DNA was discovered in the 1870s, and soon even known to be in chromosomes, but was thought to be structural, sort of like the complex polysaccharides in gristle & wood. The ATCG bases were thought to repeat the same short sequence, over and over. ("The Tetranucleotide Hypothesis") Genes were thought to be made of protein; Most people thought that DNA was too simple in structure to carry enough information, with just ATCG! (Think of the analogy to dots and dashes in the Morse code!) (Are dots and dashes too simple to carry alphabet letters, words?) Some physicists tried to deduce what kind of molecules genes must be made of; Schroedinger's book "What is Life" was published in the 1930s. He had two main conclusions: A) In order to copy themselves, genes must be made of two complementary halves, analogous to photographic positives and negatives, or in printing, or copying phonograph records & CDs. B) In order to have such low mutation rates, genes must be "an aperiodic crystal", in which covalent bonds have to be broken to change the information; big changes in free energy. In retrospect, notice that DNA has both properties! But the physicists failed to realize that DNA is a perfect fit, for both these criteria. Most people thought genes were proteins; probably enzymes that made copies of themselves. (1 scientist proposed that ATP might control genes by energy!) The discovery that genes are DNA was made studying some disease-causing bacteria; (An example of applied medical research yielding a breakthrough in "pure" science! The reverse of what usually happens!) (Also notice that they could have used harmless bacteria, differing in any number of properties!) Instead of studying the genetics of killing-power! Especially in bacteria that could infect people! These bacteria had two genetic variants: S for smooth, which form smooth colonies and kill mice. R for rough, which form rough colonies and don't kill mice (these rough colonies are also small, but s starts with an S) (we can regard these as an allelic pair of genes at the same site) (bacteria are haploid, & who knows which is dominant) The key discoveries (in the 1920s, by Griffith) were (first) that injecting mice with killed S cells at the same time as with live R cells resulting in mice dying, and (also) that live S bacteria could then be isolated from the mouse bodies. It seemed as if genes from the killed S bacteria were getting into the live R bacteria! And that was the correct explanation: These bacteria were the first "transgenic organisms". (The first to be produced in the lab! It also happens in the wild!) But people wasted time guessing that the key substance was the sugar coats that made the colonies smooth! And then Griffith was killed by German bombs, working in his London lab. So Avery, MacLeod & McCarty, at the Rockefeller Institute in NY, during WWII, continued Griffith's experiments, and found out which of the different molecules from killed S bacteria were the ones needed to "transform" R bacteria. Among other experiments, they treated extracts of killed S cells with different purified enzymes. Then they put these extracts into cultures of R bacteria, to see if any still became transformed into S bacteria. Enzymes that digest protein did NOT eliminate the development of S bacteria! Enzymes that digest RNA also did NOT eliminate the development of S bacteria. But enzymes that digested (only) DNA DID eliminate "the transforming principle" (their name for the key molecules, whatever they were) There is an excellent short paperback book about the history of this discovery, titled "The Transforming Principle". They concluded that DNA was the genetic material, at least in these bacteria. But most scientists wouldn't believe it; and certainly not that DNA were genes for everything. They didn't get the Nobel Prize. Another Professor at the Rockefeller Inst. explicitly warned the Swedes that Avery must have made some kind of mistake. Avery died in the early '50s. In the early 1950s at Cold Spring Harbor, Hershey and Chase did experiments on a bacterial virus made of DNA and protein, using radioactive phosphate to track the DNA and radioactive sulfur to track the protein, and they found that only the DNA, and not much protein, entered the infected cells. This confirmed that the genes must be DNA. {{The lesson is not to discover things too soon! Time your discovery to come out just when people are ready to believe the conclusion!}} Beadle and Tatum's did earlier genetic studies of the fungus Neurospora 1930s & 40s: They found mutants that needed various amino acids to live. (Because mutations altered genes for making these amino acids) "One gene one enzyme" and a Nobel Prize in the mid 50s. {{Beadle's wife wrote a wonderful autobiographical book titled "These Ruins Are Inhabited", about their sabbatical year in Oxford, during which the Nobel prize came. My wife and I read it just before moving to Cambridge, in 1970}} In 1952-3, Watson and Crick figured out the structure of DNA, based on Rosalind Franklin's X-ray crystallography photographs, & from their proposed structure it was almost obvious how this molecule copies itself and carries information. Why didn't Franklin discover the DNA structure? Much controversy and many books about this. Watson's "The Double Helix", among many others. Besides discrimination against women and Jews, (instead of against male anglo-saxons, like now) Watson had the advantage of reading "What is Life", so he was looking for 2 complementary parts, that could carry information in its covalent structure. Franklin was simply seeking one more structure. Having preconceptions can be a big advantage! Around 1950, Chargaff (at Columbia) had measured amounts of A, T, C and G in DNA from various different organisms. Although the relative amounts varied widely, he found that the amount of A was always the same as the amount of T, and the amount of C the same as G. Chargaff's rules: A=T and C=G One organism might have 20% A and 20%T, but 30%C and 30%G Another might have 22% of both A and T, and 28% of both C and G. He didn't even propose any hypotheses to explain either the cause or possible significance. Maybe if Chargaff had read "What Is Life", by Schroedinger! But Watson saw that their chemical structure predicts this "law". The following may be useful in life, but will not be on the tests: An alternative "Scientific Method" (instead of Observation/Hypothesis/Experiment etc.) 1) Find major questions that haven't been solved. (aging, memory, autoimmune diseases, nerve connections in the brain) 2) Read speculative books about what abstract properties the causes need to have. 3) Hang out in New York, Woods Hole, or Cambridge either one of them), where you can go to seminars and hear gossip. 4) Borrow other people's data, on which to base your theories. 5) Find bright people to use as "sounding boards" to try out ideas. 6) Submit your manuscript to Nature. 6 1/2) Don't discover things too soon, before people are ready! 6 3/4) Keep alert for odd regularities in nature (Chargaff's laws) (For example, that neural connection patterns in the brain tend to be upside-down and backwards relative to their origins; and that cancer cells have unexplained dents in their nuclei, and switch to anaerobic glycolysis, generating much lactic acid!) Notice how odd it is that bacteria will absorb DNA from outside and put it into their chromosome in place of equivalent genes! (No one has ever found out how this works, exactly!) But it only happens very rarely; one time in a million or billion; but then only those bacteria can grow in the mouse and kill it. To study genetic transformation, you need STRONG selection, so that only those one in a million will grow. Transformation also works with animal and plant cells, and has even been done with mitochondrial and chloroplast genes. (the last reported in a Science paper: Boynton, Gillham & Harris, et al.) Who shot the DNA in on buckshot, using the ATP synthase gene Science (1988) 240 pp. 1534-38 "Chloroplast transformation in Chlamydomonas with high velocity microprojectiles" Questions that you should be able to answer: 1) Explain dominance versus recessiveness of genes in terms of the "One Gene; One Enzyme" concept. 2) About what year was DNA first chemically isolated and described? 3) What other kinds of functions are sometimes served by complex polysaccharides. 4) What was the tetranucleotide hypothesis? (**Was it consistent with Chargaff's rules?") 5) What sort of chemical did almost everybody thing genes must be made out of, until the early 1950s? **6) The philosopher of science, Thomas Kuhn wrote that breakthroughs are often made people coming into a field from outside, either young students, or scientists from some entirely different field. Do you know some examples of this in the history of genetics? What advantages do newcomers have? 7) What two key predictions did Schroedinger make about the chemical structure of genes? 8) What two genetic variations, in what organisms, were used by Griffith and later Avery to discover that genes are made of DNA? *9) What is meant by transgenic animals, plants or procaryotes? "Genetically modified" is synonymous. (I think! If somebody knows of some distinction, then tell me.) 10) About what year (+-5) did Avery et al. report their evidence that genes are made of DNA. *11) Could they have made the equivalent discovery with bacteria that did not cause fatal diseases? **12) In their experiment, what seems to be the source of the selection pressure that "finds" the tiny percentage of R bacteria that got converted into S bacteria? Hint: Could they have detected transformation in the opposite direction? To judge from the larger size of the colonies of "smooth" bacteria in the photographs in the textbook, might they have been able to demonstrate transformation "in vitro" (meaning in bacterial cultures in Petri dishes, without killing mice)? 13) How did Alfred Hershey use radioactive phosphorous and sulfur to produce additional evidence that DNA is the genetic material? **14) If kinases had been discovered then, could that have dissuaded people from trying Hershey's experiment? What about if purines had a sulfur atom in them? *15) What was a special advantage of doing research on the fungus Neurospora, from Beadle and Tatum's point of view? (Other than growing rapidly, and not being cute) *16) Nearly all plants will die if unable to do photosynthesis; but a few algae can live by absorbing acetic acid, and burning it as an energy source. Can you figure out why you would need to use them in order to use genetic mutants to study the proteins used in photosynthesis? (I hope you will think hard about this one!) 17) About when (+- 5 years) did Watson and Crick propose their hypothesis about the molecular structure of DNA? *18) Whose X-ray diffraction data did they base their hypothesis on? 19) What was meant by "Chargaff's Laws"? **20) Which of Schroedinger's predictions are related (& how are they related) to Chargaff's laws. **21) Can you think of any copying technologies that don't involve complementary pairs, like photographic negatives and positives? **22) Could early geneticists have discovered that genes are made of DNA, instead of proteins, by doing chemical analyses on mutant organisms, as compared with non-mutants? (Hint: No; but why not?) *23) How should it affect the usual patterns of "branching tree" phylogeny (and taxonomy) if bacteria turn out to get many of their genes by absorbing bits of DNA from their environment? *24) How could sections of DNA be "genes" (i.e. produce different phenotypes when mutated), without actually coding for any proteins? Conversely, would you base your definition of genes on "One gene, one enzyme", or maybe "one gene, one protein". **25) If no phenotype (at all!) is produced by mutating a given section of DNA, then would you say that there are no genes there?

 

back to syllabus