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Griffith experiment (1928) - Expt 1

* Determined that DNA is the genetic material.
* Studied two strains:
- S-strains (smooth), virulent, kill mice
- R-strains (rough), non-virulent, mice live.
- R and S refer to the polysaccharide coat.
* Two types of the strains; Type II and Type III
* IIS and IIR can mutate between, and so can IIIS and IIIR, but never a type II to a type III or vice versa.


Transformation of IIR and heat-killed IIIS

* Injecting individually into mice did not kill them, but injecting both together did. IIIS were recovered from dead mice. Some heat-stable component present in IIIS transformed IIR into IIIS. Griffith didn't know what this was, so he called it the 'Transformation Principle'.


Avery, MacLeod and McCarty (1944) - Expt 2

* Heat killed S cell components added to R cells, transformation occurred.
* RNAse added to heat-killed S cell components - transformation of R cells into S cells occurred.
* Proteinases added to heat-killed S cell components - transformation of R cells occurred.
* DNAses added to heat-killed S cell components - no transformation of R cells occurred.
* Conclusion - DNA is responsible for the transformation of R cells into S cells.


Hershey and Chase (1953) - Expt 3

* Used radioactivity to independently label either the DNA or the proteins of bacteriophages.
- Took advantage of the phosphorous (only present in DNA) and sulphur (only present in proteins).
* End up with a pool of phages, some labelled with radioactive phosphorous (32P), some labelled with radioactive sulphur (35S).
* Used them to infect the EColi which were not labelled at all, so the only source of labelling was going to be the phage.
- Separate phage shells (ghosts) and DNA-infected bacteria by centrifugation - ghosts end up in supernatant, DNA ended up in pellets.
* Most Phosphorous found in pellet with cells, and most sulphur found in supernatant with phage ghosts, confirming that only DNA enters cells.
* Took it one step further and allowed the phages to replicate in the bacterial cells - new cells harvested and examined.
* Small amounts of Phosphorous found in new phages, but absolutely no Sulphur.
* Therefore, DNA being passed onto progeny, NOT protein.


What is TMV?

Tobacco Mosaic Virus


Gierer and Schram (1956)

* Purified RNA from TMV
* Injected TMV RNA into tobacco leaves = lesions
* Digested TMV RNA with RNAse = no lesions
* Therefore, RNA was genetic material in TMV.


Fraenkel-Conrat and Singer (1957)

* Did additional TMV work - isolated two stocks of TMV that possessed different protein coats.
* Coated RNA A in protein B, and got Type A progeny
* Coated RNA B in protein A, and got Type B progeny
* Conclusion - RNA always determined the end progeny type, not the protein.


Double helix structure of DNA proposed by Watson and Crick (1953), based on what two pieces of information?

1) 'Chargaff's rules', where Edwin Chargaff showed that A and T bases are present in essentially equal amounts, and G and C are also present in essentially equal amounts - the A/T and C/G ratios could differ between species though.
2) X-Ray diffraction studies by Rosalind Franklin and Maurice Wilkins, who took pictures that showed that the distance between bases was 0.34nm, and the distance between helical turns was 3.4nm.


List and describe the six features of the DNA double helix model described by Watson and Crick.

1. DNA is a right-handed double-helix. Viewed from the top, the helix winds clockwise.
2. Two chains are antiparallel
3. Sugar-phosphate backbone on the outside, bases on the inside.
4. Bases on opposite strands joined by hydrogen bonding - relatively weak - A joins with T, G
joins with C
5. Base pairs are 0.34 nm apart, a complete
turn takes 3.4 nm (=10 bp)
6. Sugar-phosphate backbone not equally
spaced - major and minor grooves i.e. 'space-filling model' - grooves permit proteins to make contact with the bases of DNA.


What is the 'space-filling model'?

Refers to the major and minor grooves of the DNA, which permit proteins to make contact with the bases of the DNA.


The 3 main experiments that established DNA as the genetic material?

1. Griffith’s Transformation Experiment
2. Avery’s Transformation Experiment
3. Hershey‐Chase Bacteriophage Experiment