-By early 1940's it had been established that DNA was the genetic material. From about 1940 to 1953 there was a race to find out what the structure of DNA was. In 1953 - Watson and Crick proposed their model of the DNA double helix.
- (6:00) They worked with two pieces of evidence that allowed them to come up with this structure (Done by other scientists)
- 1st piece of evidence: Chargaff - Base composition studies. Take DNA from an organism and analyzed the % of each of the 4 nitrogenous bases. Findings were that percentage of A and T was always equal.
- 2nd piece of evidence: Maurice Wilins and Rosiland Franklin: exposing DNA to x-rays to attempt to get an x-ray picture of DNA.
- it appeared that the DNA had a consistent width (3.4 angstroms) and it looked like a series of coins stacked on eath other.
- This suggested that DNA had a helical shape
- DNA has a 5' end and a 3' end. Our strands run anti-parallel
- (19:10) The outside of the double helix is the sugar - phosphate backbone.
- nitrogenous bases lie on the inside of the double helix like steps on a ladder. The distance between the steps is consistent (3.4 angstroms).
- It takes 10 Base pairs to go one turn around the double helix. Therefore one turn equals 34 angstroms
- Within the double helix A pairs with T and G pairs with C (complimentary - comes from work done by Chargaff). This also means that you always have a purine pairing wiht a pyrimidine. This leads to a double helix with a consistent diameter (20 angstroms). The base pairs are held together by hydrogen bonds. A-T=2H bonds . . . G-C = 3 H bonds
- overall hydrogen bonds are the weakest of the bonds.
- (28:50) Sugar Phosphate backbone is hydrophilic. The nitrogenous bases are hydrophobic
- Grooves in the structure - places where there are openings - each groove represents 5 Base pairs (major groove is larger opening and minor groove is smaller opening)
- The DNA double helix is a right handed helix
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