You are the guanine-yin to my cytosine-yang.
Source: Wikimedia Commons contributors, modified
Intro
Watson and Crick got to write a one-page paper in which they briefly mentioned that "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."[1] But how that copying mechanism works, who's to say? Watson and Crick did their abstract work, and the task of proving and elaborating their model fell on other scientists. As they stated in their paper:
The previously published X-ray data on deoxyribose nucleic acid are insufficient for a rigorous test of our structure. So far as we can tell, it is roughly compatible with the experimental data, but it must be regarded as unproved until it has been checked against more exact results.[1]
Many "more exact results" would be coming soon. One of them, called by some "the most beautiful experiment in biology"[2] (and the subject of this post), would determine which of the tree models of DNA replication popular at the time was the correct model.
Three model contenders
(No, I don't mean runway models. Please pay attention.)
According to Watson and Crick's structure, the DNA is made up of two strands, in the conformation of a double helix. But how do these strands copy themselves? Do the strands get copied to create a completely new pair, resulting in one old and one new DNA molecule (conservative model)? Do the strands get separated, with each then giving rise to a complementary strand, so that we end up with pairs that are one part old one part new (semi-conservative model)? Or do patches of DNA get copied piecemeal, resulting in a mottled DNA molecule (dispersive model)?
Is "dispersive" the new slur for "liberal"?
Source: OpenStax Biology
I might've gone for the conservative model. It seems more intuitive. But the model that was favored by Watson and Crick's structure, as well as their words, was the semi-conservative model, due to the complementary structure of the two strands of DNA[6]:
[I]t is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. [...] [I]t follows that if the sequence of bases on one chain, is given, then the sequence on the other chain is automatically determined.[1]
Watson and Crick proceeded to quote Chargaff's rule, according to which the amounts of adenine are equal to the amounts of thymine, and the amounts of guanine are equal to the amounts of cytosine.[3]
So it seemed, from this evidence, that each new strand got made from the template of an old strand. If you got one pair you can deduce the other. They fit each other like a yin fits a yang. But how could one go about deciding between the models? How could you tell what was going on at the molecular level, given the technology they had in the late 50s?
The Meselson-Stahl experiment
A real beut!
Source: Freegreatpicture
They say beauty is in the eye of the beholder, and no two people react the same when they look at a test tube. But the simplicity and ingeniousness of the Meselson-Stahl experiment led to some calling it "the most beautiful experiment in biology"[2].
Everyone knew the different models from the previous section made different predictions, depending on their different combinations of old and new strands of DNA. But how can you tell which strands are old and which ones are new?
American geneticists Matthew Meselson and Franklin Stahl devised a way for us to tell. Inspired by previous experiments in which molecules were intelligently labeled to differentiate them (as was done with sulfur and phosphorus in the Great Kitchen Blender Experiments), Meselson and Stahl thought of a new way to label old and new DNA strands.
The organism they used for their experiment was the E. Coli. The element they chose to use as a label were two nitrogen isotopes, 15N and 14N, the former heavier than the latter. Since DNA is partly made of nitrogen, in order to make more of itself it would feed on whichever isotope was present when it replicated.
So they started by growing E. Coli in medium containing 15N. They did this long enough for all the DNA to be made only of the heavy isotope.
Then they switched them to a medium containing 14N, and took samples from each successive generation of E. Coli.
Every sample they took, they centrifuged. This caused the heavier elements to go nearer the bottom of the test tube, and lighter elements to go nearer the top. That's because the solution in which they poured the samples was cesium chloride, which acted as a density gradient, with heavier elements migrating nearer the bottom end and staying there, and lighter elements migrating nearer the top end.
Knowing all this, what would your predictions be depending on which DNA replication model was right?
Let's see what happened. This is the generation 0 test tube, when the E. Coli DNA is made up entirely of 15N:
Source: OpenStax Biology, modified
There's only 15N, and so it forms a single band, as was expected.
This is the sample taken after one generation, the Gen 1 sample if you will:
Source: OpenStax Biology, modified
Even from the get-go first generation, Meselson and Stahl were able to reject the conservative model of replication. That's because, if the old strand pairs remained intact, and completely new pairs (with 14N) were made from them (as the conservative model entailed), we would expect to see two distinct bands, not one. The one single band we see indicates hybrids, and that indicates old strands combined with new ones, half 14N, half 15N, intertwined so they take the halfway position between where 14N and 15N would be if they were separate. This proves that the conservative model is wrong. (Sorry Republicans!)
That's all very good, but we still haven't decided between the semi-conservative and dispersive models. That's done in the further generations.
Source: OpenStax Biology, modified
In the second gen we see two distinct bands, indicating that the strands have separated and half of them are old paired with new 14N, so they occupy the previous level, and half are new paired with new 14N, in other words 100% 14N, so they occupy the 14N level. If the dispersive model was correct, we would still observe a single band moving up toward the 14N position, since all the DNA would be made of pieces of old and new DNA combined, so they would move in tandem.
As the generations progress, we continue to see distinct bands, as the new strands compose themselves of 14N and become more numerous. If the dispersive model was the correct model, we would see a single band moving further up the gradient. Therefore, the dispersive model is clearly out of the picture.
And there you have it folks, the elegantly simple and hence beautiful experiment by Meselson and Stahl, which you can appreciate even further in this interactive animation.
As explained here, the concept of "beautiful", when describing experiments or proofs in science, refers to simplicity, creativity, economy, imaginitiveness, and elegance. The Meselson-Stahl experiment exemplifies all of those nouns, so it can certainly be called beautiful (though I personally wouldn't call it the most beautiful, since I prefer some of the experiments described here and in future posts).
Outro
The elephant-ass, a Republican-Democrat semi-conservative combo.
Source: Pixabay
Though many at the time saw it as speculation, the Meselson-Stahl experiment helped to solidify the Watson-Crick double-helix as the correct model for DNA's structure.[9]
Following Meselson and Stahl's experiment, other scientists were able to show the same results in every other organism they studied. To this day, semi-conservative replication is the only kind of replication that is found.[6] Be you Republican or Democrat, your DNA falls right in the middle, choosing to replicate semi-conservatively!
REFERENCES
1. J. D. WATSON & F. H. C. CRICK. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid. Nature 171, 737–738 (25 April 1953). https://www.genome.gov/edkit/pdfs/1953.pdf
2. Biology's Most Beautiful https://www.aibs.org/about-aibs/030712_take_the_bioscience_challenge.html
3. Wikipedia contributors, "Chargaff's rules," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Chargaff%27s_rules&oldid=812880199 (accessed January 23, 2018).
4. OpenStax, Biology. OpenStax CNX. Dec 19, 2017 http://cnx.org/contents/[email protected].
5. Wikipedia contributors, "Semiconservative replication," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Semiconservative_replication&oldid=814500625 (accessed January 23, 2018).
6. Pray, L. (2008) Semi-conservative DNA replication: Meselson and Stahl. Nature Education 1(1):98 https://www.nature.com/scitable/topicpage/semi-conservative-dna-replication-meselson-and-stahl-421
7. Ian Glynn. Elegance in Science: The Beauty of Simplicity. https://www.huffingtonpost.com/ian-glynn/elegance-in-science_b_2819612.html
8. Meselson M, Stahl FW. THE REPLICATION OF DNA IN ESCHERICHIA COLI. Proceedings of the National Academy of Sciences of the United States of America. 1958;44(7):671-682. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC528642/
9. Tinsley H. Davis. Meselson and Stahl: The art of DNA replication. PNAS December 28, 2004
vol. 101 no. 52. http://www.pnas.org/content/101/52/17895.full
Earlier Introduction to Biology episodes:
10: The Great GATC-by: The Most Famous Science Paper of the 20th Century
9: The Great Kitchen Blender Experiments: How DNA was proved to be the seat of heredity
8: Finding, Counting, and Ordering Genes Using Incredibly Sophisticated Biomolecular Megatechnology
7: Christmas Disease — Yes, it's real, 100% scientifically proven!
6: The Most Famous All-Nighter in the History of Genetics
5: Mendel's Lucky Number Seven — The law of genetics that almost wasn't
4: How Cells Use Logic To Do The Impossible
3 : Armchair Science — The Discovery of Proteins' Secondary Structure
2 : How Cell Membranes Form Spontaneously
1 : Eduard Buchner: The Man Who Killed Vitalism
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