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1

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
- i.e. IIS, IIIS, IIR, IIIR
* IIS and IIR can mutate between, and so can IIIS and IIIR, but never a type II to a type III or vice versa.

2

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'.

3

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.

4

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.

5

What is TMV?

Tobacco Mosaic Virus

6

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.

7

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.

8

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.

9

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.

10

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.

11

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