Atoms
Atoms are the smallest building blocks of all matter. Not just in biology, in everything. Iron, oxygen, carbon, copper, these are all atoms. The word atom comes from the Greek word atomos, which means “undivided” or “uncut.”
Every atom has three parts:
Electrons want to exist in pairs. Each orbital holds exactly 2 electrons. When an atom or molecule has an unpaired electron, it becomes chemically unstable and will aggressively try to steal an electron from the nearest molecule to re-stabilise itself. A molecule in this unstable state is called a free radical.
Molecules
A molecule is two or more atoms held together by chemical bonds. Water is a molecule (two hydrogen atoms bonded to one oxygen). So is glucose, cholesterol, oestrogen, dopamine, and every protein in your body.
Some molecules are small enough to pass through cell membranes freely (oxygen, carbon dioxide, water, fat-soluble hormones like testosterone). Others are too large or too charged and need transporter proteins to get them across (glucose, amino acids, water-soluble hormones like insulin). This is why steroid hormones act inside the cell on DNA while peptide hormones act on receptors on the outside of the cell.
DNA
DNA is a very long molecule made of a sequence of smaller molecules called nucleotides. Each nucleotide contains a sugar, a phosphate group, and one of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these four bases is the genetic code. Your DNA is the instruction manual for making you, divided into segments called chromosomes and shorter individual units called genes. The sum of all your genes is the genome.
What genes specifically do is provide instructions for building proteins. Most of the functional machinery in your body is made of protein, including enzymes (which speed up chemical reactions), hormones (which carry chemical messages), antibodies (which fight infection), and structural materials (like collagen). Your genome contains roughly 20,000-25,000 protein-coding genes.
Gene expression