HISTORICAL EXPERIMENTS. DNA as Genetic Material DNA Structure

Transcription

1 HISTORICAL EXPERIMENTS DNA as Genetic Material DNA Structure

2 DNA as the genetic material Miescher (1868) Griffith (1928) Avery, McCarty, MacLeod (1944) Hershey, Chase (1952)

3 Friedrich Miescher Discovery of DNA Collected white blood cells from pus Lysed cells and isolated nuclei Found a substance he called nuclein present in every cell type he tested found in nucleus of the cells

4 1920 DNA versus Protein Debate Scientists unsure if DNA or protein was the most important hereditary molecule. The money was on protein since they knew protein had obvious function and there were 20 amino acids that could be combined in many different ways. Whereas for DNA, it was thought that 4 nucleotides were connected in the same repeated pattern.

5 1928 Frederick Griffith Studied the bacterium that causes pneumonia, Streptococcus pneumoniae or pneumococcus bacteria

6 Griffith - Bacteria Strains

7 Griffith Experiments

8 Griffith: Conclusions What did he conclude? Griffith called the observed phenomenon transformation Griffith s definition: change in a cell s function by the transfer of an unknown substance Current definition: change in genotype and phenotype due to uptake of external DNA by a cell

9 1944 Avery et al. Oswald Avery, Maclyn McCarty & Colin MacLeod Continued Griffith s idea by also using R and S strains of pneumococcus bacteria Purified DNA from the S strain bacteria Added it to non-virulent R strain bacteria Only experiments with S strain DNA resulted in S strain cells

10

11

12 Avery et al: Conclusion What did they conclude? DNA is the transforming material (not protein or RNA)

13 1952 Alfred Hershey & Martha Chase What they knew: Virus are composed of DNA and protein Virus infection reprograms a host cell to produce more virus Question: which viral component is responsible for the reprogramming, DNA or protein?

14 Hershey Chase: Materials Bacteriophage: virus that infects bacteria Radioactive phosphorus which labels DNA Radioactive sulfur which labels proteins

15

16 Hershey Chase: Conclusions What did they conclude? DNA is the material transmitted from virus to bacteria DNA is the genetic material Video: hill.com/olc/dl/120076/bio21.swf

17 DNA Structure Levene (1909) Chargaff (1947) Franklin & Wilkins (1952) Watson & Crick (1953)

18 1909 Phoebus Levene Determined correctly that: DNA made up of chains of nucleotides A nucleotide consists of 3 components: n Deoxyribose sugar n Phosphate groups (1 3) n Nitrogenous base (ATCG)

19 Nitrogenous base Rings of C & N Purine: 2 fused rings A, G Pyrimidine: single 6 sided rings C, T (DNA), U (RNA)

20 1947 Erwin Chargaff Isolated DNA from different organisms and measured numbers of each base Showed a ratio where: A =T C =G

21 1952 Rosalind Franklin & Maurice Wilkins Used x-ray crystallography to study the 3D shape of the DNA molecule

22 X-ray Crystallography A physics approach to examining biological molecules. A pure crystallized sample of the molecule is isolated. X-rays bombard the crystal sample and refract the rays in multiple directions. Diffraction pattern is analyzed to determine the molecule s 3 dimensional structure

23 X-ray interpretation DNA has: Double helix structure Backbone of alternating phosphate and sugars Nitrogenous bases are in the middle of the molecule Bases are at right angles to the backbone

24 X-ray interpretation Molecule is a uniform helix with consistency in helical dimensions Diameter Distance between one helical turn Distance between one stacked base pair Angle of helical turns

25

26 What s left to discover? Already know: Hereditary molecule is DNA. DNA molecules are polynucleotides. DNA has a double-helix structure with alternating sugar-phosphate molecules with bases in the middle. What they don t know: How are the bases arranged in the middle?

27 1953 James Watson & Francis Crick Inspired by alphahelical structure in proteins (2 o )

28 Base Pairing Determined rules for base pairing: Hydrogen bonding between A=T, G=C

29 Base Pairing Determined rules for base pairing: Hydrogen bonding between A=T, G=C Width of a purinepyrimidine pair

30 Replication Model Anti-parallel nature of strands Double helix model provided an easy method for DNA replication

31 Links: Discovery of the Structure of DNA Video: Animation: DNA Interactive Website Dolan DNA Learning Center The DNA Story Chemistry World, Issue April /April/story.asp Song:

32 Cell Cycle Overview & Central Dogma