DNA

• 1. Introduction

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  Deoxyribonucleic acid, known as DNA, is a polymer built from two chains that coil around each other into a double helix. Inside it sits the instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. A single human female nuclear genome, if pulled straight from one cell, would stretch 208.23 centimeters and weigh just 6.51 picograms. Yet for most of human history, nobody knew this molecule existed. The substance was first scraped from the pus of discarded surgical bandages. How did a chemical curiosity become the secret of life? Who decoded its shape, who fought over the credit, and how did the same molecule end up convicting murderers and storing data? The answers begin with four small chemical letters.

• 2. Four Letters

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  Cytosine, guanine, adenine and thymine are the four nitrogen-containing nucleobases that spell out every genetic message in DNA. Each base sits on a nucleotide, alongside a sugar called deoxyribose and a phosphate group. The nucleotides link into a chain through phosphodiester bonds, building an alternating sugar-phosphate backbone. The bases divide into two families. Pyrimidines, thymine and cytosine, carry a single ring. Purines, adenine and guanine, carry two fused rings. The pairing rules are strict. Adenine bonds only to thymine, held by two hydrogen bonds. Cytosine bonds only to guanine, held by three. This is complementary base pairing, and it means both strands store the same information. Because hydrogen bonds are not covalent, the two strands can be pulled apart like a zipper by force or by heat. DNA rich in guanine and cytosine resists that pulling more strongly than DNA poor in those bases. That difference matters in places like the Pribnow box in some promoters, where the strands need to separate easily.

• 3. Architecture Of The Helix

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  Both helical chains in DNA share a pitch of 34 angstroms and a radius of 10 angstroms. The strands run in opposite directions, a property called antiparallel. One end of any strand carries a terminal phosphate group on its five prime carbon. The other end carries a free hydroxyl group on its three prime carbon. Trace the spaces between the two strands and you find grooves of unequal size. The major groove measures 22 angstroms across, while the minor groove measures only 12. Because the major groove is wider, the edges of the bases sit more exposed there. Proteins such as transcription factors usually read the sequence by reaching into that wider channel. DNA does not hold one fixed shape. It can twist like a rope in a process called supercoiling, circling its own axis once every 10.4 base pairs in the relaxed state. Most natural DNA carries a slight negative twist introduced by enzymes called topoisomerases. The molecule can also fold into the A, B and Z forms, though only B and Z have been seen directly in living organisms. The Z form turns in a left-handed spiral, the reverse of the common B form, and appears where bases have been chemically modified by methylation.

• 4. Reading And Copying

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  Three-letter words called codons translate the genetic code, each formed from a sequence of three nucleotides such as ACT, CAG or TTT. Four bases in three-letter combinations yield 64 possible codons, which encode the twenty standard amino acids. Three of those codons, TAG, TAA and TGA, are stop signals that mark the end of a coding region. The journey from gene to protein runs through two steps. In transcription, RNA polymerase copies a gene into messenger RNA, swapping thymine for uracil. In translation, a ribosome reads that message against transfer RNA, which carries the amino acids. When a cell divides, it must hand each daughter a full copy of its genome. The double-stranded design makes this simple. The two strands separate, and DNA polymerase rebuilds each missing partner by complementary base pairing. That enzyme can only extend a strand in the five prime to three prime direction, so the antiparallel strands demand different copying mechanisms. Many DNA polymerases also proofread. When a mismatch breaks the expected base pairing, a three prime to five prime exonuclease activity removes the wrong base.

• 5. Damage And Defense

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  About 150,000 bases in a typical human cell have suffered oxidative damage. Mutagens drive much of this harm, including oxidizing agents, alkylating agents, ultraviolet light and X-rays. Ultraviolet light fuses pyrimidine bases into thymine dimers. Oxidants like free radicals and hydrogen peroxide attack guanosine and snap both strands at once. Double-strand breaks are the most dangerous lesions, hard to repair and able to spawn point mutations, insertions, deletions and chromosomal translocations that can cause cancer. Some mutagens slide into the gap between two adjacent base pairs, a trick called intercalation. Ethidium bromide, acridines, daunomycin and doxorubicin all work this way, forcing the bases apart and blocking both transcription and replication. Thalidomide, an intercalator, acts as a teratogen. The same poisons that damage DNA also serve in chemotherapy, where stopping replication can halt rapidly growing cancer cells. The accumulation of unrepaired damage in postmitotic tissues appears to be a major underlying cause of aging. Because the repair machinery has inherent limits, the article notes that if humans lived long enough, they would all eventually develop cancer.

• 6. Caps And Tangles

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  Telomeres sit at the ends of linear chromosomes, built in human cells from several thousand repeats of the simple TTAGGG sequence. Ordinary replication enzymes cannot copy the extreme three prime ends of chromosomes, so the enzyme telomerase extends them instead. These guanine-rich stretches can fold into stacked plates of four guanine bases, each plate called a guanine tetrad. Stacked tetrads form a stable G-quadruplex, held together by hydrogen bonds and a metal ion chelated at the center. Telomeres also curl into long loops. The single-stranded DNA folds into a circle stabilized by telomere-binding proteins, forming a T-loop. At its tip, the single strand invades a stretch of double-stranded DNA and pairs with one strand, creating a three-stranded displacement loop, or D-loop. These caps stop the repair systems from mistaking chromosome ends for damage, and they prevent neighboring chromosomes from fusing.

• 7. From Bandages To The Double Helix

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  Friedrich Miescher, a Swiss physician, isolated DNA in 1869 from the nuclei of cells in discarded surgical bandages, naming the substance nuclein. In 1878, Albrecht Kossel separated the non-protein nucleic acid and later identified its five primary nucleobases. Phoebus Levene identified the nucleotide unit of RNA in 1909, then found deoxyribose in DNA in 1929. His tetranucleotide hypothesis wrongly held that the chain was short with bases repeating in a fixed order. Frederick Griffith offered the first clear hint in 1928, showing that traits of the smooth form of Pneumococcus could pass to the rough form. In 1943, Oswald Avery, with Colin MacLeod and Maclyn McCarty, identified DNA as the transforming principle. Erwin Chargaff then published his rules, that guanine equals cytosine and adenine equals thymine in any species. In May 1952, Raymond Gosling, working under Rosalind Franklin, captured the X-ray image known as Photo 51. Maurice Wilkins passed it to James Watson and Francis Crick, and it proved critical to the correct structure. Franklin told them the backbones had to be on the outside, correcting earlier models from Linus Pauling and from Watson and Crick that placed the chains inside. On the 28th of February 1953, Crick interrupted lunch at The Eagle pub in Cambridge to announce he and Watson had discovered the secret of life. The 25th of April 1953 issue of the journal Nature published five articles on the double helix, including the famous line that the specific pairing immediately suggested a possible copying mechanism. In 1962, after Franklin's death, Watson, Crick and Wilkins shared the Nobel Prize, which is given only to the living. In April 2023, scientists concluded that Franklin had been an equal player in the discovery.

• 8. DNA On Trial

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  Sir Alec Jeffreys, a British geneticist, developed DNA profiling in 1984 by comparing the lengths of variable repetitive sections such as short tandem repeats and minisatellites. In 1986, police in the UK first used DNA analysis in a criminal investigation, asking Jeffreys at the University of Leicester to test a suspect who had confessed to a recent rape-murder but denied an earlier one. Jeffreys' testing exonerated that suspect from both crimes, and all charges were dropped. Further profiling then identified Colin Pitchfork, found guilty of both rape-murders in 1988 in the Enderby case. Forensic profiling now identifies victims of mass casualty incidents, bodies in serious accidents, and individuals in mass war graves through matching to family members. In paternity testing, the probability of parentage typically reaches 99.99 percent when the alleged parent is biologically related. Beyond the courtroom, DNA serves as a structural material in nanotechnology, folding through the DNA origami method into lattices and three-dimensional polyhedra. Catalytic DNA sequences called DNAzymes, first discovered in 1994, can speed reactions up to 100,000,000,000-fold, and the NaA43 DNAzyme, more than 10,000-fold selective for sodium, was used to build a real-time sodium sensor inside cells. The same molecule even offers a storage medium of enormous density, though slow read and write times and insufficient reliability have so far kept it out of practical use.