decades—as if providing a protracted running commentary on DNA devastation.
However definitive in retrospect, experiments on DNA and proteins in the 1940s convinced only some scientists that DNA was the genetic medium. Better proof came in 1952, from virologists Alfred Hershey and Martha Chase. Viruses, they knew, hijacked cells by injecting genetic material. And because the viruses they studied consisted of only DNA and proteins, geneshad to be one or the other. The duo determined which by tagging viruses with both radioactive sulfur and radioactive phosphorus, then turning them loose on cells. Proteins contain sulfur but no phosphorus, so if genes were proteins, radioactive sulfur should be present in cells postinfection. But when Hershey and Chase filtered out infected cells, only radioactive phosphorus remained: only DNA had been injected.
Hershey and Chase published these results in April 1952, and they ended their paper by urging caution: “Further chemical inferences should not be drawn from the experiments presented.” Yeah, right. Every scientist in the world still working on protein heredity dumped his research down the sink and took up DNA. A furious race began to understand the structure of DNA, and just one year later, in April 1953, two gawky scientists at Cambridge University in England, Francis Crick and James Watson (a former student of Hermann Muller), made the term “double helix” legendary.
Watson and Crick’s double helix was two loooooooong DNA strands wrapped around each other in a right-handed spiral. (Point your right thumb toward the ceiling; DNA twists upward along the counterclockwise curl of your fingers.) Each strand consisted of two backbones, and the backbones were held together by paired bases that fit together like puzzle pieces—angular A with T, curvaceous C with G. Watson and Crick’s big insight was that because of this complementary A-T and C-G base pairing, one strand of DNA can serve as a template for copying the other. So if one side reads CCGAGT, the other side must read GGCTCA. It’s such an easy system that cells can copy hundreds of DNA bases per second.
However well hyped, though, the double helix revealed zero about how DNA genes actually made proteins—which is, after all, the important part. To understand this process, scientists had to scrutinize DNA’s chemical cousin, RNA. Though similarto DNA, RNA is single-stranded, and it substitutes the letter U (uracil) for T in its strands. Biochemists focused on RNA because its concentration would spike tantalizingly whenever cells started making proteins. But when they chased the RNA around the cell, it proved as elusive as an endangered bird; they caught only glimpses before it vanished. It took years of patient experiments to determine exactly what was going on here—exactly how cells transform strings of DNA letters into RNA instructions and RNA instructions into proteins.
Cells first “transcribe” DNA into RNA. This process resembles the copying of DNA, in that one strand of DNA serves as a template. So the DNA string CCGAGT would become the RNA string GGCUCA (with U replacing T). Once constructed, this RNA string leaves the confines of the nucleus and chugs out to special protein-building apparatuses called ribosomes. Because it carries the message from one site to another, it’s called messenger RNA.
The protein building, or translation, begins at the ribosomes. Once the messenger RNA arrives, the ribosome grabs it near the end and exposes just three letters of the string, a triplet. In our example, GGC would be exposed. At this point a second type of RNA, called transfer RNA, approaches. Each transfer RNA has two key parts: an amino acid trailing behind it (its cargo to transfer), and an RNA triplet sticking off its prow like a masthead. Various transfer RNAs might try to dock with the messenger RNA’s exposed triplet, but only one with complementary bases will stick. So with the triplet GGC, only a
However definitive in retrospect, experiments on DNA and proteins in the 1940s convinced only some scientists that DNA was the genetic medium. Better proof came in 1952, from virologists Alfred Hershey and Martha Chase. Viruses, they knew, hijacked cells by injecting genetic material. And because the viruses they studied consisted of only DNA and proteins, geneshad to be one or the other. The duo determined which by tagging viruses with both radioactive sulfur and radioactive phosphorus, then turning them loose on cells. Proteins contain sulfur but no phosphorus, so if genes were proteins, radioactive sulfur should be present in cells postinfection. But when Hershey and Chase filtered out infected cells, only radioactive phosphorus remained: only DNA had been injected.
Hershey and Chase published these results in April 1952, and they ended their paper by urging caution: “Further chemical inferences should not be drawn from the experiments presented.” Yeah, right. Every scientist in the world still working on protein heredity dumped his research down the sink and took up DNA. A furious race began to understand the structure of DNA, and just one year later, in April 1953, two gawky scientists at Cambridge University in England, Francis Crick and James Watson (a former student of Hermann Muller), made the term “double helix” legendary.
Watson and Crick’s double helix was two loooooooong DNA strands wrapped around each other in a right-handed spiral. (Point your right thumb toward the ceiling; DNA twists upward along the counterclockwise curl of your fingers.) Each strand consisted of two backbones, and the backbones were held together by paired bases that fit together like puzzle pieces—angular A with T, curvaceous C with G. Watson and Crick’s big insight was that because of this complementary A-T and C-G base pairing, one strand of DNA can serve as a template for copying the other. So if one side reads CCGAGT, the other side must read GGCTCA. It’s such an easy system that cells can copy hundreds of DNA bases per second.
However well hyped, though, the double helix revealed zero about how DNA genes actually made proteins—which is, after all, the important part. To understand this process, scientists had to scrutinize DNA’s chemical cousin, RNA. Though similarto DNA, RNA is single-stranded, and it substitutes the letter U (uracil) for T in its strands. Biochemists focused on RNA because its concentration would spike tantalizingly whenever cells started making proteins. But when they chased the RNA around the cell, it proved as elusive as an endangered bird; they caught only glimpses before it vanished. It took years of patient experiments to determine exactly what was going on here—exactly how cells transform strings of DNA letters into RNA instructions and RNA instructions into proteins.
Cells first “transcribe” DNA into RNA. This process resembles the copying of DNA, in that one strand of DNA serves as a template. So the DNA string CCGAGT would become the RNA string GGCUCA (with U replacing T). Once constructed, this RNA string leaves the confines of the nucleus and chugs out to special protein-building apparatuses called ribosomes. Because it carries the message from one site to another, it’s called messenger RNA.
The protein building, or translation, begins at the ribosomes. Once the messenger RNA arrives, the ribosome grabs it near the end and exposes just three letters of the string, a triplet. In our example, GGC would be exposed. At this point a second type of RNA, called transfer RNA, approaches. Each transfer RNA has two key parts: an amino acid trailing behind it (its cargo to transfer), and an RNA triplet sticking off its prow like a masthead. Various transfer RNAs might try to dock with the messenger RNA’s exposed triplet, but only one with complementary bases will stick. So with the triplet GGC, only a
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