DNA Flashcards
Who was Friedrich Miescher?
- Was a chemist interested in biology. Wrote a paper on the “chemical composition of cells” and discovered NUCLEIN
What is nuclein?
Any of the substances present in the nucleus of a cell, consisting mostly of proteins, phosphoric acids, and nucleic acids.
How did Miescher purify DNA?
Started with pus which was rich in white blood cells.
Treated cells with HCl to lyse open the cells.
Removed lipids (phospholipid bilayer) with ether.
Dissolved the remains in an alkaline solution.
Obtained a DNA precipitate with acid - nuclein, which we now know as DNA and RNA.
What was nuclein originally thought to be?
A store of phosophorus in the cell, as knew it contained phosphorus.
Also speculation that it might contain hereditary material.
Why did people suspect that proteins were the hereditary material?
Showed greater potential for diversity as there are 20 amino acids as opposed to 4 nucleotides.
What had been shown by early 1900s in terms of DNA?
DNA was nuclear.
DNA was a component of chromosomes.
Didn’t know that it contained genes.
What did Fred Griffith prove with his experiments?
1928 - Griffith’s experiment.
Demonstrated bacterial transformation, where a bacterium distinctly changed its form and function.
Knew there was a transforming factor, later shown to be DNA.
What was the Griffith’s experiment?
Interested in child pneumonia. Studied both rough (harmless) and smooth (pathogenic) strains.
1928. Gave mice the smooth version - dead. Heat treated smooth - alive. Rough version - alive. Rough + heat treated smooth - dead.
What is transformation?
The ability of a substance to change the genetic characteristics of an organism.
What did Avery, MacLeod and McCarty show in 1944?
The results of Griffith’s experiments in a different way.
Combined rough bacteria with:
Smooth protein - alive.
Smooth carbohydrate - alive.
Smooth DNA - dead
Used enzymes to rule out contamination by proteins:
Destroyed protein - dead.
Destroyed DNA - alive.
Showed that DNA was RESPONSIBLE for transformation.
How was the pathogenic bacterium created?
The DNA which makes the smooth capsule (allows it to be infectious) is transferred to another bacterium.
What did Martha Chase and Alfred Hershey show?
Repeated the Avery experiment with different methodology.
Infected bacterial cells with bacteriophages.
The protein in the bacteriophages were radioactively labelled with sulphur. Bacteriophages after infection still contained almost all of the radioactive sulphur.
Labelled nucleic acids with phosphorus. Showed that the bacteriophages lacked radioactive phosphorus after infection.
Showed that only the bacteriophage DNA enters the cell, and that this is the transforming factor.
What did Rosalind Franklin show?
X-Ray diffraction photos of DNA showed that the molecule was spiral shaped or helical.
The diameter was roughly twice the width of a single nucleotide, suggesting the DNA molecule might be composed of two side by side DNA chains.
Crick & Watson?
Suggested that their structure of DNA provides answers for the copying mechanism of DNA.
Predicts semi-conservative.
One purine with one pyrimidine.
“it has not escaped our notice that the specific pairing we have postulated immediately suggests a copying mechanism for the genetic material”
Meselson and Stahl experiment?
Used nitrogen isotopes.
Grew E.Coli in heavy (N15) and then transferred into normal (N14).
Caesium Chloride forms a density gradient at low speeds.
