DNA

Question 1

Mendel - 19th century

Answer 1

indicated the genetic material is contained in cells

Question 2

Frederick Griffith - 1928

Answer 2

• demonstrated the transmission of genetic instructions by a process we now call the ‘transmission principle’

Question 3

Avery, MacLeod and McCarty - 1944

Answer 3

suggest DNA is the ‘transforming factor’ and not proteins or other material

Question 4

Hershey and Chase - 1952

Answer 4

proved that DNA was the genetic material in bacteriophage

Question 5

Frederick Griffith experiment

Answer 5

• the first experiment demonstrating that molecules can transfer genetic info from one organism to another
• vaccine for pneumonia - heat killed microorganisms, could be injected into patients to elicit the immune response of live cells, without the risk of disease
• observed the bacterium and had two forms when grown on agar plates: smooth (S) and a rough (R) form.
• S phenotype due to a polysaccharide capsule
• In mice, R bacteria were harmless, but the S bacteria caused disease, heat killed S cells are also harmless, when the two are mixed , the injected mice got sick and died
• when he isolated bacteria from the dead mice, they had the appearance of the virulent strain (S), even though he had injected nonvirulent R bacteria

Question 6

Conclusion of Frederick Griffith experiment

Answer 6

• non virulent strain bacteria can be transformed into a virulent strain of bacteria by an unknown molecule from the virulent cells
• the unknown molecule carries information that causes virulence

Question 7

1944 - Avery et al, experiment

Answer 7

1 - Remove lipids and sugars from a solution of heat-killed S cells. Proteins, RNA and DNA remain
2 - Treat solutions with enzymes to destroy protein, RNA and DNA
3 - Add to culture containing living R cells, observe for transformation by testing for the presence of virulent S cells

Question 8

1952 - Hershey and chase - Bacteriophage

Answer 8

• Hershey and Chase used bacteriophages labelled with radioactive isotopes:
• ³⁵S to label protein (sulphur in protein coat)
• ³²P to label DNA (phosphorus in DNA)
  Method:
  1- Phages with labelled protein (³⁵S) or DNA (³²P) infected E. coli.
  2- Blending and centrifugation separated the phage coats from the bacterial cells.
  3 - Radioactivity from ³²P (DNA) was found inside the bacteria; ³⁵S (protein) remained outside.
• Conclusion:
  DNA, not protein, is the genetic material transmitted by bacteriophages to bacteria.

Question 9

James Watson and Francis Crick - 195

Answer 9

• Watson and Crick built models of a double helix to conform to the X rays produced by Franklin
• determine there is an outer sugar-phosphate backbone, with the nitrogenous bases paired in the molecule’s interior
• built a model in which the backbones were antiparallel (their subunits run in opposite direction)