By Riya Gohil
The Most Beautiful Experiment in Biology
One of my favorite analogies of learning is the expanding circle metaphor — if a circle represents your knowledge, then as your circle of knowledge expands, so does your circumference of darkness. I think we often get caught up in the complexity of science – as we learn more about how things work, we’re often left with more questions than we had when we started. Even still, it amazes me how simple some answers to scientific questions can be, and it amazes me even more how we use our limited (yet ever expanding) knowledge to find solutions to the mysteries of the universe.
As a genomicist, I’m partial to the beauty and mysteries of DNA. DNA (or deoxyribonucleic acid) is the hereditary molecule that is passed down between generations. It serves as a template, encoding instructions for the development, growth, and function of every living organism (and even nonliving agents like viruses!). DNA is a double helix consisting of two strands that coil around each other. Each strand is made up of a sugar-phosphate backbone with four bases (A – adenine, G – guanine, T – thymine, and C – cytosine). Rosalind Franklin, James Watson, and Francis Crick revealed the double helical structure of DNA in 1953, nearly seventy years ago, leading to a Nobel Prize in Physiology or Medicine.
After the discovery of the DNA structure, the next question in the field was how more DNA is made. For instance, processes like cell division and reproduction need to duplicate DNA to pass on the same genetic information to a new cell. This brings us to the most beautiful experiment in the field of biology – the Meselson and Stahl DNA replication experiment. At the time, there were three hypotheses in the field about how DNA replication occurred: the semi-conservative model, the conservative model, and the dispersive model.
The semi-conservative model (proposed by Watson and Crick) suggests that the two strands of DNA are unwound and each strand is used as a template to make a new strand. In this way, each molecule of DNA is always composed of one template strand and one newly synthesized strand, retaining a sense of “memory” of the original template. The conservative model suggests that the entire double helical DNA is used as a template to generate a new DNA molecule. This model keeps the original DNA molecule intact and produces a completely new DNA helix (i.e., each double strand is newly synthesized). Finally, the dispersive model suggests that DNA is replicated in ‘batches’. This model posits that DNA replication occurs in many short pieces across the molecule, alternating between the template and newly synthesized strands.
To test which hypothesis more accurately models DNA replication, Meselson and Stahl took advantage of the structure of DNA. Each of the four bases are composed of nitrogen atoms. Meselson and Stahl grew E. coli in isotope N15 (a form of the nitrogen atom that has an extra neutron, which increases its atomic mass compared to naturally occurring N14) to allow the bacterial cells to incorporate N15 into their DNA. After growing the E. coli in N15 for a few generations, they started their experiment. They switched the media they were growing the E. coli in to contain the N14 isotope. Thus, as new DNA is synthesized, N14 would now be incorporated into the new DNA instead of N15. In this way, they could track the original template DNA from generation 0 (labelled with N15) and the newly synthesized DNA from generation 1 (labelled with N14). They tracked the levels of N15 vs. N14 in DNA replication over a few generations to deduce which model their results followed. N15 and N14 have different densities, so when DNA is put in a CsCl gradient (a density gradient that allows separation of molecules based on density alone), one can observe a distinct band of either N15 or N14 isotope in a test tube.
Meselson and Stahl found that after 1 generation, there was a single band between the N15 and N14 densities. This suggested that after 1 generation of DNA replication, the resultant DNA consisted of both N15 and N14 labeled strands. This result aligned with either the semi-conservative or the dispersive model. After 2 generations, they found two bands at equal intensity: one at N14 and one between N15 and N14. This suggested that one DNA molecule is composed of newly synthesized with N14 at both strands, while one DNA molecule is still composed of N14 and N15 strands. This result contradicted the dispersive model and supported the semi-conservative model. Meselson and Stahl continued this experiment for a few more generations and calculated the ratio of N14 and N15 levels to ensure that the ratios they witnessed fit the semi-conservative model, which it indeed did.
Many scientists dubbed this experiment to be the most beautiful experiment in biology due to the simplicity in its design and abundantly clear results. It is rare that the results from an experiment almost flawlessly fit a hypothesis – oftentimes, we find that our experiments are telling us that something more complex than we originally thought is at play. The Meselson and Stahl experiment provided a physical basis for how DNA is copied, passed on to new cells, and how mutations can arise in this mode of replication. DNA replication is a field that is still being heavily studied. Understanding the mode of DNA replication led scientists to ask what physical units/proteins are engaged during this process, what their specific roles are, and what happens when they malfunction. We’ve learned that a lot of genetic diseases arise from malfunctions in DNA replication and this has prompted the scientific community to expand to understand why errors occur and how we can therapeutically fix these errors. As our knowledge of DNA replication builds, our ability to understand what else we don’t know also expands and allows us to further dive into the ‘darkness’ in hopes to shine a light on it.
