NOTES - Ch. 16 (part 1): DNA Discovery and Structure

By the late 1940’s scientists knew that chromosomes carry hereditary material & they consist of DNA and protein. (Recall Morgan’s fruit fly research!)

*proteins have functional specificity *little was known about DNA Most researchers thought PROTEIN was the genetic material because: *proteins have functional specificity *little was known about DNA *properties of DNA seemed too uniform to account for the multitude of inherited traits…it seemed too simple!

● Frederick Griffith (1928) performed experiments which provided evidence that the genetic material was DNA *Griffith (a British medical officer) was studying the bacterium that causes pneumonia in mammals: Streptococcus pneumoniae

● 2 strains of pneumococcus: -smooth, encapsulated strain (S): PATHOGENIC -rough, nonencapsulated strain (R): NONPATHOGENIC

4 experiments (see fig. 16.2)

DISCOVERY… **the living R pneumococci had been TRANSFORMED into virulent S-strain organisms!!!

● Griffith was unable to determine the chemical nature of the transforming agent but hinted that it was not protein because heat denatures protein yet it did not destroy the transforming ability of the genetic material in the heat-killed S cells

● Avery, McLean and MacLeod (1944) discovered that the transforming agent was DNA (purified chemicals from heat-killed bacteria and tried to transform live nonpathogenic bacteria w/each chemical…only DNA worked!)

the genetic material of bacteriophage (T2) in 1952 ● Hershey and Chase discovered that DNA is the genetic material of bacteriophage (T2) in 1952 *a virus consists of DNA enclosed by a protective protein coat; *to reproduce, a virus must infect a cell and “take over” (reprogram) the host cell’s metabolic machinery to make new viruses *viruses that infect bacteria are called BACTERIOPHAGES, or just PHAGES. 1953

*these scientists did not know which viral component (DNA or protein) was responsible for reprogramming the host bacterial cell (see experiment Fig. 16.4)

*Results: viral protein remains outside the host cell and viral DNA is injected into the host cell; therefore nucleic acids rather than proteins are the hereditary material: -radioactive P-32 found inside host cell -radioactive S-35 found outside host cell

● Experimental evidence for DNA as the hereditary material in eukaryotes came from the lab of Erwin Chargaff (1950); using paper chromatography to separate nitrogenous bases, Chargaff reported the following: 1930

Chargaff’s Results: *DNA composition is species-specific; the amounts and ratios of bases vary from one species to another *in every species he studied, there was a regularity in base ratios where: the # of A residues equaled the # of T residues, and the # of G equaled the # of C

...I believe that the double-stranded model of DNA came about as a consequence of our conversation; but such things are only susceptible of a later judgment...." 1930

● Watson and Crick discovered the double helix by building models to conform to X-ray data *Watson saw an X-ray photo of DNA produced by Rosalind Franklin; Watson and Crick deduced from Franklin’s data that:

a) DNA is a helix with a width of 2 nm; b) purine & pyrimidine bases are stacked 0.34 nm apart; c) the helix makes 1 full turn every 3.4 nm along its length; d) there are 10 layers of bases in each turn of the helix.

Original DNA demonstration model ● Watson and Crick’s proposed structure is a ladder-like molecule with sugar-phosphate backbones as uprights and pairs of bases as rungs. ● the 2 sugar-phosphate backbones of the helix are ANTIPARALLEL (they run in opposite directions) Original DNA demonstration model

 to be consistent with a 2 nm width, a purine on one strand must pair (by H-bonding) with a pyrimidine on the other strand  base structure dictates which pairs of bases can form hydrogen bonds

PURINES & PYRIMIDINES:  PURINES = double-ringed nitrogenous bases;  adenine (A) and guanine (G)  PYRIMIDINES = single-ringed nitrogenous bases;  thymine (T) and cytosine (C)

Chargaff’s Rules:  Purines (double-ring) must pair with pyrimidines (single-ring)  A pairs with T  G pairs with C

*5-carbon sugar: deoxyribose *nitrogenous base: A, G, C, or T ● DNA is made up of subunits called NUCLEOTIDES   ● each nucleotide is made up of 3 basic parts: *5-carbon sugar: deoxyribose *nitrogenous base: A, G, C, or T *phosphate group