DNA technology

Thirty rounds of PCR turn one piece of DNA into about a billion copies.

Copying DNA in a tube

Most ways of studying DNA need far more of it than a drop of blood or a hair holds. The polymerase chain reaction, or PCR, solves that. NHGRI calls it molecular photocopying: a fast and inexpensive way to copy a chosen small segment of DNA. It uses the same base-pairing rules as a cell. The sample is heated so the two strands separate, then cooled so short starter pieces called primers can stick on either side of the target, then a heat-tolerant DNA polymerase builds a new partner strand on each template. That is one cycle.

Each cycle can double the number of copies, so the count grows fast. After 10 cycles one molecule could become 2¹⁰ = 1,024 copies, after 20 cycles 1,048,576, and after 30 cycles 1,073,741,824, about a billion. Real reactions fall a little short of perfect doubling, but the idea holds. Kary Mullis, who devised PCR in 1983, received the Nobel Prize in Chemistry in 1993. NHGRI lists its uses: most mapping work in the Human Genome Project, DNA fingerprinting, detecting bacteria and viruses, and diagnosing genetic disorders.

Sorting pieces by size

To see what a reaction made, labs often use gel electrophoresis. OpenStax explains it. A slab of agarose gel, made from a substance found in seaweed, is soaked in a salt solution, and DNA samples are loaded into small wells at one end. DNA carries a negative charge because of its phosphate groups, so when an electric current runs through the gel, the DNA moves toward the positive electrode. The gel acts like a tangle that small pieces slip through quickly while large pieces lag behind. After a while the smallest fragments are farthest from the wells. A dye then makes the bands visible.

Reading the letters and the whole genome

Reading the order of bases is called sequencing. Frederick Sanger developed the chain-termination method, which uses slightly altered nucleotides that stop a growing strand wherever they are added; sorting the stopped pieces by size reveals the sequence one base at a time. He received the Nobel Prize in Chemistry in 1980, and his method was the one used for the Human Genome Project. That project began officially in October 1990 and was completed in April 2003, more than two years ahead of schedule. It covered about 92 percent of the genome, as far as the technology of the time allowed. The last 8 percent, full of repeats, was finished by the Telomere-to-Telomere consortium in 2022, giving a complete sequence of 3.055 billion base pairs.

Comparing genomes shows how alike people are. NHGRI notes that any two people's genomes are often said to be about 99.9 percent identical. With about 3 billion bases, 0.1 percent is still about 3 million differences, enough to make each person's DNA pattern distinctive.

Editing DNA

Genome editing tools act like scissors that cut DNA at a chosen spot; scientists can then remove, add or replace DNA there. NHGRI describes CRISPR as simpler, faster, cheaper and more accurate than older editing methods, and many scientists who edit genomes now use it. Emmanuelle Charpentier and Jennifer Doudna shared the 2020 Nobel Prize in Chemistry "for the development of a method for genome editing". Researchers use it to switch off single genes in animals such as mice and zebrafish to learn what those genes do. Treatments are being developed, but NHGRI points to open problems: tools sometimes cut in the wrong place, and edits to eggs, sperm or embryos would pass to future generations. Many countries restrict that kind of editing, and the NIH does not fund research to edit human embryos.