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history of DNA

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BIOL 2030

February 3rd (Lecture 1) Milestones in the history of DNA SEE SLIDE 10 (BENTZEN 10) Although the scientific community has been aware of DNA for more than 150 years, it is only in the last 60 years that DNA was identified as playing a key role in our genetic information Mid 1980's -- when the power of DNA analysis for identification purposes was revealed The first use of DNA testing for human identification anywhere in the world was in the UK as part of a non-clinical case in 1985, Sarbah vs. The home office, 1985 (the Ghana immigration case). In this case DNA testing was used to prove the mother-son relationship between Christiana Sarbah and her son Andrew All started in the 1860s when Mendel performed a set of experiments that pointed to the existence of biological elements called genes Johann Friedrich Mischer in 1869 while experimenting on pus cells noted the presence of something that "cannot belong among any of the protein substances". He also showed that the new substance was derived from the nucleus of the cell alone and consequently named it "nuclein". Later renamed DNA Finding was important because it was instrumental in disproving the current theory that all cells were made up of proteins Protein was favoured to be the genetic material Proteins were abundant in cells Proteins had sufficient complexity to convey genetic information We will begin by looking at how scientists discovered the nature and structure of the genetic material These discoveries led to an explosion of knowledge about molecule aspect of biology Some of the major milestone are outlines here from the time Mendel first made his observations to at least a hundred years later when the structure of DNA was solved. A lot of scientists were involved in the long process to establish DNA as the genetic material 1865 Mendel presents his results 1869 DNA isolated by Meisner 1888 Chromosomes first seen 1903 Chromosome theory of heredity 1900 Mendel rediscovered 1. DNA as the source of genetic information Characteristics of genetic material For a molecule to serve as the genetic material, it must be able to Replicate accurately Store large amounts of information Allow for phenotypic variation (traits) Historical background: Discovery of DNA Johann Meischer: Isolated a weakly acidic substance, "nuclein", from nuclei of human white blood cells. Later renamed nucleic acid Nuclein couldn't be protein, because it lacked sulfur but had lots of phosphorous and nitrogen. At the time, the popular theory was that cells were made up only of proteins Frederick Griffith: Demonstrated that cells could be transformed Transformation: uptake of genetic material from an external source resulting in new traits Griffith's experiment SEE SLIDES 15,16 (BENTZEN 1) Frederick Griffiths was a British bacteriologist who worked with bacteria that cause pneumonia Two trains of a pneumonia causing bacteria, rough (R) and smooth (S) Rough: Type R bacteria had a rough appearance in culture, non virulent (doesn't kill) Smooth: Bacteria had a smooth appearance in culture, virulent (kills) The capsule alone did not cause pneumonia Heat killed S strain was avirulent Ability to escape immune detection and multiply When heat-killed strain was mixed with living R- strain --> the mouse dies of pneumonia Encapsulated strain (S) recovered from dead mouse --> now a live strain The R strain had somehow acquired the ability to produce the polysaccharide capsule Transformation Ability to produce coat was an inherited trait --> daughter cells also produced capsule Performed several experiments whose results can be summarized as follows: Mouse + live S = died, autopsy revealed infestation of S strain bacteria Mouse + live R = lived a happy life These 2 experiments illustrate the effects of normal R and S strains Mouse + heat-killed S --> lived a happy life Experiments illustrate that dead bacteria by themselves are harmless Mouse + heat-killed S + live R = died. Autopsy revealed infestation of S strain Apparently something from the heat-killed S changed the live R to make them virulent - this was called transformation Griffith didn't know what is was that transformed the R strain into the S strain, but he demonstrated that it could be done The nonvirulent R cells had somehow acquired the ability to cause the disease The nonvirulent R cells have been transformed A "transforming factor" is present in the heat killed virulent S cells Substance responsible.. Is taken up by the R strain Causes a permanent genetic change (heritable) What is the transforming principle? Historical background: DNA carries genetic information Oswald Avery, MacLeod and McCarthy: the first demonstration that DNA was the genetic material Avery, MacLeod and McCarthy's experiment SEE SLIDES 19,20 (BENTZEN 1) Simple but effective experiment Performed the experiment similar to Griffiths, only they used tissue cultures growing in petri dishes Heat-killed S + live R + protease = Live R and S
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