From a 19th-century monk counting peas to a CRISPR pipette in a high school lab. The story of how a four-letter alphabet runs every life on Earth.
Gregor Mendel, an Augustinian friar in Brno, crossed thousands of Pisum sativum plants between 1856 and 1864 and discovered three things that contemporaries failed to grasp:
Published 1866 in an obscure Brünn journal. Rediscovered 1900 by de Vries, Correns, and Tschermak — the same year Bateson coined "genetics."
3 : 1 phenotypic ratio. 1 : 2 : 1 genotypic.
Watson, Crick, and Wilkins shared the 1962 Nobel for the structure; Rosalind Franklin's Photo 51 had given it away. Two anti-parallel sugar-phosphate strands, paired bases inside, 10.5 bp per turn, ~2 nm wide.
RNA polymerase II reads a DNA gene and synthesizes a complementary mRNA. Eukaryotic mRNA is then capped, polyadenylated, and spliced.
Ribosome reads mRNA codons (3 nt each). 64 codons → 20 amino acids + stop. tRNAs deliver. Result: a polypeptide.
The peptide chain folds (sometimes with chaperones) into a 3D shape — α-helices, β-sheets, motifs, domains. Function follows form.
The genetic code is universal (with a handful of exceptions): the same codons mean the same amino acids in E. coli, in oak, and in you. Strong evidence for common descent.
Marshall Nirenberg, Har Gobind Khorana, and Robert Holley cracked the codon table by 1966 (Nobel 1968). Most amino acids are encoded by multiple codons — a redundancy that buffers against mutations at the third position ("wobble").
AUG: methionine, also START. UAA, UAG, UGA: STOP.
The code's redundancy is not random — chemically similar amino acids share similar codons, so single-base errors tend to be conservative.
Each human somatic cell carries 46 chromosomes — 23 pairs, 22 autosomal and one sex pair (XX or XY). Each chromosome is a single, long DNA molecule wound around histones into nucleosomes, then into 30 nm fibers, into loops, into chromatids.
Sex chromosomes evolved from a regular autosome pair ~166 Mya in mammals; the Y has shed most of its content and now carries ~70 protein-coding genes vs. the X's ~800.
For decades the non-coding portion was called "junk DNA"; the ENCODE Project (2012) reported biochemical activity in > 80 % of the genome, though "function" remains contested. Much of the regulatory grammar — enhancers, insulators, lincRNAs — lives there.
Epigenetics: heritable changes in gene expression that don't alter the DNA sequence. The two main marks:
Cellular differentiation is largely epigenetic: a liver cell and a neuron in your body share the same genome but read it differently. Some marks survive mitosis; a small fraction may even pass through meiosis (transgenerational, debated).
Substitution of one base. Synonymous (silent), missense, or nonsense. ~1 in 10⁹ per base per division after proofreading.
Insertion or deletion. If not a multiple of 3 in coding region, frame-shift; usually catastrophic.
Inversions, translocations, duplications, copy-number variants. Important in evolution and in disease (BCR-ABL, Charcot–Marie–Tooth).
A human zygote inherits ~70 de novo mutations relative to the parental genomes — most neutral, a few deleterious, very rarely a beneficial one.
1822–84 · particulate inheritance.
1866–1945 · fly chromosomes; sex-linkage.
1877–1955 · DNA as transforming principle.
1920–58 · X-ray crystallography of DNA.
1953 · structure paper.
1902–92 · transposons.
CRISPR-Cas9 · Nobel 2020.
RNA interference · Nobel 2006.
Mendel reads "Experiments on Plant Hybrids" to the Brünn Society.
Friedrich Miescher isolates "nuclein" from white-blood-cell pus.
Mendel rediscovered. Bateson coins "genetics" 1905.
Avery, MacLeod, McCarty: DNA, not protein, is the genetic material.
Watson & Crick model the double helix from Franklin's data.
Genetic code fully decoded.
Sanger sequencing; first viral genome (φX174) read.
PCR — Kary Mullis amplifies DNA in a tube.
Human Genome Project: $3 B, 13 yr, 92 % of euchromatin.
1000 Genomes Project; HapMap completes; GWAS era begins.
Doudna & Charpentier publish CRISPR-Cas9 as a programmable nuclease.
mRNA vaccines deployed against SARS-CoV-2 within 11 months.
T2T consortium completes the gapless human genome (3.055 Gbp).
First base-edit therapy approved for sickle-cell disease (Casgevy).
CRISPR-Cas9 is a programmable molecular scissor. The Cas9 protein, guided by a 20-nt guide RNA you design, cuts DNA at a specific site. The cell repairs it — by error-prone NHEJ (knock-out) or HDR with a template (knock-in).
Originally a bacterial immune system: archived viral DNA (CRISPR repeats) as templates to chop matching invaders. Doudna and Charpentier showed in 2012 it could be retargeted at will. Awarded the 2020 Nobel.
Base editing (Liu, 2016) and prime editing (Liu, 2019) edit without double-strand breaks. They underlie 2024's approved therapies for sickle-cell disease and β-thalassemia.
Single-gene disorders. ~7,000 known. Sickle-cell (HBB), cystic fibrosis (CFTR), Huntington's (HTT). Highly penetrant; relatively rare.
Hundreds to thousands of variants of small effect. Type 2 diabetes, schizophrenia, height. Polygenic risk scores predict probabilistically.
Mutations accumulated over a lifetime. Driver vs. passenger. KRAS, TP53, BRCA1/2, EGFR. Tumor genomics now routine.
The 2018 birth of CRISPR-edited twins by He Jiankui shocked the field — heritable germline editing in humans had been a self-imposed red line since the 1975 Asilomar conference. He served three years in Chinese prison. The moratorium on germline edits remains in force; somatic edits are progressing rapidly.
Other live debates: gene drives in wild populations; DTC genetic testing privacy; reproductive screening; equitable access to expensive therapies (Casgevy: $2.2 M/dose).
Each line is a moving negotiation between technology, regulation, and values.
A reference graph that captures structural variation across populations, replacing the linear GRCh38.
Fetal CRISPR for monogenic diseases — animal proof of concept; first human trials approaching.
Yeast Sc2.0 nearly complete; bacterial genome write at the megabase scale routine.
Kurzgesagt's "Genetic Engineering Will Change Everything Forever – CRISPR" — the field's most-watched explainer.