Episode 28 recAPs some of the great scientific minds of history.
Episode 28 recAPs some of the great scientific minds of history. Atoms are the building blocks of matter, with work by greats such as Dalton, Rutherford, and Bohr (1:08). Gregor Mendel is now renowned as the father of heredity, but began as a pea farmer (2:30). DNA was identified as the molecule of heredity by comparing chemical properties through experiments by Griffith, Avery, Hershey and Chase (3:28). You likely remember Watson and Crick’s double helix model, but what about Franklin and Chargaff? (4:30). No episode on scientists is complete without mentioning Charles Darwin and the theory of evolution by natural selection (5:37).
The Question of the Day asks (7:03) “What mainland South American country is closest to the Galapagos Islands? ”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Scientists!
Lets Zoom out:
Many brilliant minds have contributed to our understanding of the natural world. While each discovery and breakthrough often leads to more questions than answers (such is the scientific process), each scientist built on the experiments of the past. Not all discoveries are happy stories and not everyone’s work is accepted during their lifetime. You do not need to memorize discovery dates and specific experiments for the AP Exam - but appreciate the how and why behind the content. So here’s a nod to the past for our scientists of the future.
Lets Zoom in:
Where else could we start but with the building block of all matter - the atom. The original question posed by ancient philosophers, including Democritus and Aristotle - What is stuff made of? In fact, the word atom comes from the greek word for “indivisible.” Thousands of years later, the Quaker thinker Dalton proved that common substances always broke down into elemental proportions. Atomic theory was now accepted by the broader scientific community. Thomoson later won a nobel prize for electron discovery and a chocolate chip cookie model of the atom (where protons are the dough and electrons the chips). This model was quickly discarded however, by the work of Thompsons own students - Rutherford and Bohr. Rutherford’s gold foil experiment showed that most of the atom’s mass was concentrated in the center (termed nucleus), with only a few electrons spread out. Bohr went even further, stating that electrons orbit the nucleus at fixed distances and energies - a planetary model. Later, Heisenbeg debunked Bohr’s theory with his uncertainty principle, stating that the exact location and speed of an electron cannot be determined at the same time. While our understanding of atoms is continuously changing (we are currently on the quantum model), the fact that atoms are the building blocks for all matter - has not. In AP Biology, we will primarily follow the electrons, as bonds break and form.
The father of heredity - Gregor Mendel. He was an Augistian monk and farmer in Austria. A scientist at heart, Mendel began to perform experiments with pea plants, monitoring traits such as flower color, pea shape, plant height etc. Peas were a great choice of study because many of their traits are determined by a single gene, they can self-fertilize, reproduce quickly, and produce many offspring. The patterns he noticed paved the way for modern genetics. Three important conclusions
The peas inherited a pair of genetic instructions (he called them factors, we know them as alleles). Mendel also noticed that some alleles were stronger or more dominant than others, recessive. Lastly, the passing of traits was consistently random. Mendel summarized these findings as the Law of segregation. By crossing plants and following two traits at once, Mendel developed the law of independent assortment. The passing on of each trait did not influence any other.
On to DNA Discovery - The first challenge was to figure out what was the molecule of heredity. Most of the toss up fell between protein and DNA. Griffith did his experiment injecting mice with bacteria. His transforming principle got so far as to say that something from the heat-killed bacteria “transformed” the rough bacteria and made them lethal. Avery took his work a step farther to determine what the transforming molecule was. Through chemical tests observing the ratios of nitrogen to phosphorus, Avery showed that the chemical composition of the molecule matched DNA, not protein. In addition, adding enzymes that break down DNA made the transforming principle inactive. The addition of enzymes that break down proteins had no effect. Hershey and Chase then came onto the scene with bacteriophages. A bacteriophage is a spacecraft shaped virus made of protein and DNA. They tagged the protein with radioactive Sulfur and the DNA with phosphorus. Results showed that only the radioactive phosphorus had entered the bacteria, again confirming that DNA and not protein was the molecule of heredity.
Ok - so DNA is in charge - but what does it look like? What is its structure? Much of the prevailing minds couldn't fathom that the most important part of the molecule, the nitrogenous bases - could be tucked away. Chargaff provided a valuable piece of data. Known as Chargaff’s rule, the relative amount of adenine and thymine was always equal in a DNA sample. Same for guanine and cytosine. Another valuable clue was Franklin’s photo 51 - produced by X-Ray crystallography, it showed a repetitive diffraction pattern. There was disagreement in the scientific community whether DNA was a single, double or triple helix (see the work of Pauling and Wilkins) Watson and Crick unified the ideas, noting that A and T bond with two hydrogen bonds and C and G fit with three hydrogen bonds, all forming a double helix structure. Watson, Crick and Wilkins (Franklin’s colleague), earned a Nobel Prize for this discovery. This sounds like a happy ending, but Franklin’s work had been shown to Watson and Crick without her awareness. She also could not win the Nobel prize, having passed away from cancer, likely due to her research methods (5:37).
Closing up with our friend Chucky D. I know - evolution is not a unit of study on the 2020 exam - but evolution is one of the four big ideas. So let’s briefly recap the work of Charles Darwin and his vacation to the Galapagos Islands. Darwin was an English naturalist who collected multiple animal species on his five year trip aboard the Beagle as it mapped the coast of South America. Darwin noticed variation of characteristics between different tortoises and finches. He found fossils of extinct animals that were similar to modern ones and found fossil shells in the mountains. Building upon the work of other scientists who had made observations about the natural world (Linneaus, Buffon, Lamark, Lyell, Malthus, Hutton, Cuvier), he came to the assumption that species are sumhow adapted to their environment. We know this now as the theory of Evolution by Natural Selection. Spoiler alert - Darwin wasn’t the first to propose this theory, but he was the first to publish, and sometimes that’s all it takes (sorry Wallace).
To recap….
Atoms, pea plants, DNA and Darwin - there have been many steps to discovery in the history of science. Focus on the thematic understanding and purpose behind the discoveries. How does their work relate to what we know now? Remember, Watson was only 25 when he had his breakthrough DNA discovery. Your Nobel Prize might be right around the corner!
Today’s Question of the day is about geography.
Question. What mainland South American country is closest to the Galapagos Islands?