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

*properties of DNA seemed too uniform

to account for the multitude of inherited

traits…it seemed too simple!

Most researchers thought PROTEIN

was the genetic material because:

● 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!)

● 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

1930

...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...."

● Watson and Crickdiscovered 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.

● 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

● 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