From Cells to Species

The cell is the basic unit of life, and every new cell arises from a pre-existing one.

DNA carries genetic instructions in a double helix structure first described by scientists in 1953.

Mitochondria generate a cell's chemical energy and are believed to descend from ancient free-living bacteria.

Natural selection favors helpful traits over generations, illustrated by peppered moths shifting color amid industrial pollution.

Evolution describes gradual change across generations, driven largely by natural selection acting on genetic variation.

A species is generally defined by the ability of its members to interbreed and produce fertile offspring.

Genes are DNA segments coding for specific traits, passed from parents to offspring during reproduction.

Chromosomes package DNA tightly enough to fit inside a cell, with humans carrying twenty-three pairs.

Gregor Mendel's pea plant experiments identified inheritance patterns now known as Mendelian genetics.

A mutation is a random change in DNA that can be neutral, harmful, or occasionally beneficial.

Homeostasis lets an organism maintain stable internal conditions, such as the body regulating its own temperature.

Bacteria are single-celled organisms found almost everywhere on Earth and rank among the earliest forms of life.

Viruses cannot reproduce alone and must hijack a host organism's cells, as the common cold virus does.

Stem cells can develop into many different cell types and are used in medicine to repair damaged tissue.

Cloning produces a genetically identical organism, as demonstrated when Dolly the sheep was cloned from an adult cell.

Taxonomy, developed by Carl Linnaeus, organizes living things into a hierarchy running from kingdom down to species.

An ecosystem links living organisms with their physical environment, as in a coral reef supporting thousands of species.

Symbiosis describes a close, long-term relationship between species, like clownfish sheltering among stinging anemones.

Extinction occurs once the very last member of a species dies out, as happened to the dodo bird.

Charles Darwin developed his theory of natural selection partly from observations made on the Galápagos Islands.

An adaptation is a trait shaped by natural selection to fit an organism to its environment, like a polar bear's thick fur.

Invertebrates lack a backbone yet make up the vast majority of all known animal species.

Photosynthesis converts sunlight into chemical energy in plants and releases oxygen as a byproduct.

Proteins are built from chains of amino acids and include structural molecules like keratin in hair and nails.

Enzymes act as biological catalysts, speeding up reactions such as the digestion of food.

A food chain traces how energy passes from one organism to the next, as grass feeds rabbits that feed foxes.

Fossils preserve remains or traces of ancient life across long geological timescales, like dinosaur bones in rock layers.

Metamorphosis transforms an animal's body form during development, most famously a caterpillar becoming a butterfly.

Vertebrates are animals defined by having a backbone, a group spanning fish, birds, and mammals.

Mammals nurse their young with milk, and nearly all give birth to live young rather than laying eggs.

Fungi form their own biological kingdom, distinct from plants and animals, including mushrooms and baking yeast.

Biodiversity measures the variety of life in an ecosystem, reaching its peak in places like tropical rainforests.

Pollination transfers pollen between flowers, often carried by bees, enabling many plants to reproduce.