The words you need to know

The basic unit of life. Every living thing is made of cells. Bacteria are a single cell; humans are around 37 trillion. The cell is where everything happens: energy production, protein manufacturing, DNA storage, reproduction. Understanding the cell is understanding life at its most fundamental level.

The two fundamental categories of cell. Prokaryotes (bacteria and archaea) have no nucleus. Their DNA floats free in the cell. Eukaryotes (everything else: plants, animals, fungi) have a nucleus that contains and organises the DNA. This is one of the most important distinctions in all of biology. Antibiotics work on bacterial cells specifically because prokaryotes and eukaryotes are different enough to target separately.

The membrane-bound compartment inside a eukaryotic cell that contains the DNA. The cell's control centre. When a cell needs to make a protein, the instructions are read from DNA in the nucleus and carried out elsewhere in the cell.

The organelles that produce most of a cell's energy, in the form of ATP. Often called the powerhouse of the cell. That is accurate. Mitochondria have their own DNA, separate from the cell's nucleus, which is evidence that they were once free-living bacteria absorbed by larger cells over a billion years ago.

The boundary of a cell: a thin, flexible layer made of lipids that controls what enters and leaves. Every cell has one. Many organelles inside the cell have their own membranes too. The membrane is not just a wall; it's an active system that regulates the cell's environment.

A specialised structure inside a cell with a specific function, the cellular equivalent of an organ. The nucleus stores DNA. The mitochondria produce energy. Ribosomes make proteins. The Golgi apparatus packages and ships proteins. Each organelle does one job and does it constantly.

The molecule that carries the genetic instructions for the development, function, and reproduction of all known living organisms. A long double-stranded helix made of four chemical bases: adenine, thymine, cytosine, guanine. Their sequence sequence encodes information. The order of these bases is the code. Change the code and you change the organism.

A specific sequence of DNA that encodes instructions for making a protein. The human genome contains around 20,000 genes, scattered across 23 pairs of chromosomes. Genes are inherited from parents. They are not destiny. Most traits are influenced by many genes interacting with the environment, but they are the starting point.

A tightly packaged structure of DNA and protein. Humans have 46 chromosomes in 23 pairs. Each chromosome carries hundreds or thousands of genes. When a cell divides, chromosomes are copied and distributed so each new cell gets a full set. Errors in this process are the cause of many genetic conditions.

The complete set of genetic material in an organism, all its DNA. The human genome contains about 3 billion base pairs. Sequencing a genome means reading the full order of those base pairs. The Human Genome Project, completed in 2003, was the first full read of the human genome. Today it takes hours and costs hundreds of dollars.

A variant version of a gene. For most genes, you inherit one copy from each parent. If the two copies are different variants, you have two different alleles. Some alleles are dominant: their effect shows even with one copy. Others are recessive: they only show when both copies are the same. Eye colour, blood type, and many genetic conditions work this way.

Genotype is the genetic sequence an organism has. Phenotype is what actually shows: the physical and behavioural traits that result from the genotype interacting with the environment. Identical twins have the same genotype. Their phenotypes can differ significantly based on how they've lived. The relationship between the two is one of biology's central questions.

A change in the DNA sequence. Mutations happen constantly, through copying errors, radiation, chemical exposure. Most are harmless or quickly corrected. Some affect how a gene functions. A few are the raw material of evolution: a mutation that gives an organism a survival advantage can spread through a population over generations.

The two types of cell division. Mitosis produces two identical copies of a cell: how you grow, repair tissue, and replace old cells. Meiosis produces sex cells (eggs and sperm) with half the normal number of chromosomes. When egg and sperm combine, the full number is restored. Meiosis is also where genetic shuffling happens, mixing parental DNA to produce variation in offspring.

The fundamental principle of molecular biology: DNA is transcribed into RNA, and RNA is translated into protein. Information flows in one direction. DNA is the archive. RNA is the working copy. Protein is the product. Francis Crick named it the central dogma in 1958. It has been refined but not overturned.

A molecule closely related to DNA that plays several roles in the cell. Messenger RNA (mRNA) carries instructions from DNA to the ribosomes where proteins are built. Transfer RNA (tRNA) brings the right amino acids to the ribosome. RNA is also the genetic material of some viruses, including influenza and SARS-CoV-2, mRNA vaccines work by delivering synthetic mRNA that instructs cells to produce a viral protein, triggering an immune response.

The workhorses of the cell. Proteins carry out almost every biological function: enzymes that speed up chemical reactions, structural proteins that give cells shape, antibodies that fight infection, hormones that carry signals between organs. A protein is a chain of amino acids folded into a specific three-dimensional shape. The shape determines the function.

A protein that speeds up a chemical reaction. Without enzymes, most of the chemistry of life would happen too slowly to sustain it. Enzymes are highly specific: each one catalyses one reaction. Digestion, DNA replication, energy production: all depend on enzymes. Many drugs work by blocking specific enzymes.

The process by which the information in a gene is used to produce a functional product, usually a protein. Not all genes are active all the time. Which genes are expressed, in which cells, at which times, is regulated by a complex system of switches. Identical DNA in every cell of your body produces very different cells: liver, neuron, skin. Different genes are expressed in each different genes are expressed in each.

A gene-editing technology that allows scientists to cut DNA at a precise location and modify it: removing, inserting, or correcting sequences. Originally a bacterial immune system, repurposed as a molecular tool. Faster and cheaper than previous gene-editing methods by orders of magnitude. In clinical use for conditions including sickle cell disease. The implications for medicine, agriculture, and the future of the human genome are still unfolding.

The mechanism by which evolution works. Individuals vary. Some variations are heritable. Some variations improve survival and reproduction. Individuals with those variations leave more offspring. Over generations, advantageous traits become more common in the population. Darwin's key insight, published in 1859, remains the central explanation for the diversity and adaptation of life on Earth.

A trait that improves an organism's ability to survive and reproduce in its environment. Adaptations are produced by natural selection over many generations. The eye, the wing, antibiotic resistance in bacteria: all adaptations. An adaptation is not a response to a conscious need. It is the result of random variation being filtered by survival.

A group of organisms that can interbreed and produce fertile offspring. The most common definition, though it has genuine edge cases. Lions and tigers can mate and produce offspring (ligers), but ligers are infertile. So lions and tigers are different species. The definition works in most cases and breaks down in interesting ones.

The process by which a new species forms. Usually happens when a population becomes separated by geography, behaviour, or timing, and the two groups evolve independently until they can no longer interbreed. Given enough time and selection pressure, two populations of the same species become two different species.

The shared ancestral organism from which two or more species descended. Humans and chimpanzees share a common ancestor that lived around 6 million years ago. All life on Earth shares a common ancestor if you go back far enough, estimated at around 3.5 to 4 billion years. Evolution is the branching story of how that ancestor became everything alive today.

All the chemical reactions taking place in an organism to maintain life. Two directions: catabolism breaks molecules down to release energy (digestion); anabolism builds molecules up using energy (growth, repair). Metabolic rate is how fast this happens. A fast metabolism burns energy quickly; a slow one more conservatively.

The maintenance of a stable internal environment despite changes in the outside world. Body temperature at 37°C. Blood pH at 7.4. Blood glucose within a narrow range. The body has constant feedback systems: hormones, nerves, organs. They detect deviations and correct them. Homeostasis is what keeps you alive when the world changes around you.

A chemical messenger produced in one part of the body that travels through the bloodstream to affect another part. Insulin signals cells to take up glucose. Adrenaline prepares the body for action. Oestrogen and testosterone regulate development and reproduction. Hormones are slow compared to nerve signals but they coordinate long-term changes across the whole body.

The body's defence against pathogens, bacteria, viruses, fungi, parasites. Two layers: the innate immune system responds immediately and non-specifically to anything that looks foreign. The adaptive immune system is slower, specific, and remembers previous infections. Vaccines work by training the adaptive immune system to recognise a pathogen before you encounter it for real.

A protein produced by the immune system that recognises and binds to a specific molecule, usually on the surface of a pathogen. Each antibody is specific to one target. When antibodies bind to a pathogen, they mark it for destruction by other immune cells and can neutralise it directly. Antibodies produced after infection or vaccination persist in the body, providing future protection.

Any organism or agent that causes disease. Bacteria, viruses, fungi, parasites: all can be pathogens. Not all microorganisms are pathogens, the human body hosts trillions of bacteria that are harmless or beneficial. The distinction matters: antibiotics kill bacteria but have no effect on viruses, which is why taking antibiotics for a viral infection achieves nothing.

A community of living organisms and the physical environment they interact with, functioning as a system. A forest, a coral reef, a pond. The living and non-living components, organisms, soil, water, climate, are all connected. Remove one part and the system adjusts, sometimes catastrophically.

A position in the food chain. Plants are primary producers, they capture energy from the sun. Herbivores are primary consumers. Predators of herbivores are secondary consumers. Each level up loses about 90% of the energy from the level below, which is why ecosystems can support far more plants than herbivores, and far more herbivores than apex predators.

The variety of life in a given area, measured in number of species, genetic diversity within species, and diversity of ecosystems. High biodiversity is generally a sign of a healthy ecosystem. It also makes ecosystems more resilient: more species means more ways to absorb disruption. Current extinction rates are estimated to be 100 to 1,000 times the natural background rate.

The maximum population size that an environment can sustain indefinitely, given the food, water, habitat, and other resources available. Populations that exceed their carrying capacity crash. The concept applies to every species, including humans, though human ingenuity has repeatedly extended the apparent carrying capacity of the planet through agriculture and technology.