Lecture 5- DNA and Heredity

Lecture 5- DNA and Heredity - DNA: Discovery and...

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DNA: Discovery and Replication By the turn of the century it was clear that the mysterious essence from males and females that combined in the embryo and determined the inherited traits of their offspring was not something otherworldly but rather a physical substance. In fact, with the help of a microscope, you could actually see it. Just watch a cell in mitosis. You can see the chromosomes, the carriers of heredity, segregate to each cell, bringing with them the genotype of the daughter cells. With the discovery that the genetic material was in the chromosomes came the realization that it might be possible to discover the chemical nature of the genetic material just as it had been possible to discover the chemical nature of many cell components. In other words, it might be possible to purify a gene. To do so you needed two things: 1) a method of isolating cell components, and 2) an assay for genetic materialness. What about genetic material. As it happens the first problem, isolating cell components had already been solved in 1868 by Friedrich Miescher. He figured out a way to isolate the nuclei from cells found in pus and found that the main constituent of the nucleus was a compound he called nuclein. Nuclein turned out to be DNA. Thus nuclein became a candidate for being the genetic material but not the only candidate. After all, chromatin contains lots of protein too. Deciding which was the genetic material required an assay. This was a trickier problem. Ideally you would like to be able to take the candidate genetic material (protein or DNA or carbohydrate or whatever you think might be the genetic material) from, for instance, a round pea and and give it to a wrinkled pea plant and show that the wrinkled pea now became round. But you can't expect to just dip the wrinkled pea in round pea genetic material and expect it to become a round pea, can you? In general, "No". But in specific cases the answer is, amazingly, yes. Frederick Griffith was interested in developing a vaccine to Streptococcus pneumoniae so he spent a lot of time injecting live or heat-killed S. pneumoniae bugs into mice. He could culture the bugs on a petri plate too and worked on two strains S (smooth) and R (rough). In addition to having smooth and rough colonies on petri plates, the smooth bugs were virulent, they killed the mice and the rough bugs were nonvirulent, the mice killed them. Of course heat killed S bugs were dead so they did not kill the mice. For some reason Griffith decided it would be a good idea to inject heat-killed S-bugs along with live R-bugs. This killed the mice dead, just as if he injected live S-bugs. In fact, when he looked in the dead mice that's what he found, live S-bugs. The R-bugs had been transformed into S-bugs.
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Lecture 5- DNA and Heredity - DNA: Discovery and...

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