Every protein you are made of is spelled out in an alphabet of four bases. Below is the real opening of a real human gene — HBB, which makes the beta chain of haemoglobin. Copy it into mRNA, read it codon by codon into a chain of amino acids, then change one letter and watch what happens. Lab 13 shows you the nucleus; this is what is written inside it.
The two backbones run in opposite directions — that is what antiparallel means, and it is why the ends are labelled 5′ and 3′ the other way round. A pairs only with T (two hydrogen bonds) and G only with C (three), so the lower strand is fully determined by the upper one. Transcription copies the lower, template strand, which is why the mRNA comes out reading like the upper strand with U in place of T. The helix drawing is to scale in bases, not in nanometres.
Three bases give 4 × 4 × 4 = 64 possible codons, and there are only 20 amino acids to spell plus an instruction to stop. The spare capacity is not wasted: most amino acids have two, four or six codons, and they nearly always differ only in the third letter. That is why a change in the third position of a codon is so often silent. The table below is the standard genetic code, used by almost every organism on Earth — the codon being read is highlighted as the ribosome moves.
Rows are the first base, columns the second, and the four lines inside each block are the third. AUG is both methionine and the start signal; UAA, UAG and UGA code for no amino acid at all and end the chain.
You have two copies of nearly every gene, one from each parent, and you pass on one of your two at random. A Punnett square is just the four ways that can combine. Pick two parents and read off what their children can be — then try the traits at the bottom of the list, where the tidy 3 : 1 story you were probably taught stops working.
3.1 billion base pairs, and only about 20,000 of them are protein-coding genes — the pieces that actually end up in a protein come to roughly 1.5% of the total. The rest is regulatory switches, introns, structural DNA, and an enormous archive of transposable elements: copies of copies of selfish sequences make up close to half the genome.
46 chromosomes: 22 matching pairs plus one sex pair, one of each pair from each parent. Chromosome 1 is 249 million bases and about 2,000 genes; the Y is a shrunken 57 million with roughly 45. Having two copies is the whole reason a recessive allele can hide. Chimpanzees have 24 pairs — our chromosome 2 is two of theirs fused end to end, and the old telomere sequence is still visible in the middle of it.
Mitosis: one cell becomes two identical cells with 46 chromosomes each — growth and repair. Meiosis: one cell becomes four different cells with 23 each — eggs and sperm. Both exist because building a body needs exact copies, while making a new person needs shuffled halves. Without the halving, every generation would double the chromosome count.
A stretch of DNA whose sequence specifies a product, usually a protein. An average human gene spans about 27,000 bases, but only around 1,300 of them survive into the protein — the introns in between are cut out after transcription. Splicing the surviving pieces in different orders lets 20,000 genes make well over 100,000 proteins. The largest gene, dystrophin, runs to 2.2 million bases and takes about 16 hours to transcribe.
A guide RNA about 20 bases long carries the Cas9 protein to the one place in three billion where the sequence matches, and Cas9 cuts both strands. The cell then repairs the cut — sloppily, which breaks the gene, or by copying a template you supply, which rewrites it. The 2020 Nobel Prize in Chemistry went to Charpentier and Doudna for it; Casgevy, licensed in the UK and US in late 2023 for sickle-cell disease, is the first approved CRISPR therapy. It does not repair the HBB letter — it switches fetal haemoglobin back on instead.
Four bases is two bits, so 3.1 billion bases is 775 megabytes — about a fifth of a DVD. A 128 GB phone could hold 165 copies of you. And because any two people are about 99.9% identical, storing only your differences from a reference genome takes a few tens of megabytes. The raw output of a sequencing machine is far bigger, a few hundred gigabytes, because every base is read over and over to be sure of it.
Copying three billion bases is not perfect. Each of us carries roughly 70 new mutations that neither parent had, most of them in the 98.5% that codes for nothing, and almost all of them harmless. That background rate is the raw material evolution works with — and it is why the sickle allele, once it appeared, had something to be selected on.
| Organism | Genome, base pairs | Protein-coding genes |
|---|
Genome size and complexity are barely related — biologists call it the C-value paradox. A small fern with fifty times your DNA is not fifty times more complicated; it simply has far more repeated and duplicated sequence. Your mitochondria carry their own tiny 16,569-base genome, a leftover from the bacterium they descend from.