DNA - The genetic material

Question 1

Who discovered DNA and how?

Answer 1

Friedrich Miescher in 1869
- Isolated nuclei from leukocytes (from pus on bandages)
- Broke cells open with detergent, treated with acid to form a pp of ‘nuclein’

Question 2

Altmann’s contributions

Answer 2

The name changed to nucleic acid
- Also found purine and pyrimidines

Question 3

Miescher’s chemical analysis

Answer 3

Chemical analysis - found nuclein was made of phosphorus , carbon, oxygen etc
=> also found when a cell divides it doubled

Question 4

Issues with Miescher’s experimental procedure

Answer 4

• Determined egg and sperm had diff amounts of N
• Concluded nuclein was not genetic material as a result

Question 5

Griffith’s Experiment (1928) - Transforming non-lethal strain of Streptococcus pneumoniae to lethal

Answer 5

• S strain live cells (pathogenic) killed mice while R strain did no harm
• When S strain was heat-killed, mouse lived
• When heat-killed S was injected with R, mouse lived and S strain live cells were found
• The extract of S strain transformed R strain (transforming principle) to be pathogenic do to the DNA passing on the genes for such

Question 6

Avery, Macleod & McCarty showing DNA is the transforming factor (1944)

Answer 6

• Combined S strain extract w/ R strain with:
• no components destroyed like Griffith = live S strain recovered
• Polysaccharides, lipids, RNA destroyed by enzymes = strain recovered
• Protein destroyed = strain survived (surprising as was thought to be genetic material)
• DNA destroyed = no live S strain + only cond. mouse lives

Showed DNA is genetic material

Question 7

Acceptance of DNA being genetic material

Answer 7

• Avery, Macleod & McCarty was ignored by scientists (esp. those looking into what was the genetic material) at the time
• Hershey and Chase confirmed their findings in 1952 => and this was when scientists accepted it

Question 8

Evidence that DNA is the genetic material in phage: the Hershey–Chase experiment (1952)

Answer 8

• Used radioisotopes to give DNA + protein labels that could be tracked during infection
• Phosphorus (32^P) into DNA + Sulfur (35^S) into proteins of separate phage cultures
• Infected E.coli cultures , sheared empty phage ‘bodies’ off bac. cells with blender
• Separated cells from phage bodies in centrifuge + measures radioactivity in solid bacteria + liquid phage ghosts

Question 9

Results from Hershey-Chase Experiment (1952)

Answer 9

• When 32^P phages infected E.coli, radioactivity was inside bac. cells - DNA entered cells
• When 35^S phages were used, radioactivity was in phage ghosts - protein did not enter the cell
• Also, progeny of 32^P phages remained labeled, while 35^S phages didn’t

All indicates DNA is hereditary material