The molecules of life: DNA, RNA & protein

Welcome! Almost everything a living cell does comes down to three molecules working together: DNA, RNA, and protein. In this lesson we will meet each one, see how it is built, and learn a few key words along the way. You need no biology and no coding background to follow along.

DNA: life's instructions

DNA (short for deoxyribonucleic acid) is the molecule that stores the instructions for building and running a living thing. It is a long chain made of small repeating units called nucleotides. Each nucleotide carries one of four chemical letters, called bases: A (adenine), C (cytosine), G (guanine), and T (thymine).

In most cells DNA is two strands twisted around each other into a shape called the double helix, like a ladder gently twisted into a spiral. The two strands hold together by base-pairing, and the pairing follows a strict rule: A always pairs with T, and G always pairs with C. Because of this rule the strands are complementary, meaning if you know the letters on one strand you can work out the other.

5' A C G T A A G C 3'
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3' T G C A T T C G 5'

Notice the little labels 5' and 3' (say them as "five prime" and "three prime"); they are simply names for the two different ends of a strand, coming from small differences in its chemistry. The two strands run in opposite directions, so where one reads 5' to 3', its partner reads 3' to 5'. You do not need the chemistry details yet; just remember that DNA has a direction, and by convention we read a strand starting from its 5' end.

RNA and proteins

RNA (ribonucleic acid) is a close cousin of DNA, with two key differences. First, RNA is usually single-stranded, one strand on its own rather than a twisted pair. Second, RNA uses a base called U (uracil) in place of T, so when RNA pairs up, A pairs with U and G still pairs with C.

The third key molecule is protein, and proteins are the cell's main workers: they build structures, carry signals, and speed up chemical reactions. A protein is a chain made from smaller building blocks called amino acids, and living things use 20 different amino acids. The particular order of those amino acids folds the chain into a specific shape, and that shape determines what the protein does.

A gene is a stretch of DNA that holds the instructions for one particular job, very often the recipe for a single protein. Genes sit along much longer DNA molecules called chromosomes, which package and organize the instructions like chapters in a book. All of an organism's DNA taken together, every chromosome, is called its genome.