HERE'S...FOLKS.
HERE'S an interesting idea: It was once believed that if you would leave out food to rot, living creatures like maggots and rats would poof into existence. Someone forgot to eat their celery? *POOF*A maggot! Some grains spoilt? *POOF* A rat! The burrito in your lunchbox lies forgotten? *POOF* Another maggot! Isn't that nice! Sadly, it was proven false- Yep, the Latin phrase in your textbooks- "Omne Vivum Ex Vivo"- was proposed. Our Awesome Friend Charles Darwin hypothesised that well, simple creatures can evolve into complex creatures under the right conditions. A special shoutout to Russian biochemist Alexander Oparin, who conceived the idea of progression from simple chemistry to cells. He imagined the early oceans of Earth as a primordial soup- basically, its fancy speak for a large number (by large I mean huuuge) of molecules floating around having the time of their life. Chemical reactions could take place in this soup and that would eventually lead to the formation of living cells. J.B.S Haldane, a brilliant polymath, was one of the founders of the theory on the chemical origin of life, and independently came up with the same idea(s). But there, my friends, was where the problem lay. There was practically no way to verify any of these theories until a very important someone arrived.
1952. Quite a lot of things happened that year, don't you think? An edition of the Summer Olympics, Elizabeth II was proclaimed Queen of the United Kingdom, the first time a woman scored a number 1 hit on the American country chart... the list goes on. Biology, in particular had a nice, well-deserved cool dude emote (I'm just kidding around). In all seriousness, it was a spectacular year. The astute among you would have recognised it as the year Photo 51 was clicked (go check out the previous post!). Before proceeding onto the 70s and onward, I would want to particularly look at another key experiment- which gave us insights into the origins of life.
Earth had a... CRAZY COOL beginning, if you ask me. After planetary accretion, it was an extremely chaotic place- it's literally called the Hadean Eon- after the Greek God of the underworld, Hades, and the underworld itself. It accurately describes the hellish conditions on Earth- the planet had just formed, there was an abundance of radioactive elements, and matter from space constantly bombarded Earth¹. After that, the Archean Eon set in (Don't worry, I'm gonna gloss over that bit for now.) By this eon, water had formed and the ancient water cycle- evaporation, condensation and precipitation- was set in motion².
Back to the problem statement: unless you had a time machine, or some fossils of that era- both virtually impossible- we had no idea how life came to be.
In September 1951, a young man arrived at the University of Chicago for his graduate studies. Due to financial constraints, he needed funding support and this could only be done through teaching assistantships. The young man shot out an application to the University, and was overjoyed when he was accepted. Apart from enrolling into the required courses, Stanley Miller started to look for a possible thesis project. Stanley was originally working on how elements were synthesised by stars. During his first semester in the fall of 1951, Stanley attended a seminar in which the Nobel laureate and chemistry professor Harold Urey presented his ideas about the origin of the solar system and the chemical events associated with this process. Stanley was super super intrigued, but stuck to his PhD topic. The man who was supervising his thesis suddenly announced he was leaving Chicago to start a weapons laboratory. Stanley was then left to search for a new thesis ³.
Miller in the lab with his experimental setup.
Stanley began to think about Urey's talk, and approached Urey⁴ about the possibility of doing a prebiotic synthesis experiment- Urey at first was not too encouraging, feeling that Stanley was biting off more than he could chew, but relented. Miller came up with an amazing idea- he could simulate early Earth conditions in a lab and observe what happens. In 1952, Miller, together with Urey, designed an apparatus to simulate the water cycle. The water was gently boiled to represent evaporation, gases like methane, hydrogen and ammonia were chosen to mimic the atmosphere. They added a condenser to cool the atmosphere, allowing water molecules to condense and fall back as precipitation. The whole setup- to model Earth, would have required a source of energy, like the Sun was for the Earth. They induced sparks in the atmosphere to emulate lightning. Within a week, their "ocean" had turned a queer brownish black. Analysis revealed that many complex molecules had been generated- among these were (the reason I brought this up) AMINO ACIDS! Which, as we know are pretty fundamental to life. Miller's⁵ stroke of genius gave rise to a whole new field of science, called Prebiotic Chemistry!
Another notable experiment is the Hershey-Chase Experiment, performed by Alfred Hershey and Martha Chase (and yes, this *also* happened in 1952)⁶. Many scientists still assumed at the time that proteins carried information for inheritance even though DNA was discovered, because DNA appeared to be an inert molecule, and since it's located in the nucleus, its role was assumed to be phosphorus storage. In their experiment, Hershey-Chase showed that when bacteriophages (viruses that infect bacteria, composed of DNA and protein) infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not.
Martha Chase and Alfred Hershey.
Hershey and Chase needed to be able to examine different parts of the phages they were studying separately, so they needed to distinguish the phage subsections. Viruses were known to be composed of a protein shell and DNA, so they chose to uniquely label each with a different elemental isotope. This allowed each to be observed and analysed separately. Since phosphorus is contained in DNA but not amino acids, radioactive phosphorus was used to label the DNA contained in the T2 phage (it's a phage that affects E. coli- and chances are you've heard of E. coli a bunch of times so I'm not detailing it here.) Radioactive sulfur was used to label the protein sections of the T2 phage, because sulfur is contained in protein but not DNA. Hershey and Chase inserted the radioactive elements in the bacteriophages by adding the isotopes to separate media within which bacteria were allowed to grow before bacteriophage introduction.
An example of a phage under an electron microscope. Fascinating!
When the bacteriophages infected the bacteria, the progeny contained the radioactive isotopes in their structures. The labeled progeny were then allowed to infect unlabeled bacteria. The phage coats remained on the outside of the bacteria, while genetic material entered. Disruption of phage from the bacteria by agitation in a blender followed by centrifugation allowed the separation of the phage coats from the bacteria. Bacteria which were infected by radioactive (DNA) phages were radioactive but the ones that were infected by radioactive protein viruses were non-radioactive. Resultant radioactive and non-radioactive bacteria infer that the viruses that had radioactive DNA transferred their DNA to the bacteria but viruses that had radioactive protein didn’t get transferred to the bacteria. Thus, the Hershey–Chase experiment helped to confirm that DNA, not protein, is the genetic material.
That was a mouthful.
Anyway, this brings us to the conclusion of partie-3 of our conquest into the history of genetics! Peace out folks.
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¹ I'm probably gonna make a series starring the origins of life and life on Earth, so look out for that! This would be detailed there :D
² Fun fact: the water you're drinking right now might have once been dinosaur pee. You're welcome :)
³ The guy leaving was probably a blessing in disguise for Stanley, since the origin of the elements were soon to be detailed by Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle- all of whom did pathbreaking work.
⁴Urey had actually won the Nobel a couple of years ago, for his discovery of deuterium and work on isotopes.
⁵Miller was actually nominated for a Nobel more than once in his life (which is a huge achievement in itself). Sadly, he didn't win it.
⁶Regrettably, I haven't covered the Avery-MacLeod-McCarty Experiment (1944) in detail. To the enthusiasts, I recommend Google-ing it out. Interesting experiment involving bacterial transformation and other cool stuff.
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