Cells, genes and the basics of life

Every living thing is made of cells, the smallest units capable of carrying out the processes of life. Some organisms, such as bacteria, consist of a single cell; others, such as humans, are made of trillions of cooperating cells. The cell theory, developed in the nineteenth century, states three things: all living things are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. That last point is worth pausing on: every cell in your body descends, through unbroken divisions, from a single fertilised egg, and ultimately from cells that lived billions of years ago.

Cells come in two broad kinds. Prokaryotic cells, such as bacteria, have no nucleus; their genetic material floats in the cell. Eukaryotic cells — those of plants, animals, and fungi — keep their DNA inside a nucleus, and contain specialised compartments called organelles. Mitochondria generate most of a cell's energy, ribosomes build proteins, and in plant cells, chloroplasts capture sunlight for photosynthesis. This division of labour inside the cell mirrors the division of labour in a body: specialised parts cooperating for the whole.

The instructions for building and running a cell are stored in DNA, a long molecule shaped like a twisted ladder, the double helix. Its rungs are pairs of chemical bases: adenine with thymine, cytosine with guanine. The sequence of these bases spells out genes, sections of DNA that code for proteins or functional RNA. In 1953, James Watson and Francis Crick published the double-helix model in the journal Nature, building on X-ray diffraction work by Rosalind Franklin and Maurice Wilkins. The model's power was that it explained copying: each strand can serve as a template for a new partner strand, which is how genetic information is passed on when cells divide.

Genes do not determine everything. The same genetic instructions produce different results depending on environment, diet, and chance, and most traits involve many genes. Mutations — changes in DNA sequence — are usually neutral or harmful, but occasionally useful, and they are the raw material of evolution. Modern genetics also raises practical questions: genetic tests can predict disease risk, but the information is sensitive, and deciding who may access it, and for what purpose, is as much an ethical question as a scientific one.