between nucleic acid strands: adenine with thymine (or uracil in RNA) and cytosine with guanine.

Variation in which a trait can take any value within a range, with many intermediates, such as human height; usually controlled by many genes and affected by the environment.

A vacuole in freshwater protists such as Paramecium that collects excess water entering by osmosis and expels it from the cell.

The independent evolution of similar characteristics in unrelated lineages exposed to similar selection pressures, producing analogous structures.

A chemical bond in which two atoms share a pair of electrons; covalent bonds are strong and hold the atoms of a molecule together.

One of the folds of the inner mitochondrial membrane that project into the matrix and increase the surface area for the reactions that produce ATP.

Electron microscopy of samples frozen so rapidly that water forms glass-like ice, with many images combined by computer to give three-dimensional structures of proteins at near-atomic resolution.

A photosynthetic bacterium that releases oxygen; chloroplasts are thought to have evolved from a cyanobacterium taken in by endosymbiosis.

Everything inside the plasma membrane except the nucleus: a watery fluid (cytosol) containing dissolved substances, enzymes, ribosomes and, in eukaryotes, organelles.

A pyrimidine base (C) found in DNA and RNA, which pairs with guanine by three hydrogen bonds.

A network of protein filaments (microtubules and microfilaments) in the cytoplasm of eukaryote cells that maintains shape, holds organelles in place and moves structures.

Reasoning from a general statement or theory to a specific prediction, such as predicting that a newly discovered organism will be made of cells.

The permanent removal of forest and its conversion to another land use, such as agriculture, plantations or urban development.

The pentose sugar in DNA nucleotides, which has a hydrogen atom instead of an –OH group on carbon 2′.

The ratio of deuterium (heavy hydrogen) to ordinary hydrogen atoms in a sample of water, used to identify where water came from; abbreviated D/H.

An identification tool made of a series of steps, each offering a choice between two contrasting descriptions of an observable feature, leading to the name of an organism.

Natural selection acting in different directions on two populations because they experience different selection pressures, so that they diverge.

The difference between the two ends of a nucleic acid strand, the 5′ end with a phosphate and the 3′ end with a free –OH; strands are built 5′ to 3′.

Variation in which individuals fall into a small number of distinct categories with no intermediates, such as the four ABO blood groups; also called discontinuous variation.

The process by which species descended from a common ancestor become increasingly different as they adapt to different environments, producing homologous structures with different functions.

Deoxyribonucleic acid: a double-stranded nucleic acid with deoxyribose and the bases A, C, G and T, which is the genetic material of all living organisms.

The identification of species using the base sequence of a short, standardised region of DNA, such as part of the COI gene in animals, compared with a reference database.

The highest rank in the classification of organisms; all life is divided into three domains, Bacteria, Archaea and Eukarya.

The shape of a DNA molecule: two antiparallel strands of nucleotides coiled around each other, with the backbones outside and paired bases inside.

The variety of habitats, communities and ecosystems within a region.

A species that is both Evolutionarily Distinct and Globally Endangered, and so is given priority for conservation by the EDGE of Existence programme.

A microscope that uses a beam of electrons focused by electromagnets to form an image; its very short wavelength gives a resolution below 1 nm, but specimens must be dead and in a vacuum.

The tendency of an atom to attract the shared electrons in a covalent bond towards itself; oxygen is much more electronegative than hydrogen.

The process by which a prokaryote cell taken into a larger cell survived and became a permanent part of it; mitochondria and chloroplasts are thought to have originated in this way.

DNA that has been shed by organisms into their surroundings, such as water, soil or air, and can be collected and analysed to identify the species present; abbreviated eDNA.

The domain containing all organisms whose cells have a nucleus and membrane-bound organelles: animals, plants, fungi and protists.

An organism whose cells have a nucleus enclosed in a double membrane and membrane-bound organelles; animals, plants, fungi and protists are eukaryotes.

Change in the heritable characteristics of a population over many generations.

A measure of how much unique evolutionary history a species represents, based on how few close relatives it has and how long ago its lineage separated from others.

The conservation of species outside their natural habitat, for example in zoos, botanic gardens or seed banks.

The removal from an organism of waste products of its metabolism, such as carbon dioxide, urea or excess water; the removal of undigested food (egestion) is not excretion.

The permanent loss of a species, when its last individual dies.

A transparent scale with arbitrary divisions placed in the eyepiece of a microscope; once calibrated with a stage micrometer it is used to measure specimens.

The rejection of a hypothesis or theory when evidence contradicts a prediction made from it; a scientific idea must be testable in a way that could show it to be false.

The circular area of a specimen that is visible through the eyepiece at one time; its diameter decreases as the magnification increases.

A long whip-like projection from a cell that moves it through liquid; eukaryote flagella have a microtubule core and are usually one or two per cell, while prokaryote flagella are simpler protein fibres.

A dye that absorbs light of one wavelength and emits light of a longer wavelength, so that the structures it binds to glow against a dark background in a fluorescence microscope.

The preserved remains or traces of an organism from the past, such as bones, shells, imprints or structures like stromatolites, usually found in sedimentary rock.

An electron microscopy technique in which a specimen is frozen rapidly and split so that the fracture runs through the centre of membranes, showing the proteins embedded in them.

The processes carried out by all living organisms: metabolism, nutrition, growth, response to stimuli, movement, excretion, homeostasis and reproduction.

A section of DNA with a specific base sequence that codes for a polypeptide or a functional RNA molecule and influences a characteristic.

The transfer of alleles from one population to another when individuals or their gametes move between the populations and interbreed.

The set of rules by which the sequence of base triplets (codons) in mRNA specifies the sequence of amino acids in a polypeptide; it is almost universal.

The variety of alleles and genes within a species, both within a population and between populations of that species.

The whole of the genetic information of an organism, consisting of the entire base sequence of its DNA (or RNA in some viruses).

A group of closely related species that share many features and a recent common ancestor; the first part of a binomial name.

Reproductive isolation of populations caused by a physical barrier, such as a mountain range, river or sea, that prevents individuals from meeting and interbreeding.

The range of distances from a star within which a planet's surface temperature allows water to exist as a liquid; also called the habitable zone.

A stack of flattened, curved membrane sacs (cisternae) that receives proteins from the rough ER in vesicles, modifies and sorts them, and sends them out in new vesicles.

A bacterium with a thick cell wall of peptidoglycan and no outer membrane, which stains purple with the Gram stain; Bacillus and Staphylococcus are examples.

A stack of thylakoids in a chloroplast; grana are joined to each other by unstacked thylakoid membranes called intergranal lamellae.

A purine base (G) found in DNA and RNA, which pairs with cytosine by three hydrogen bonds.

The clearing, draining, burning or fragmentation of a habitat so that it can no longer support the species that lived there.

Describes a characteristic that is passed from parents to offspring through the genes in their gametes.

The 1952 experiment that used T2 bacteriophages labelled with ³⁵S (protein) or ³²P (DNA) to show that DNA, not protein, enters bacteria during infection and is the genetic material.

A basic protein around which eukaryote DNA is wound to package it into chromosomes; prokaryote DNA is not associated with histones.

A pair of chromosomes, one from each parent, with the same length, centromere position and sequence of genes, though not necessarily the same alleles.

A structure that is similar in basic arrangement in different species because it was inherited from a common ancestor, even if it now has a different function.

The transfer of genetic material between organisms other than from parent to offspring, for example between different bacterial species by conjugation, transformation or transduction.

The offspring of a cross between two different species (or two genetically different varieties); hybrids between species are often sterile.

The inability of a hybrid between two species to produce viable gametes, usually because its chromosomes from the two parent species cannot pair in meiosis.

The attraction between a partially positive hydrogen atom in one polar molecule and a partially negative atom, such as oxygen or nitrogen, in another polar molecule or another part of the same large molecule.

Attracted to water and able to dissolve in it; ions and polar molecules are hydrophilic because their charges attract water molecules.

Not attracted to water and insoluble or poorly soluble in it; non-polar substances such as fats and oils are hydrophobic.

One of the thin, branching threads that make up the body of most fungi; aseptate hyphae have no cross walls, so many nuclei share one continuous cytoplasm.

A tentative, testable explanation of an observation, from which predictions can be made and checked by experiment or further observation.

A light microscopy technique in which antibodies carrying a fluorescent marker bind to one specific protein, showing where that protein is located in a cell or tissue.

The conservation of species in their natural habitat, for example by protecting and managing ecosystems in nature reserves and national parks.

Reasoning from many specific observations to a general statement or theory, such as concluding that all organisms are made of cells after observing cells in many different organisms.

An attraction between separate molecules, such as a hydrogen bond, which is much weaker than the covalent bonds holding atoms together within a molecule.

A species moved by humans, deliberately or accidentally, to an area outside its natural range.

An atom or group of atoms with an overall electrical charge because it has gained or lost electrons, such as Na⁺ (a cation) or Cl⁻ (an anion).

The International Union for Conservation of Nature's assessment of species' risk of extinction, placing each in a category from Least Concern to Extinct using standard criteria.

An image of the chromosomes of a cell, arranged in homologous pairs in order of decreasing length, used to show the karyotype.

The number and type of chromosomes present in the nucleus of a cell of an organism.

The hypothesis, proposed by Lamarck, that characteristics acquired by an organism during its lifetime through use or disuse are inherited by its offspring.

The most recent population of organisms from which all organisms living today are descended; it was already a cell with DNA, ribosomes and the genetic code.

One of the four scales at which biology is studied: molecules, cells, organisms and ecosystems; each level is built from the one before it.

A microscope that uses visible light focused by glass lenses to form a magnified image; its resolution is limited to about 0.2 µm, but it can be used to view living cells in colour.

The short stretch of DNA that connects one nucleosome to the next.

The cycle in which phage DNA is integrated into the host chromosome as a prophage and replicated with it at every cell division, without making new virions, until induction triggers the lytic cycle.

A small membrane-bound vesicle in animal cells containing hydrolytic (digestive) enzymes, which break down food, worn-out organelles and engulfed bacteria.

The cycle in which a virus enters a host cell, uses the cell's machinery to replicate its genome and make viral proteins, assembles new virions and releases them by lysis, killing the cell.

The number of times larger an image is than the real object; magnification = image size ÷ actual size, with both sizes in the same units.

An event in which a large proportion of the Earth's species, usually 75% or more, become extinct in a geologically short time.

A thin solid filament made of the protein actin, part of the cytoskeleton; microfilaments change cell shape and are involved in cell movement and division of the cytoplasm.

A photograph or digital image taken through a microscope; an electron micrograph is taken with an electron microscope and a light micrograph with a light microscope.

A hollow cylinder made of the protein tubulin, part of the cytoskeleton; microtubules form the spindle in cell division and the core of cilia and flagella.

A tiny finger-like fold of the plasma membrane on the surface of some animal cells, such as those lining the small intestine, that increases the surface area for absorption.

The 1953 experiment in which methane, ammonia, hydrogen and water vapour were exposed to electrical sparks in a closed apparatus, producing amino acids and showing that carbon compounds can form from inorganic molecules.

The fluid inside the inner membrane of a mitochondrion, containing enzymes of the Krebs cycle and link reaction, 70S ribosomes and naked circular DNA.

A double-membrane organelle in which aerobic cell respiration produces ATP; the inner membrane is folded into cristae around a fluid matrix, which holds 70S ribosomes and circular DNA.

Using the number of differences between base or amino acid sequences of two species to estimate the time since they diverged, assuming differences accumulate at a roughly constant rate.

The definition of a species as a group of organisms that share the same set of observable structural features (morphology).

The condition of an organism made of many cells that stay together, communicate and are specialised for different functions; it has evolved independently many times.

An area of land or water that is protected and managed for the conservation of its wildlife, habitats and natural features.

A nitrogen-containing ring molecule attached to carbon 1′ of the pentose in a nucleotide; the five bases are adenine, cytosine, guanine, thymine and uracil.

A branch point on a cladogram, representing the last common ancestor of the groups that branch from it and the point at which its lineage split.

The double membrane that surrounds the nucleus of a eukaryote cell, perforated by nuclear pores.

A protein-lined opening through both membranes of the nuclear envelope that controls movement of molecules such as mRNA and proteins between the nucleus and cytoplasm.

A polymer of nucleotides that stores or transmits genetic information; the two nucleic acids are DNA and RNA.

The region of a prokaryote cell that contains its main DNA molecule, a naked, circular loop not enclosed by a membrane.

A dense region inside the nucleus of a eukaryote cell where ribosomal RNA is made and ribosome subunits are assembled.

The unit of DNA packing in eukaryotes: about 147 base pairs of DNA wrapped about 1.65 turns around an octamer of eight histones, held in place by histone H1.

The monomer of nucleic acids, consisting of a pentose sugar, a phosphate group and a nitrogenous base.

The organelle of eukaryote cells that contains the chromosomes, enclosed by a double membrane with pores; it controls the cell through the genes that are transcribed.

An organism or agent that can reproduce only inside the living cells of a host, using the host cell's metabolism; all viruses are obligate intracellular parasites.

Harvesting a species by hunting, fishing, logging or collecting faster than its population can replace the individuals removed.

A fundamental change in the accepted framework of ideas in a field of science, when evidence accumulates that the existing framework cannot explain.

A group that contains a common ancestor and some, but not all, of its descendants, such as the traditional reptiles, which exclude birds.

The principle that, of all possible cladograms for a group of organisms, the one requiring the fewest evolutionary changes is the most likely to be correct.

A charge smaller than that of an electron or proton, written δ+ or δ−, found on atoms at the ends of a polar covalent bond because electrons are shared unequally.

A limb with one upper bone, two lower bones, a group of wrist or ankle bones and up to five digits, found in amphibians, reptiles, birds and mammals and inherited from a common ancestor.

A sugar with five carbon atoms; the pentose in DNA is deoxyribose and in RNA is ribose.

The polymer of sugars cross-linked by short chains of amino acids that forms the cell walls of bacteria; it is thick in Gram-positive bacteria.

Medical care in which the prevention, diagnosis or treatment of disease is tailored to an individual, using information such as their genome sequence.

The negatively charged group in a nucleotide, attached to carbon 5′ of the pentose, that links to the next sugar in the sugar–phosphate backbone.

The covalent bond, formed by a condensation reaction, linking the phosphate of one nucleotide to carbon 3′ of the pentose of the next in a nucleic acid strand.

A branching diagram of the evolutionary relationships of a group of organisms; when drawn to scale, branch lengths represent time since divergence or amount of genetic change.

The partially permeable membrane of phospholipids and proteins that surrounds the cytoplasm of every cell and controls what enters and leaves; also called the cell membrane.

A small, circular DNA molecule in many prokaryotes, separate from the main loop of DNA; plasmids carry a few genes, such as antibiotic resistance, and are not essential.

A covalent bond in which the electrons are shared unequally because one atom is more electronegative, giving one end a partial negative charge and the other a partial positive charge.

A molecule with an uneven distribution of electrical charge, so that one part carries a partial negative charge and another part a partial positive charge.

Describes a group whose members evolved from more than one ancestral source, so they do not share a single most recent common ancestor; viruses are thought to be polyphyletic.

The condition of having more than two complete sets of chromosomes in each cell, such as three sets (triploid) or four sets (tetraploid).

A mechanism acting after a hybrid zygote has formed, such as hybrid inviability, hybrid sterility or hybrid breakdown, so that alleles do not pass between species.

A mechanism that prevents mating or fertilisation between individuals of different species, so that no hybrid zygote is formed.

How close repeated measurements of the same quantity are to each other; precise measurements show little spread, though they may still be inaccurate.

The hypothesis that viruses arose from pieces of genetic material that escaped from cells and gained a protein capsid; also called the escape hypothesis.

An organism whose cells have no nucleus and no membrane-bound organelles; its DNA is a naked loop in the cytoplasm. Bacteria and archaea are prokaryotes.

Phage DNA that has been inserted into the chromosome of a host bacterium and is replicated with it, without producing new virions.

A hypothetical early stage in the origin of cells: a vesicle enclosing self-replicating molecules such as RNA, able to grow and divide.

A nitrogenous base with a double-ring structure; adenine and guanine are purines, and each pairs with a pyrimidine.

A nitrogenous base with a single-ring structure; cytosine, thymine and uracil are pyrimidines, and each pairs with a purine.

Observations described in words rather than measured, for example the colour, shape or texture of cells seen with a microscope.

Observations recorded as numbers with units, obtained by measuring or counting, for example the length of a cell in micrometres.

A radioactive form of an element, such as ³²P or ³⁵S, whose radiation can be detected so that molecules containing it can be traced.

A method of finding the age of rocks or fossils from the known, constant rate of decay of radioactive isotopes into their decay products.

A biconcave blood cell packed with haemoglobin that carries oxygen; in mammals the mature cell has no nucleus and no mitochondria.

An atmosphere with little or no free oxygen, rich in gases such as hydrogen, methane and ammonia that can donate electrons or hydrogen, which favours the formation of carbon compounds.

The hypothesis that viruses descend from small parasitic cells that lost most of their genes while living inside host cells; also called the reduction hypothesis.

The separation of two populations so that they no longer interbreed and no gene flow occurs between them.

The smallest distance between two points at which they can still be seen as separate points; it determines how much detail a microscope can show.

An RNA virus that uses the enzyme reverse transcriptase to make a DNA copy of its RNA genome, which is then inserted into a chromosome of the host cell; HIV is a retrovirus.

A viral enzyme that synthesises DNA using RNA as a template; it has no proofreading activity, so it makes frequent errors.

The pentose sugar in RNA nucleotides, which has an –OH group on carbon 2′.

A small structure of RNA and protein, not bounded by a membrane, that synthesises polypeptides; prokaryotes, mitochondria and chloroplasts have 70S ribosomes, while the eukaryote cytoplasm has larger 80S ribosomes.

An RNA molecule that acts as a catalyst, such as the RNA in the ribosome that catalyses the formation of peptide bonds.

Ribonucleic acid: a usually single-stranded nucleic acid with ribose and the bases A, C, G and U, involved in expressing genetic information.

The hypothesis that early life used RNA both to store genetic information and to catalyse reactions, before DNA and proteins took over these roles.

The base of a cladogram, representing the common ancestor of all the organisms shown on it.

A network of flattened membrane sacs (cisternae) with 80S ribosomes attached to the outer surface, which synthesise proteins that are then transported in vesicles, often for secretion.

A line drawn on a micrograph or drawing and labelled with the actual length it represents, so that sizes can be worked out whatever the image is enlarged to.

A facility that stores dried seeds at low temperature so that they remain viable for many years, conserving the genetic diversity of plant species and crop varieties.

Deliberate choice by humans of individuals with desired heritable characteristics as parents, so those characteristics become more common in a domesticated population over generations; also called artificial selection.

The spontaneous organisation of molecules into larger ordered structures without outside direction, such as fatty acids forming bilayers and vesicles in water.

The production of copies of a molecule using the molecule itself as a template, as RNA can do by complementary base pairing; needed for heredity and natural selection.

Arranging base sequences of a gene, or amino acid sequences of a protein, from different organisms in rows so that corresponding positions line up and differences can be identified.

A chromosome that determines the sex of an organism; in humans the X and Y chromosomes, with females XX and males XY.

A characteristic that evolved in the most recent common ancestor of a clade and is found in its descendants but not in more distantly related organisms.

A living phloem cell with no nucleus and very little cytoplasm, joined end to end with others through perforated sieve plates to form a sieve tube that transports sap.

An index of species diversity, $D=N(N-1)\mathrm{/}\sum n(n-1)$, where N is the total number of individuals and n the number of each species; higher values mean greater richness and evenness.

A position in the genome where the base differs between individuals of the same species, for example C in some individuals and T in others; abbreviated SNP.

A network of membrane tubules without ribosomes that synthesises lipids such as phospholipids and steroids and, in liver cells, detoxifies drugs.

A substance that is dissolved in a solvent to form a solution, for example glucose or sodium ions dissolved in water.

The process in which solvent molecules surround and separate the particles of a solute; in water, a shell of water molecules forms around each ion or polar molecule.

A liquid in which other substances (solutes) dissolve; water is the solvent in which most reactions of life take place.

The formation of new species by the splitting of an existing species into two or more populations that become reproductively isolated from each other.

A group of organisms with shared traits that can interbreed to produce fertile offspring; the basic unit of classification.

The variety of species in a community, depending on both the number of species (richness) and how evenly individuals are distributed among them (evenness).

A measure of how similar the abundances of the different species in a community are; evenness is high when all species have similar numbers of individuals.

The number of different species present in a community or area.

The amount of energy needed to raise the temperature of 1 g of a substance by 1 °C; for water it is 4.18 J g⁻¹ °C⁻¹.

A coloured chemical that binds to particular cell structures and increases contrast, such as iodine for starch in plant cells or methylene blue for nuclei in animal cells.

A skeletal muscle cell: a very long, cylindrical, multinucleate fibre formed by the fusion of many cells, with a striped (striated) appearance due to its regularly arranged proteins.

The fluid inside the inner membrane of a chloroplast, surrounding the thylakoids, where the enzymes of the Calvin cycle are found.

A layered rock structure built up by mats of microorganisms, mainly cyanobacteria; fossil stromatolites about 3.48 billion years old are among the oldest evidence of life.

The chain of alternating pentose sugars and phosphate groups, linked by covalent bonds, that forms the structural core of a strand of DNA or RNA, with the bases projecting from it.

The tendency of a liquid surface to behave like a stretched elastic film, caused by cohesion pulling surface molecules inwards; it lets small animals such as pond skaters stand on water.

The formation of new species from populations living in the same area, through reproductive isolation caused by behavioural or temporal differences or by polyploidy.

A group of organisms at any level of a classification, such as a species, genus, family or kingdom.

The science of identifying, naming, describing and classifying organisms into groups.

A region of repeated short DNA sequences at each end of a eukaryotic chromosome that protects the chromosome from damage and from shortening during replication.

A bacteriophage that can follow either the lytic cycle or the lysogenic cycle; bacteriophage lambda is an example.

Reproductive isolation caused by populations breeding or flowering at different times of day, season or year, so that they do not interbreed.

A specimen placed in a drop of liquid (water or stain) on a glass slide and covered with a cover slip, for viewing with a light microscope in a single session.

A branch of a cladogram that ends at a tip, leading to a single species or group that is not divided further on the diagram.

The disproved hypothesis that DNA is a simple repeating unit of the four bases in equal amounts, which would make it unable to carry genetic information.

In science, a well-tested explanation of a wide range of observations that is supported by a large body of evidence and is used to make predictions; it is more than a guess.

The ability of a substance to transfer heat by conduction; water conducts heat more than 20 times faster than air.

The classification of all cellular organisms into three domains, Bacteria, Archaea and Eukarya, based mainly on differences in the base sequences of ribosomal RNA.

A flattened membrane sac inside a chloroplast that holds chlorophyll and is the site of the light-dependent reactions of photosynthesis.

A pyrimidine base (T) found only in DNA, which pairs with adenine by two hydrogen bonds.

The presence of three copies of a particular chromosome in a cell instead of the normal two, for example trisomy 21, which causes Down syndrome.

A pyrimidine base (U) found only in RNA, where it replaces thymine and pairs with adenine.

A membrane-bound sac in the cytoplasm; plant cells usually have one large permanent sap vacuole, bounded by the tonoplast, which stores solutes and keeps the cell turgid.

Differences in the traits of individual organisms, both between members of the same species and between different species.

A small sac of membrane that transports or stores substances inside a cell; secretory vesicles carry proteins to the plasma membrane for release by exocytosis.

A phospholipid bilayer surrounding the capsid of some viruses, taken from a membrane of the host cell as the virus buds out, with viral glycoproteins inserted in it.

A complete virus particle outside a host cell, consisting of its nucleic acid inside a capsid and, in some viruses, an envelope.

A non-cellular infectious particle made of genetic material (DNA or RNA) inside a protein coat, which can reproduce only inside a host cell.

The resistance of a fluid to flowing; water is about 55 times more viscous than air, so it exerts more drag on a moving animal.

The process of determining the complete base sequence of all the DNA in the genome of an organism.